Eylais warkawae n.sp. (Hydracarina) and Some Features of Its Life History and Anatomy
[Read before Wellington Branch, February 3. 1953: received by Editor, February 6, 1953.]
Eylais warkawae is described and found to be closely related to the previously described New Zealand species, Eylais schauinslandii. The principal differences include the absence of a pan of bristles on the eye-bridge; the absence of coarse Innition with scattered giaining on the skin of the body; the angled not rounded anterior point of the mandible; the more slender P. III with no projection from the distal flexor surface; the outer row of five, not nine, short spines on P.IV, and the presence of three pairs of genital plates in the female.
A general account of development in the Hydracarina is included and the life history of E. wraikawae described and shown to agree with the general pattern. Details of the life history such as the method by which the larva hatches, its attachment to the host and the emergence of the nymph and adult are described.
An account of the anatomy of the female of Eylais waikawae is given and agrees with that given by Croneberg for E. extendens in 1878 except in the shape and structure of the mouth glands, the more extensive branching of the excretory system and the presence of a median unpaired nerve innevating the pharynx. The internal organs have undergone modification from the normal acarid plan, especially in the development of a large sac-like ventrieulus and extensive branchings of the exeretory and reproductive systems.
Introduction and Technique
The Hydracarina have been neglected in New Zealand, where only one species. Eylais schauinslandii Koenike 1900, has been so far described. This was collected at D'Urville Island by Professor Schauinsland during a voyage in 1896 and 1897. The species is listed by Hutton (1904) and by Lamb (1952). White (1900) refers to minute arachnids travelling rapidly over the surface of the water, but not entering the water. These probably belonged to the Genus Notaspis, a member of a terrestrial family but often found on the surface of the water or on water plants. Two species of the closely related marine mites, Halacaridae, have been described by Chilton (1883) from Lyttelton Harbour. In Australia the Hydracarina are a little known group. The major work is by Lundblad (1947) who describes species sent to him by H. Womersley. Lundblad's paper includes nearly all the Hydracarina described from Australia previously to 1941, but does not mention the three species described by Rainbow (1905–1907), Eylais maccullochi Rainbow, Hydrachna odontognathus Canest., and Atax cumberlandensis Rainbow.
Hydracarina are probably widespread in New Zealand, although the number of species may not be large. In this work for the study of the external morphology both canada balsam and Diaphane gave satisfactory mounting media. For anatomical studies Schaudinn's fixative, with three to four additional drops of acetic acid, was used, the chitin being softened by immersion in chlorinated
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water for a few minutes until the animal bleached. For dissection, the specimen was fixed to the bottom of a syracuse watch glass by a layer of glycerine jelly and dissected in 2 1/2% formalin. Tinting the specimen with 1 in 200 methylene blue for six hours made the dissection easier. For serial sectioning, an imbedding wax consisting of a mixture of three parts paraffin to one part beeswax, gave best results. Sections cut well at 10 mu but were inclined to crumple at 8 mu. Picro-nigrosin and Delafield's haematoxylin counterstained with eosin were the most satisfactory stains. Reconstructions of the anatomy were drawn, using serial coloured diagrams of the sections together with information from dissections.
Superfamily: Limnocharae Viets, 1926 (part).
Family: Eylaidae Leach, 1815 (Eylaides).
Subfamily: Eylainae Claus, 1880.
Genus: Eylais Latr., 1796.
In the Genus Eylais the animal swims with a quick, gliding movement, without employing the fourth pair of legs, which are carried trailing behind. All four eyes are placed in the middle of the front part of the back. These Hydracarina are mostly large, red animals. In this genus the most important characters for the determination of species are those of the eye-capsules, especially the intercapsular bridge, the maxillary organ, the palps, the form and sculpturing of the epimera and the presence and form of the genital “hair plates”. This New Zealand species is near to the previously described New Zealand form. E. schauinslandii Koenike and to the closely related widespread species E. infundibulifera Koenike. Koenike's account of E. schauinslandii is inadequate especially in illustration, and it has not been possible to obtain specimens from the locality from which his specimens were collected—namely, D'Urville Island, but it seems fairly certain that at least two New Zealand species exist. The specimens dealt with here differ from nearly all European species and from E. schauinslandii in the absence of a pair of bristles on the eye-bridge. Other differences from E. schauinslandii include the absence of coarse liniation with scattered graining on the skin of the body; the angled, not rounded, anterior point of the mandible; the more slender P.III with no projection from the distal flexor surface; the outer row of five, not nine, short spines on P.IV. and the presence of three pairs of genital plates in the female. The descriptions of about 55 species of this genus have been compared with the specimens collected.
Eylais waikawae n.sp.
Adult. (Text-figs. I and II.)
Body: Oval in outline (Text-fig. I, Figs. 1 and 2) and dorsally compressed. Up to 3 mm. in length and 2.5 mm. in breadth, bright red in colour.
Skin: Smooth, with the pores of the dermal glands very small and each bearing a small, fine hair. Several papilliform projections (Fig. 3), 0.12 mm. in length, and with four curved hooks (H) at the end occur irregularly over the body surface.
Text-fig. I. Eylais waikawae n.sp.
Fig. 1.—Dorsal view of female.
Fig. 2.—Ventral view of female.
Fig. 3.—Papilliform projection from skin.
Fig. 4.—Eye-capsules and bridge, dorsal view.
Fig. 5.—Genital area of female, ventral view.
Fig. 6.—Genital plate of male, lateral view, porosity confined to a restricted area. Scale in mm.
Eylais waikawae n.sp.
Fig. 1.—Mouth parts, ventral view.
Fig. 2.—Maxillary organ, ventral view.
Fig. 3.—“Luftkammer” (air chamber). lateral view, showing part of trachea.
Fig. 4.—Mandibles, lateral view.
Fig. 5.—Right palp
Fig. 6.—Left fourth leg.
Porosity omitted except in Fig. 5, Scale in mm.
Eyes: Two pairs contained in capsules, there being one capsule on each side of the body (Fig. 1, E). The eye-capsules (Fig. 4) relatively short and broad, squarely oval and set slightly at an angle. Each capsule 0.22 mm. to 0.24 mm. in length and 0.17 mm. to 0.18 mm. in breadth, the entire width of the eye area, including both capsules and bridge is 0.38 mm. to 0.42 mm. The chitin of the capsules coarsely granulated and thickened at the edges; the outer capsular margins only very weakly convex, their anterior margins straight. Intercapsular bridge (B) short and narrow, its length being equal to nearly half and its width, 0.04 mm. to 0.05 mm., to nearly one-third that of the capsules. The process for muscular attachment short, broad and projecting only very slightly beyond anterior margin of bridge. No hairs or bristles are present on either the bridge or the capsule.
Mouth parts: Total length 0.72 mm. (Text-fig. II, Fig. 1) Maxillary organ (Fig. 1, MO; Fig. 2) 0.46 mm. in length not including the posterior processes (VP), with the smallest breadth 0.31 mm. and the breadth in the middle 0.39 mm. The sides widen out anteriorly to form a broad shoulder at the articulation of the palps. The anterior edge has a moderately shallow bay, with the processes on either side of it short and forming a point. The dorsal anterior processes (DP) long and strong, thickened at the ends but not forming a point. They are directed posteriorly and reach almost as far as the base of the ventral posterior pair. The short, broad ventral posterior pair (VP) are expanded and bluntly rounded at their distal ends. The mouth is almost circular, with a broad outer ring and a broad inner mouth “frill” (MF). The mouth opening measures 0.09 mm. by 0·05 mm., the outer ring 0.27 mm by 0·25 mm. The basal plate is porous posteriorly but reticulately sculptured round the mouth opening.
Pharynx (Fig. 1, PH) broad, tapering very slightly at each end and extending approximately to posterior edge of the ventral processes. The pharynx measures 0.42 mm. in length and the posterior end is chitinised. The air-chambers (“Luft-kammer”) (Fig 1, L; Fig. 3) which he alongside the pharynx, extend back to posterior edge of the latter and are moderately strong, being of nearly equal width throughout but near the mouth curved and forming a point.
The mandibles (Fig. 1, M; Fig. 4) lie both anterior to the maxillary organ and dorsal to it, with the dorsal processes of the maxillary organ on either side of them. Each mandible measures 0.37 mm. in length and 0.22 mm. at its greatest depth. Dorsally the mandibles end in a porous, chitinous cap forming a medial point, ventrally they are composed of two thick and a third more delicate chitinous projections (Fig. 4, HY), all of which meet medially at the ventral, or mouth, end in a “shield” (S). The ventral tip of the median projection forms a dark brown rounded hook, which is hyalme (HY).
Palpi: Up to 1.06 mm. in length (Text-fig. I, Figs. 1 and 2, P; Text-fig. II, Fig. 5), which is approximately equal to three and a-half times length of body. They are not as strongly developed as the segments of the first pair of legs, and taper slightly towards the distal end. The flexor surface of the small P. I is longer than the extensor surface. P. II and P.III are approximately equal in length and each is twice as long as P. I, while P.IV is twice as long as P.III, and P.V is straight and slightly longer than P.III. Two bristles are present on the extensor surface of P. I, P. II has seven bristles on the extensor surface, four on the flexor surface, and one medially at distal end. There are six bristles on the extensor surface of P.III, six on the distal end of the flexor
surface and six more further outwards. P.IV has five bristles on the extensor surface, ten on the flexor surface, and 11 medially, some of the medial ones tending to be pectinate. Six short, sturdy spines are present at the tip of P.V. and three bristles on the flexor surface with three more medially. The chitinous structure of the palps is very coarsely porous.
Epimera: Occupy the anterior half of the ventral surface (Text-fig. I, Fig. 2, EP). The first and second epimera on each side lie one against the other and are long and slender. The third epimera is also narrow and extended, meeting the more triangular fourth epimera medially, so that a slit is left on the lateral margin. All pairs are of approximately equal breadth, and the second and third are the longest. The fourth epimera is expanded on the posterior outer corner. All epimera have a wavy network of striations and thickened margins, the inner end of each having a thick subcutaneous margin.
Legs: Length of first pair 2.3 mm.; of second pair 2.5 mm.; of third pair 2.9 mm., and of fourth pair 3.2 mm. (Fig. 2). Abundant bristles are present on all segments but increase slightly in number from the first to the fourth pair of legs, some of them tending to be pectinate. The bristles on the fourth pair are slightly shorter than the others (Text-fig. II, Fig. 6). At the distal end of each leg is a pair of strong claws, each claw with a small accessory claw-like projection arising near its base. The claws have approximately the same size and shape on all four pairs of legs, and four to eight small fine bristles are also present at the distal tip of each leg. The chitinous structure of the legs, like that of the palpi, is very coarsely porous, and each pore is of an irregular quadrilateral shape.
According to Daday, a sexual difference is shown by the legs in this genus; in the male both the upper surface and the under surface of segments three to five of the first three pairs of legs carry “swimming hairs,” but in the female only the upper-surface has these. In the specimens dealt with here there is no such difference in the leg bristles of male and female, since they are present on the upper surface and the under surface of segments three to five of the first three pairs of legs in the female as well as the male.
Genital area: Genital aperture (Text-fig. I, Fig. 2 and 5, GA) narrow and approximately 0.08 mm. long, situated between the two groups of the first and second epimera. Anteriorly there are three pairs of plates (Fig. 5, GP), the first 0.07 mm. long carrying 12 setae, the second much smaller with three setae, and the third slightly smaller again with three setae.
Excretory pore: Lies between the two groups of the third and fourth epimera (Fig. 2), with its centre about 0.34 mm. behind the genital aperture and surrounded by a chitinous ring.
Agrees with the female except in the genital area, in which the genital organ is situated between the first two pairs of epimera and extends slightly posterior to them. It (Fig. 6) is funnel-shaped with the genital aperture at the apex, surrounded by numerous strong hairs, the anterior part longer than the posterior part and expanding at the tip to end bluntly. It is 0.42 mm. long and 0.2 mm. high. The chitin is porous as on the legs and palps.
E. waikawae is near to both E. infundibulifera Koenike and E. schauinslandii Koenike, but differs in the following respects.
Koenike gives the length for E. schauinslandii as 2. 7–3.5 mm., and the greatest breadth 2–2.8 mm., while E. infundibulifera is up to 5 mm. in length, so that E. waikawae is probably slightly smaller than both these species.
In E. schauinslandii the body skin shows a coarse liniation with scattered graining, but this is not present in E. waikawae.
Koenike does not give a diagram of the eyes in E. schauinslandii, but describes them as having the same shape as in E. infundibuliera except that the projection for muscular attachment is weaker. His description of E. infundibulifera refers to the weakly convex outer margin and weakly concave inner margin of the capsule, and the distance between the capsules as very nearly equal throughout. The width Koenike gives for the intercapsular bridge is wider than in E. waikawae but Soar and Williamson (1925) point out that “this species appears to be liable to considerable variation, particularly as regards the form of the intercapsular bridge” The anterior projection is longer than in E. waikawae, and the length of the bridge is also longer. A pair of bristles is present on the bridge as in most Eylais species, but these bristles are absent in E. waikawae.
In E. infundibulifera the extremities of the dorsal anterior processes of the maxillary organ broaden out, and the ventral posterior processes are longer than in E. waikawae. The anterior half of the maxillary organ is closely set with large pores, not reticulately sculptured as in E. waikawae. The pharynx extends beyond the posterior ventral processes in E. infundibulifera but the air-chambers, which are slender, do not extend as far as the posterior extremity of the pharynx.
Koenike compares the mouth parts of E. schauinslandii with those of E. infundibuliera, and states that the mouth “frill” protrudes forward considerably further than it does in E. infundibulifera. Although Koenike gives no diagram of this, the “frill” does not, in E. waikawae, protrude forward any further than it does in E. infundibulifera, as figured by Soar and Williamson (1925). As in the latter species, there are large pores on the maxillary organ behind the mouth opening. The mandible in E. schauinslandii measures 0.32 mm. in length and the hind end of the “basal” part (the anterior point) is slightly rounded, whereas it is angled in E. waikawae. In his diagram, Koenike shows a moderately large space between the two mandibles, about halfway along their length, but this is not present in E. waikawae. Koenike's two diagrams of the mandibles, ventral and lateral views, do not agree with each other, and his description is not clear, but none of the three agree with the mandibles of E. waikawae.
In E. infundibulifera P.III is very stout and the distal flexor surface projects slightly, unlike that of E. waikawae P.IV has an outer row of nine short spines instead of five as in E. waikawae, and P.V curves well towards the flexor surface. There is a non-porous area on the flexor surfaces of P.I and P.II which is absent in E. waikawae.
In E. schauinslandii P. I has a strong chitinous flexor surface developed as a short, sturdy appendage which from the diagram appears to be larger than in E. waikawae P.I also has a pectinate bristle, while neither of the bristles on P. I in E. waikawae are pectinate. There is a non-porous ring, 0.016 mm. wide, around the junction of P.I and P.II The distal expanded portion of the flexor surface of P.III carries nine short undivided bristles, in E. waikawae it carries six. The bristles on P.IV are entirely different, and although the remaining
bristles are not described in the text, the diagram shows that there are marked differences from E. waikawae.
The epimera of E. infundibulifera are not described and for E. schauinslandii there is no description but a diagram of the ventral surface of the animal shows the outlines only, and these do not agree with those of E. waikawae.
There is no description of the legs of either E. infundibulifera or E. schauinslandii.
The genital area of the female of E. infundibulifera is not described, and in his description of the female of E. schauinslandii Koenike makes no mention of the three pairs of plates.
The genital organ in E. infundibulifera is similar to that of E. waikawae except that the aperture is surrounded by a single circlet of hairs, whereas there is more than one circle in E. waikawae. In E. schauinslandii the genital aperture is 0.062 mm. long and the “funnel” 0.304 mm. long and 0.176 mm. in height, which is smaller than the average for E. waikawae, but Koenike's specimens may have been younger and smaller. This may also be the explanation for Koenike's measurements of the male, 2·0 mm. in length and 1·8 mm. in breadth, which are smaller than those of most E. waikawae males.
Spherical and measuring on the average when laid 0.14 mm. to 0.16 mm. in diameter. Bright red in colour and rich in yolk. The egg-case is composed of three layers, the middle one thick and with radial divisions, the outer and inner layers thin.
Larva. (Text-fig. III. Fig. 1.)
Length 0.165 mm., breadth of body 0.116 mm. The larva is red in colour with the body elongate oval, dorsoventrally somewhat flattened. The skin is soft and not enclosed by chitinous plates, and there is no dorsal shield. Ventrally the body is almost completely covered by the expanded epimera (EP) and the excretory (EX) is surrounded by a plate, so that only a small area of the ventral surface is left uncovered. There are numerous long hairs on the body.
The capitulum (CAP) is not large and is firmly attached to the body. There are two mandibles (M) which lie inside a maxillary organ. The front edge of this organ is almost straight, although in his description of the larva of this genus Viets (1936) describes a moderate snout. Viets' description is based almost entirely on the larva of one species, E. hamata, and he does not claim that it is complete. The mouth opens just ventral to the outer edge of the maxillary organ. The palpi (P) are attached to the side of the capitulum and project forward slightly in front of the maxillary organ, although Viets says that they project scareely or not at all in front of it. Each palp has five segments, with P. V. bent back against P.IV. There is no strong forked appendage on the outer end of the extensor surface of P.IV, as was described by Viets.
On the body, double eyes are present on either side at the anterior end of the dorsal surface. All three epimera (EP) on each side are coalesced except at the lateral edges where divisions are present for about half the breadth of each epimera. The epimera of the two sides are separated medially by a moderately wide space, which broadens both anteriorly and posteriorly. Each of the six legs has five segments, not six as Viets maintains, and bears bristles. The distal end of the last segment is narrowed and carries two different claws, one long and
sickle-shaped, the other smaller, thicker and slightly bent at the distal end. The third, smallest, bristled claw-like formation described by Viets is not present. There are several small bristles at the distal end of the segment. The hyaline, bent, tube-like, rounded at the end appendage (“tactile appendage” or “Tastan-hang”) on the proximal end of the extensor surface of the end segment of the second leg described by Viets is absent. A “urstigma” (UR) occurs on each side, laterally and just behind the articulation of the first leg with the epimera. Its function is unknown and there does not appear to be a movable lid. The excretory pore (EX) lies on a small elongated plate which narrows to a point at its anterior end.
Approximately 1·64 mm. in length and 1·5 mm. in breadth. The projection from the front edge of the eye-bridge is absent. Otherwise the only difference from the adult is in the absence of sexual apparatus. There is the same number of bristles on the palps, and the mouth “frill” and mandible are the same, although according to Viets (1936) these characters may be different in the nymph.
The nymph of E. schauinslandii is 1·6 mm. in length and 1·4 mm. in breadth, and differs from the nymphs collected in the same characters as those in which the adults differ.
Holotype: Maramau Lagoon, South Wairarapa; paratypes, fresh water pool near Waikawa Beach.
South Wairarapa Lagoon: Monthly collections were made between March, 1951, and July, 1952 inclusive. Nymphophans were collected on July 5, 1951, and April 12, 1952, and probably occur throughout the winter from about April until the end of the year. Nymphs were present in small numbers in July, 1951, became more numerous in August, and continued to be present until April, 1952. Adults occurred from October, 1951, until March, 1952, when, having laid their eggs, they died.
Waikawa Beach Pool: On the two collections made, on April 14 and 15, 1952, and June 2, 1952, both nymphophans and adults were present, but nymphs were only present in the April collections. In the laboratory nymphs were still metamorphosing into adults in May. Thus in this locality the adults occur at a time when they are absent from the other locality.
The Life History. (Text-fig. III.)
Henking in 1882 presented a scheme outlining the normal development of the Acarina, followed by the Hydracarina, and naming the “skins” in which the resting stages are enclosed apodermata. The egg develops into a schadonophan which is enclosed inside a special secondary egg-skin, the “deutovummembrane” or schadonoderma. The six-legged larva which hatches from inside the deutovum-membrane is always very different from the adult and resembles the terrestrial mites more closely than does the adult. The larva begins to feed, which it may do so copiously that the original shape is lost. Most hydrachnid larvae attach themselves to a host, usually an insect, and feed on its tissues, remaning attached to the host until the emergence of the nymph. Feeding soon ceases, and the larva becomes immobile, passing into the nymphochrysalis resting stage,
which may be found as pear-shaped bodies on water insects. The appearance of an apoderma marks the beginning of the nymphophan stage, during which a complete metamorphosis takes place. The nymph emerges from inside the old larval skin, which remains adhering to the host. The nymph always has the habitus of the adult and the principal difference is that the internal organisation is still not completely developed, especially the genital system. A complete metamorphosis is undergone by the nymph, which passes into the teleiochrysalis stage, usually spent attached to a plant. A second postembryonic apoderma is formed, the teleioderma, and the animal steps into the teleiophan stage. The adult emerges by breaking the teleioderma and the old nymphal skin. The development according to Henking's scheme is:—
|Resting Stages||Free-living stages|
|1. Egg||→||Appearance of the Apodermas.||→||Schadonophan||stage||→||Throwing off of the Apodermas.||→||Larva|
Following Lundblad (1927), if we do not take into consideration the less important nymphochrysalis and teleiochrysalis stages, which are essentially only immobile, and in the beginning of their formation the larva or, respectively, the nymph, the following stages are seen:—
The most useful works on development in the Hydracarina are those of Wesenberg-Lund (1918), Lundblad (1927), Viets (1936), Vitzthum (1940) and Masuda (1934).
Knowledge of the development in the genus Eylais is limited and a great deal of the information in the early literature is incorrect. Summaries of the early observations are given by Wesenberg-Lund (1918) and Lundblad (1927). Most of the nymphophans reported on a wide variety of insect hosts probably did not belong to this genus, and there are few accounts of developmental stages definitely identified as belonging to this genus. When the species is given it is usually Eylais extendens O. F. Müller.
There are no accounts of the method of copulation in this genus, but it has been observed several times in E. waikawae. The male and female come together with their ventral surfaces opposing, clinging to each other with their legs, but facing in opposite directions, the anterior end of one being opposite to the posterior end of the other. In one instance recorded, copulation began at 10.5 a.m., and the animals parted at 4.50 p.m., but this is slightly longer than the average length of time taken. On several occasions, often up to six times in one day immediately after the collection, clumps of six to thirty swarming Eylais have been observed in a constantly moving ball which has hydrachnids clambering all over the outside, and is formed of individuals clinging tightly together, usually with two copulating in the centre. The hydrachnids forming the cluster are
in most cases nearly all females. Since the males are markedly rarer than the females, it is possible that when a male is recognised, perhaps by some excretion, the females are stimulated to copulate.
The process of egg-laying has been recorded by Neuman, in 1880, and Thon, in 1901 and 1906, and Thon's account has been criticised by Sokolow in 1924 (Lundblad, 1927). It was not observed in the New Zealand species, and probably takes place at night or early in the morning. In a 10 cm. culture dish set up on October 21, 1951, with an Anacharis branch and four Eylais adults, two females and two males, from Maramau lagoon, eggs were laid in the evening of October 28. The eggs occurred in thick clusters, containing about three to four hundred eggs, covering the Anacharis stem near to the apex and the undersides of three adjacent leaves. They were deep pink to red in colour, full of yolk and covered with a transparent gelatinous layer. Later about three hundred eggs were laid by a female of this species collected from the Waikawa Beach pool, and again they were laid between 10 p.m. (on April 20, 1952) and 9 a. m. (on April 21, 1952). In the morning a female was crawling on top of the egg cluster but did not lay any eggs. These eggs were laid on a leaf of Ruppia maritima, a plant which is common in this locality. The cluster was approximately 1·0 mm. long and encircled the leaf. The eggs were bright red and each was enclosed in its own individual gelatinous casing, the thickness of which was approximately equal to the diameter of the egg. The egg-laying has been observed in January, April, June, October and November, being most prevalent in the late summer. It probably does not take place between June and October. Several authors, including Wesenberg-Lund and Lundblad, believe that the eggs may partially hibernate, so that eggs laid in July (in Europe) do not hatch until the following spring, but the larvae then are at the same stage of development when hatched as those which hatch in the autumn. All the eggs laid by the New Zealand species hatched within approximately the same time span.
When offered a variety of plants, including Potamogeton, Nitella and Anacharis, individuals from Maramau lagoon always laid their eggs on the Anacharis, except in one instance when the eggs were laid on the side of the trough. Individuals from the Waikawa Beach pool, when supplied with plants from their natural habitat laid their eggs on Ruppia. However, if there was also some Anacharis in the culture dish the eggs were laid on this plant, atlhough it is not present in the locality from which they were collected.
About twenty to thirty eggs, removed from a stem and placed in an embryo-logical watch glass, were kept under observation. On about the sixth day the eggs began to show differentiation into a darker central area and a paler outer area, and during the next day or two the edge of the darker area formed into stumpy protrusions of varying shapes. By the eleventh day, three areas of different shades of red were visible, darkening towards the centre of the egg. The protrusions lengthened and narrowed, and by the fifteenth day six of them had become segmented and leg-like, while the body had rounded off into a more compact shape. Leucocytes were visible but not in large numbers. The auterior protrusions, which will form the mouth parts and palps, still showed no signs of segmentation. In four more days the eyes were visible, and in another three to four days, when the eggs were 23 to 24 days old, the mouth parts had differentiated and a dark gland was present at the posterior end of the body. The larvae now began to move their legs intermittently. In other batches of eggs, move-
ments of the larvae were not observed until 35 days after the laying of the eggs, these eggs having been laid later in the season. The larvae begin to hatch between the 24th and the 37th day, and the hatching process was watched several times. The dark gland at the posterior end of the body discharged its contents, and the larva remained still for a few minutes before moving round and eventually pushing out a soft area near to one end. This area expanded and the larva inserted its head and front pair of legs into it, the back pair of legs pushing against the opposite side of the egg-case. Apparently without a great deal of effort, the larva pushed its head and front pair of legs through the expanded portion and crawled out. It ran forward about twice its length from the empty case and then remained still, before commencing to swim actively and climb over the egg clusters. A dark area remained inside the empty case. The temperature was 57° and the hatching took eight minutes, after the discharge of the gland. The hatching of this larva was rapidly followed by the hatching of five more, and larvae hatched at any time of the day or night but usually in the morning. Some of the larvae were not active but remained still or with just the legs moving for most of the time, with only occasional short runs. Within six days of the hatching of the first larva, all of the original three to four hundred except for about a dozen had hatched.
The further activity of the larva (Text-fig. III, Fig. 1) has been observed by only a few authors. The larvae do not swim about the dish for long, but collect at the surface of the water or at the sides of the dish and there move to and fro with a crawling or running movement. They often congregate together, usually on the surface of the water, and frequently at the side nearest to the source of light. The New Zealand larvae were never seen to climb with the aid of air bubbles from water plants, although Musselius in 1914 reports that Krendowski stated that this occurred (Lundblad, 1927).
The host during the nymphophan stage had been previously determined by the collecting of all sizes of this stage on the backswimmer, Anisops wakefieldi, but when the larvae were introduced into a petrie dish containing individuals of this species of host they showed no signs of attaching themselves. Even when larvae and hosts were kept together in a culture dish from just before the time of hatching of the larvae, very few larvae attached themselves to a host, often only about six or eight from a batch of 300 to 400 eggs. The conditions obtaining in the laboratory are probably in some way responsible for this failure to find hosts. The larvae die within about two days after hatching if they have not attached themselves to a host and larvae which hatched on July 31, 1952, and showed no sign of any reaction to the host on August 1 were dead on August 2. There is probably a large mortality at this stage of the life history.
The Eylais larvae attack both waterboatmen and backswimmers. No Eylais nymphophans were found on any of the other insects or possible hosts examined. This is particularly interesting in the case of the dragonflies and damsel-flies, Uropetala, Procordulia, Austrolestes and Xanthocnemis, of which about 70 were examined but none gave any indication of hydrachnid parasites. According to the literature the larvae parasitise land insects, especially Tipulidae and Odonata, but they have also been found on and hatched from off waterbugs, Graphoderes, and the beetles, Gyrinidae and Dytiseidae, as well as, according to Lundblad, Corixidae. Wesenberg-Lund (1918) suggests that a change in host
occurs in this genus, the Eylais larva at first attaching itself to an aerial insect but leaving this host while still in the six-legged stage by changing to a water insect. Wesenberg-Lund points out that the Tipulidae, especially, are delicate insects and could not carry the relatively heavy Eylais nymph. In one place Wesenberg-Lund writes that “a change of host must take place before the first pupal stage, the six-legged larva leaving the aerial insect for search of another, upon which it could reach its full size.” Later he expresses doubt that the stage hatching from the water insect is a nymph, and thinks that it may be an adult. Two original ideas are involved, the change of host during the larval or early nymphophan stage and the continuance of a parasitic life up until the adult stage. Lundblad (1927) disagrees with Wesenberg-Lund's conjecture that a host change takes place, but he refers only to the second observation of Wesenberg-Lund, that is the hatching of the Eylais from the water insect as an adult, and not to Wesenberg-Lund's suggestion of an earlier change of host. In the New Zealand species, neither of Wesenberg-Lund's conjectures need apply, since the whole of the nymphophan stage can be carried out on the one host, from which the Eylais hatches as a nymph. Soar and Williamson (1925) also suggest that there may be two hosts.
On the waterboatmen the nymphophans always occur on the back, underneath the elytra. On the backswimmers they are usually on the underneath, or morphologically dorsal, surface. Lundblad (1927) also reports that the Eylais nymphophans observed by him were always fastened on to the back of the Cymatia and covered by the hemielytra.
The development of the nymphophan follows the general plan for hydrachnids. The parasitic stage in Eylais is little known, and the principal worker has been Piatakov, in 1916. A few days after the larva has attached itself to the host, the body begins to swell (Text-fig. III, Fig. 2). The swelling continues until the nymphophan is large and bag-like, an almost shapeless mass attached to the host by the mouth parts of the larva and with the legs and epimera of the larva still visible at the anterior end, except that some of the legs are usually lost. Inside this bag the nymph develops in the normal way, but the time taken for its development varies greatly, depending to a certain extent on the time of the year. If the larvae hatched during late November or December, the nymphs may hatch out at any time during the following year between April and October or November. When the larvae have not hatched until August, the nymphs probably hatch about April of the following year.
The nymph is visible through the larval skin for a few days before hatching and faces towards the anterior end of the larva. On the day of hatching its legs begin to move and it turns slowly round to face the posterior end. Stretching its legs and moving them vigorously, it breaks the skin enclosing it by a circular tear close to the posterior end. The nymph crawls out and swims away. It may rest for a time on a plant, or it may immediately begin to swim round the culture dish.
The length of time spent in the nymphal stage varies considerably, and m the case of Eylais waikawae the most important factor is the time of the year. If the nymphs hatch early in the summer, in November, December or January, they may remain in this stage for up to two months, but later in the season the nymphs usually metamorphose into adults after about a week. Wesenberg-Lund's
conclusion that the Eylais must emerge as adults from off the water insects because the specimens he was observing had not attached themselves to plants two weeks after hatching, is therefore unjustified. His specimens emerged on June 6, equivalent to the beginning of December in New Zealand, and the nymphs would probably not metamorphose for at least another month. The way of life of the nymph is similar to that of the adult.
In order to undergo the final metamorphosis, the nymph attaches itself to a plant, that most favoured in this species being Nitella (Fig. 3). The nymph clings to the Nitella stem (ST) with its legs and palps, which hook round the thin cylindrical stem and the rest of the body hangs free. The Eylais now enters the teleiochrysalis stage.
The development of the teleiophan takes only three or four days. On the second day the leg and mouth parts protrusions (DL, DM) are visible. Just previous to emergence, the legs begin to move and the adult starts to turn round inside the nymphal skin. The skin usually tears before the hydrachnid has turned round completely, and the tear is near to the posterior end of the enclosing skin. It is a smooth tear as in the case of that formed when the nymph emerges. The adult crawls out and swims away, being immediately active, without any quiescent period.
The time of the year when the various stages occur differs between the two localities. Eggs have been laid in the culture dishes in October, November, January, February, April and June, the batches early in the season being laid by specimens from Maramau lagoon, and those later in the season by specimens from the Waikawa Beach pool. Eggs have never been observed in the natural habitats, but they are probably mostly laid during February, March and April. The larvae hatch just under four weeks later from eggs laid in the early summer, but the length of time taken for their development increases later in the season, so that eggs laid in June take about seven weeks to hatch, and the time of hatching extends until August. The nymphs emerge at almost any time of the year, but principally between July and April, and the adults also at any time of the year, but principally from March until October. The adults die a few weeks after they have laid their eggs. The different stages of the life history are therefore not restricted to any particular time of the year. The species overwinters principally in the nymphophan stage, but eggs may still be developing in July. Dispersal is possible in the nymphophan stage, when the hydrachnid is carried from pond to pond by the host.
This account of the complete life history of a single species of Eylais agrees with most of the observations of overseas workers. The hosts are both native, and it is of interest that the Dytiscidae are not parasitised in New Zealand, although they are overseas. Anisops and Arctocorisa seem to have been the only water insects subjected to attack by larval hydrachnids in New Zealand. The possibility that the early part of the nymphophan stage may be passed on a dragonfly or damsel-fly is not completely excluded, and the infrequency of the attack on the Anisops and Arctocorisa hosts by the larvae in the culture dishes may be due to a preference for the aerial insects by the recently hatched larvae. The water insects may be attacked only as a last resort. Attempts to bring the Eylais larvae and the delicate aerial insects together were unsuccessful.
Internal Anatomy of the Female of Eylais waikawae n.sp. (Text-figs. IV, V and VI.)
The major work on the anatomy of the Acarina is that by Vitzthum (1940), who obtains most of his information on the Hydracarina from Sig Thor (1904) and Schmidt in 1935. Sig Thor studied the anatomy of several species of Acarina, including two species of Eylais and Hydrachna globosa. Other less important papers are by Nordenskiold in 1898 and Reuter in 1909. The most recent publication on the anatomy of the Acarina is by André (1949), but references to the Hydracarina are brief and apparently taken directly from Vitzthum. The only original work, besides that of Thor, published on the anatomy of the genus Eylais is by Croneberg in 1878. It has not been possible to refer directly to this work, but it is quoted extensively by Michael (1895) in a paper on the anatomy of Thyas petrophulus.
The anatomy of hydrachnids such as Eylais is greatly modified from that of other Acarina in relation to the adoption of an aquatic life and the reception of food in the liquid form. An intermediate stage is shown by hydrachnids such as Thyas petrophilus.
The Alimentary System.
In the Trombidiformes the alimentary system consists of a stomodaeum and a mesenteron or midgut. In Eylais waikawae a small round mouth is situated just below the anterior margin of the body in the ventral midline. It leads to a small lanceolate pharynx, 0·42 mm. long, whose walls are thickened with chitin. In transverse section the pharynx is crescent shaped, the ventral wall consisting of a strongly chitinised groove, the dorsal wall enclosed in the ventral groove and not as strongly ehitinised. According to Vitzthum (1940), the lumen is wider behind than in front, and in the case of Eylais species often much wider. In Eylais waikawae, the pharynx is only very slightly wider anteriorly than posteriorly. The pharynx is richly supplied with muscles, and tendons attach long, perpendicular muscles to its upper wall.
The pharynx leads into a narrow non-muscular oesophagus (Text-fig. IV, Figs. 1 and 2, OE), 1·3 mm. long and of approximately the same diameter throughout its length. The pharynx and the oesophagus together comprise the stomodaeum. Less than half way along its length, the oesophagus loops back anteriorly before passing between the two nerve ganglia (Fig. 2, N), which are separated only by the oesophagus. The oesophagus enters the large sac-like ventriculus (Text-fig. IV, Figs. 1 and 2; Text-fig. V, Fig. 1, Text-fig VI, Fig. 3; VE) in the midline, 0·05 mm. behind the nerve ganglia. It extends into the ventriculus for a short distance as a tube with a slightly thicker wall than the remainder of the oesophagus. The oesophagus is lined with small flat cells containing inconspicuous nuclei, referred to as hypodermis by Vitzthum and Thor.
The digestive organ of the alimentary system is the large, thin-walled, nonmuscular ventriculus (VE) which takes the form of a sac furnished with numerous short diverticula arising from the dorsal surface and the lateral walls, a condition also described by Croneberg in 1878 (Michael, 1895). In the immature individual the ventriculus fills the greater part of the body, extending anteriorly as a large median lobe (Text-fig. IV, Figs. 1 and 2; Text-fig. V. Fig. 1), and with diverticula, which are usually paired, filling all the available space. In the mature individual, the reproductive organs have increased in size to encroach
Index to Text-Figs.
An, accessory nerve; B, eye-bridge; CAP, capitulum; CC, chitinous capsule; CE; chitinous exoskeleton, DL, developing legs; DM, developing mouth parts; DP, dorsal process of maxillary organ; DU, duct; E, eye; ED, excretory duet; EG, egg; EO, excretory organ; EP, epimera; EX, excretory pore; F, femur; FC, femuricollum; G, genu; GA, genital aperture; GC, glandular cells; GP, genital plate; GR, “ground plate”; H, hooks; HA, glandular hair; HO, host; HY, hyaline area; L, “Luftkammer” (air-chamber); LC, lighter cell; LE, leucocytes; LS, lens; M, mandible; MF, “mouth frill”; MG, mouth gland; MO, maxillary organ; N, nervous system; NE, nerve to eye; NI, nerve to internal organs; NL (I-IV) nerve to leg (I-IV); NM, nerve to mandible; NMO, nerve to maxillary organ; NO, nerve to oesophagus and pharynx; OE, oesophagus; OT, ovarian tissue; OV, oviduct (or uterus); P, palp; PH, pharynx; RE, retina; RH, rhabdomes; S, “shield”; SBO, suboesophageal ganglionic mass; SG, skin gland; SPO, supiaoesophageal ganghonic mass; ST, stem, or leaf, of waterplant; T, trachea; TA, tarsus; TI, tibia; TR, trochanter; UR, “urstigma”; VE, ventriculus; VP, vential process of maxillary organ; Y, yolk.
on some of the space formerly occupied by the ventriculus. The diverticula are now narrower with the sides approximately parallel. Besides the median anterior lobe there are usually three median dorsal lobes and, on each side, six lateral dorsal lobes and two large lateral diverticula with a further still larger diverticulum ventral to them. The number, size and position of the diverticula varies in different specimens. The wall of the ventriculus (Text-fig. VI, Fig. 3, VE) is composed of a basal Malphigian layer of squarish cells with deeply staining nuclei giving off large vacuolated cells which exhibit holocrine secretion and project into the lumen. These cells are loose and rounded and may become detached. Vitzthum (1940) says that the digestion in hydrachnids is intracellular, and the cells agree with Vitzthum's diagram for Hydrodroma despiciens (O. F. Müller) but not with his diagram of Diplohydrachna globosa (de Geer) taken from Thor (1904), nor with Thor's other drawings. The most important character of the alimentary system in the trombidiforme type, to which the hydrachnids belong, is the absence of small intestine, colon, rectum and anus, so that the ventriculus is closed behind.
Only fluid food is eaten, in the form of plant and animal juices. The juices pumped in by the muscular pharynx pass through the narrow oesophagus (OE), which acts solely as a conducting tube, to the ventriculus (VE), where digestion takes place.
The Mouth Glands (MG).
Mouth glands are present in most Acarina, but reach their greatest development in Hydracarina. In Eylais waikawae they are situated on either side of the anterior median lobe of the ventriculus and take the form of three large roseate-shaped glands (“munddrusen” of Vitzthum) (Text-fig IV, Fig. 2, MG). The most dorsal gland is slightly elongated, measuring approximately 0·45 mm. by 0·33 mm., the anterior ventral gland is 0.55 mm. in height and 0.4 mm in length, and the posterior ventral gland is spherical, being 0.45 mm. in diameter. The glands each consist of a single layer of large vacuolated secretory cells with large, deeply staining nuclei. The inner ends of the cells approach close together at the centre of each gland so that the lumen is very small. The glands discharge by short ducts which join to form a single large duct, with a thick glandular wall and a narrow cavity, opening into the alimentary system close to the entry of the oesophagus into the ventriculus. The secretion of the mouth glands is injected into the prey or the plant cell, where it dissolves the tissues enabling them to be pumped into the animal in the liquid form.
According to Michael (1895), Croneberg found three pairs of glands, each pair bilaterally symmetrical, two pairs being more or less kidney-shaped, while the third pair is more sausage-shaped. He draws the kidney-shaped glands as composed of numerous, largish, closely pressed secreting cells with clear nuclei, and the sausage-shaped glands as composed of a single layer of squarish cells surrounding a small lumen. The description of his drawing of the ducts corresponds to the condition found in the New Zealand species. In Thyas the mouth gland is formed of large secreting cells radiating almost from a centre, a condition similar to that found in Eylais waikawae. Michael says that in Thyas the size and shape of the gland varies a good deal in different individuals and probably at different times, and this is possibly true also in Eylais. Michael also doubts whether there is sufficient evidence to justify a positive assertion that the function of these glands is salivary only, and this is apparently just as true now.
The Excretory System.
Since in the Trombidiformes there is no anal opening, excretion is active and the products are removed through an excretory pore. The excretory pore often opens at the hind end of the body, but in hydrachnids it opens in the midventral line, in Eylais between the fourth pair of epimera (Text-fig. I, Fig. 2, EX). According to Vitzthum, in principle the excretory organs of the Trombidiformes consist of a single tube which lies in the midline of the body above the ventriculus and fairly tight under the dorsal exoskeleton and sinks down behind the ventriculus to open through the excretory pore. The form of the excretory pore depends on the shape of the hind end of the excretory tube. In some hydrachnids, including Eylais, the lumen remains almost the same size as the previous duct, and the excretory pore is thus fairly large in circumference and often has two chitinous flaps, or valves. The flaps are raised by their own musculature, so that the lumen of the excretory tube lies free and the excretory mass is ejected with a pigment. This is moderately frequent. The front end of the anterior portion of the excretory tube forms a Y-shaped fork in most Hydracarina and some terrestrial Prostigmata, the branches sometimes spreading out so widely from one another as to form the figure of a T.
The schema is essentially the same in all Trombidiformes, but the excretory tube is very diversely shaped, even in individuals of the same species. Its form is modified by the development of the genital organs and by the size of the ventriculus and its diverticula. and above all by the quantity of the excretory mass present. The uneven distribution of the excretory mass causes irregular widening out of the tubes and outpocketing at their ends. In Eylais waikawae the ramification of the excretory tubes is greatly developed, and they contain hard white excretory matter. Extending inwards from the thick-lipped excretory pore the tube (Text-fig. IV. Fig. 2, ED) is for a short distance of approximately equal width, it then widens to form a reservoir (EO) which divides into two very short broad branches, each of which sends off a number of tubes. These tubes lie in the depressions of the ventriculus between the diverticula, and they expand slightly in the broader depressions at the bases of the diverticula and also at their ends.
Vitzthum points out that the tissue structure of the excretory organs shows clearly several differences in the case of different genera, but in the ground plan it seems to agree fairly well in all Trombidiformes. He describes a thin basal
membrane and in the anterior part of the organs, that is all the tubes, with high cells of a glandular character which probably are the only site for the essential production of the excretory mass. Further back towards the excretory pore, the cells become flatter and their delimitation may even completely vanish. Here the glandular function ceases, the tube serving only as a reservoir for the excretory mass. This reservoir can become greatly extended. Only in the neighbourhood of the excretory pore do the cells become higher again and narrower. In all its parts the excretory organ is surrounded by longitudinal musculature which is very strong in the case of Eylais, although usually thin.
In the New Zealand species, the cells are not high and glandular in the tubes or near to the excretory pore, but remain flat and indistinctly delimited throughout the whole organ. The organ is surrounded by longitudinal musculature which is not strong throughout, but special striated muscles are present near to the hind end of the tube leading to the excretory pore. The tissue structure probably differs considerably in the different genera and Vitzthum's diagrams are of Piona and Limnesia.
According to Michael, Croneberg draws and describes a single clavate sac overlying the ventriculus in the median line. This ends bluntly in front without any entrance into the ventriculus, but passes along the median line of its dorsal surface to bend down behind it to the opening on the ventral surface marked “an” in Croneberg's diagram. He describes this organ as being filled with the white matter so abundantly found in the malphigian vessels of many Acarina. No such single clavate sac occurs in E. waikawae.
The Genital System of the Female.
In Acarina the ovary is generally a paired, but often unpaired, organ at the inner end of the oviduct, while in the Parasitengona the paired ovaries join to form a horseshoe and in some cases the two ends of the horseshoe join to form a ring. The germinal layer lies on the inner side of the horseshoe or ring, while the eggs develop only on the outer side and are often “stalked” (“gestielt”). They receive their chitinous shell during development, and the shell forms from the egg itself (Vitzthum, 1940). In the fully mature females the ovary spreads out as numerous branches, which extend throughout the body in many directions. Consequently the eggs develop in long chains in all parts of the body. This situation is present in most of the Hydracarina which have been investigated, including Eylais. The New Zealand species agrees with Vitzthum's account and the branching is developed extensively, with the result that the ovarian tissue (OT) occurs as a mass of tissue which surrounds the inner end of the oviduct (OV) and extends outwards as ramifications following the surface of the ventriculus (VE) throughout the body but not extending to the tops of the diverticula. It lies close to the wall of the ventriculus except at the bases of the depressions where the excretory organ (EO) separates the two structures. A lateral extension forms a moderately thick layer of tissue which reaches the exoskeleton. The germinal tissue is on the side next to the oviduct, and developing eggs increase in size away from the oviduct. At first the eggs are small, spherical cells with a large, very deeply staining nucleus and deeply staining cytoplasmic granules. As they increase in size the eggs stain less deeply and yolk begins to appear, first close to the nucleus but eventually filling the egg except for a small clear space around the nucleus. The shell which develops,
consists of thin inner and outer membranes and a thick, chitinous middle layer with closely spaced parallel partitions giving a cellular appearance. According to Sig Thor (1904) a great amount of difference exists between the shells of the eggs in different species, and his description of the egg in Hydrachna differs from that of Eylais waikawae especially in regard to the middle layer.
In most Acarina the oviduct is paired, and lies on the ventral side of the ovary following the ovarian branchings, so that it becomes very long in some species. The oviducts from each side join and lead to the vagina, which is sometimes described as an ovipositor. The presence of a receptaculum seminis is stated for Eylais by Schmidt, who describes it as a strongly chitinised cavity, which empties by a canal (the “ductus receptaculi”) into the genital opening side by side with the vagina. In Eylais waikawae the oviduct (OV) is paired in the immature female, and as maturation approaches it branches, following the ramifications of the ovary. In the mature individual it is therefore paired for only the short distance for which it extends back to the posterior end of the body. Here it broadens considerably and turns dorsally before dividing into branches which extend between the diverticula of the ventriculus and dorsal to the ovarian tissue. They expand slightly in the depressions of the ventriculus and are joined medially between the diverticula, while laterally they extend as lobes between the outer diverticula of the ventriculus. Anteriorly the two branches join just behind the anterior median lobe of the ventriculus (VE) passing down either side of this lobe as a broad branch, lateral to the mouth glands (MG). These two branches turn posteriorly and pass ventral to the two main lateral diverticula of the ventriculus to join posteriorly with the original paired oviduct (OV). The oviduct therefore extends throughout the body, lying on top of the ovary (OT), except that it does not extend lateral to the ovary. It is a large tubular cavity with very thin walls and ovarian tissue often closely adhering to the wall (Text-fig. VI, Fig. 3). The eggs (EG) enter the oviduct (OV), when they have developed a shell and are full of yolk (Y), apparently through openings at the ends of the branches and on the slight expansions of the oviduct, but this is difficult to establish. The method by which the eggs enter the oviduct is not mentioned by Vitzthum (1940), Sig Thor (1904) or André (1949). When the two sides of the oviduct join to form a single tube leading to the genital aperture, the lumen becomes a narrow duct with a thick, muscular wall. This part of the oviduct, the only unpaired portion, is sometimes referred to as the uterus, but it is in the nature of a vagina. There are long, striated muscles attached to the vagina on either side of the genital aperture. Lying side by side with the vagina is a small tube, closed at its inner end, whose walls have a thin layer of chitin on the inner side and on the outer side a thick muscular layer. It opens into the genital aperture next to the vagina, and this is the “receptaculum seminis,” but it lacks the strong chitinisation described by Schmidt and has no canal. There are no glands near to the base of the vagina and Thor (1940) says that these are rare in females. The genital aperture is surrounded by a chitinous cap carrying long hairs. Michael does not mention Croneberg's description of the female genital system.
The Muscular System.
The muscular system is divided by Vitzthum into four parts, comprising the musculature of the body, of the legs (he includes the musculature of all the appendages under this heading), of the alimentary system and of the genital
system. The principal muscles of the body are the dorso-ventral muscles, which are nearly vertical and extend the height of the body. They pass between the remaining organs and usually close to the ventriculus. In the Parasitengona there are eight pairs of dorso-ventral muscles, but in the Hydracarina fusion has taken place to give four pairs, and sometimes, as in Arrenurus, even less. Eylais wai-kawae possesses four pairs of strongly developed dorso-ventral muscles which are inserted close together on to small, chitinised apodemes situated on either side of the midventral line in the region of the legs and slightly posterior to this region. The two members of the anterior pair are further apart than the other pairs. At their origins on the dorsal exoskeleton the muscles are more widely separated, giving a fan-like arrangement, and they are attached by tendons to thickened regions of the chitinous exoskeleton, which usually show a hook-like structure. The most anterior pair origmates dorsally just in front of the eyes, and the most posterior pair near to the posterior end of the body. The striations of the muscles are clearly visible.
The muscles of the appendages are well developed. There are two strong muscles to each epimera, lying parallel with the epimera, and therefore at an angle to the midventral line. Each muscle is inserted at either end on to the epimera. The muscles of the legs and palps follow the normal arachnid pattern, comprising a series of extensor and flexor muscles for each segment originating in the previous or following segment, and special muscles for the tarsal claws. Chitinous apodemes extend inwards for the attachment of the proximal muscles of the legs. The muscles of the mouth parts are strongly developed and consist of numerous retractors, protractors, flexors and elevators and stretch principally from the mandibles to the maxillary organ. Neither Vitzthum, Sig Thor nor André gives more than a mention of this musculature, and only Thor mentions the musculature of the epimera.
The extrinsic musculature of the alimentary system consists almost entirely of well-developed muscles inserted on to the pharynx and originating principally on the posterior processes of the maxillary organ. The oesophagus and ventriculus are essentially non-muscular, and this condition is probably true in all hydrachnids in which the ventriculus is sac-like.
The extrinsic musculature of the genital system consists of the muscles in the region of the vagina on either side and near to the genital aperture.
The Nervous System. (Text-fig. VI, Figs. 1 and 2)
The nervous system is condensed to two ganglionic masses, the supraoe-sophageal and the suboesophageal, which are intimately and fully applied to one another, so forming a single mass perforated by the oesophagus. From the anterior end of the supraoesophageal ganglionic mass (Fig. 2, SPO), a small median unpaired nerve passes along the dorsal surface of the oesophagus (NO) and innervates the pharyngeal muscles. On either side of this nerve and nearly half way back on the ganglionic mass, the two nerves to the mandibles (NM) are given off and these nerves do not branch until they reach the mandibular muscles. They are slightly larger than the oesophageal nerve and leave the ganglionic mass almost vertically. A short way further back in the midline, the large paired nerves to the eyes (NE) are given off, close together and leading almost vertically to the eyes, dividing into two a short way before reaching each double eye. Vitzthum (1940) believes that these two optic nerves probably have a common
source inside the ganglionic mass, but in Eylais waikawae there is no evidence of this condition either from dissection or serial sections.
From the suboesophageal ganglionic mass (Fig 1, SBO), the nerves to the maxillary organ (NMO) arise at the anterior end, slightly to each side. These nerves, probably also supply the palps. According to Vitzthum these nerves arise from the anterior edge of the fused ganglia, but Vitzthum decides, on analogy with the Mesostigmata (although not the Ixodides), that they must arise from the suboesophageal ganglionic mass. From dissection, it is apparent that the nerves to the maxillary organ arise from the ventral part of the front edge of the fused ganglia and from below the oesophagus, and therefore they must arise from the suboesophageal ganglionic mass, as Vitzthum believed. The largest nerves from any part of the ganglionic masses are the four pairs of nerves to the legs (NL. I-IV), which arise laterally from the suboesophageal ganglionic mass along the entire length of its lateral wall. They innervate the muscles of the legs and each is accompanied by a small accessory nerve (AN) which innervates the muscles of the epimera. Posteriorly, and just median to the nerves to the fourth pair of legs but slightly more ventral, there is given off a median-sized pair of nerves (NI) which supplies the internal organs, principally the genital organs.
This account of the nervous system in Eylais waikawae agrees in all essentials with that of Vitzthum (1940) who apparently relies on the description of the nervous system of Eylais given by Croneberg in 1878 for his information on the nervous system in all Hydracarina.
According to Michael. Croneberg says that the pharynx in Eylais is innervated from the first pair of nerves from the supraoesophageal ganglionic mass, which also supply the mandibles. This is not the case in Thyas nor is it the case in Eylais waikawae, where a separate median unpaired nerve innervates the pharynx, as in Thyas. Croneberg also describes the much smaller accessory nerves running parallel to the leg nerves and springing from the “brain” itself, which are found in the New Zealand species. In Thyas the accessory nerves branch from the principal nerve a short distance from the “brain”, and Michael believes that this was the original condition. In most respects the nervous system in Thyas is more elaborate than in Eylais.
The Skin Glands. (Text-fig. V, Fig. 2.)
Among the Acarina, skin glands are present only in Hydracarina. They are arranged symmetrically, principally on the dorsal side of the body. They do not usually occur where the chitin is thickened. The number of skin glands varies considerably in different genera, and there are several different types, Eylais being the only genus of its type at present known. Vitzthum gives a description and a diagram of the skin glands in Eylais extendens (Müller), after Schmidt, which agree in most respects with the condition in the New Zealand species, where there are about twenty glands irregularly spaced on the body wall. Each consists of a chitinous tube (DU) whose opening on the surface of the skin is closed by a small gland plate which carries a hair (HA.). On the outside of the tube are a large number of long, thin cells (GC) which stain fairly deeply but do not, according to Vitzthum and Schmidt, belong to the secretory cells of the gland. Scattered among them are a few lightly staining, shorter cells (LC). These two groups of cells are covered by a basal membrane of long, very
thin cells, with, according to Schmidt, long nuclei, and they form the “Polsterung” (bolstering or stuffing) around the chitinous tube (Vitzthum, 1940). The secretory part of the gland is contained inside the chitinous tube and consists of spindle-shaped glandular cells, which in Eylais waikawae have deeply staining nuclei and are slightly larger than in Eylais extendens. The cells nearest the pore are slightly fatter and shorter than the others. At the proximal end of the tube is a cluster of leucocytes between the gland cells according to Vitzthum, but these are not present in the New Zealand species. The glands are said to exude a repellant which serves as a defence mechanism, especially against fish. If the hydrachnid is taken into the mouth of the fish the taste of the secretion is said to be such as to cause the fish to expel it (Elton, 1922).
No unpaired glandular trachealis was observed in Eylais waikawae, although its presence is known in Eylais and a diagram of the condition in E. extendens, after Schmidt, is given by Vitzthum.
The Hydracarina lack most of the other glands present in the remaining Acarina. A pair of glands of unknown function lies in Eylais waikawae, along the dorsal surface of the ventriculus, one on each side of the median diverticula and extending ventrally into one of the depressions of the ventriculus. Each consists of large, distinctly nucleated, closely packed squarish cells of approximately 0.9 mm. in diameter. These cells are of the same form throughout the gland. There is a narrow lumen especially in the more ventral part, but apparently no duct. These do not seem to have been mentioned by Croneberg, but Michael says that they are present in Thyas, and he also finds no duct.
The Circulatory System.
The circulatory system of the Acarina, in so far as such a “system” exists, is an open one. The “blood” (Vitzthum; coelomic fluid of Sig Thor), which is in the form of a colourless fluid containing leucocytes, bathes the internal organs and muscles and flows through all available spaces. The leucocytes are generally round, as in Eylais waikawae, although somewhat uneven, and they originate from the hypodermis (Sig Thor, 1904). The flow is most noticeable around the muscles, especially the dorso-ventral muscles (Vitzthum, 1940). There is no account in the available literature of any further details of the circulatory system in the Parasitengona, but “hearts” are described for other Acarina and short descriptions of these are given, for example by Vitzthum. In Eylais waikawae there is a small spherical “heart”, 0.04 mm. to 0.05 mm. in diameter, with a very thick muscular wall. The “heart” has a number of short, thin “vessels” leading away from it, all of which end before reaching the internal organs, and the whole muscular wall, both on its outer and its inner surface, is irregularly indented. A very thin “pericardium”, with the walls of the long thin cells indistinct, is present around the “heart”.
The Respiratory System.
In Eylais there is a pair of trachea carried on the “Luftkammer” (air chambers) which are associated with the mouth parts. In most other genera the trachea are associated with the mandibles. The trachea show the characteristic chitinous spiralling (Text-fig. II, Fig. 3, T), but they are generally believed to be vestigial, and no observer has ever seen hydrachnids come to the surface of the water in order to take air into the trachea. Vitzthum (1940) says that in what way the respiration of the Hydracarina takes place is not clear. The larvae
possess a pair of trachea, which may be functional since, for example, in Eylais the larvae spend most of their free existence at or near the surface of the water. It is most likely that Hydracarina rely principally, or probably wholly, on dissolved oxygen in the water transpired through their skin, especially the thinner patches between the plates.
The Eyes. (Text-fig. V, Fig. 3.)
Visual organs are absent from most Acarina, but eyes are usually well developed in the Prostigmata. Generally two pairs are present, of which the two on the same side are more or less perfectly fused and sessile. In some genera, a median unpaired eye is present, which is of the same simple type, and there are no very great modifications. In Eylais waikawae the eyes lie close together in single capsules (Text-fig. V, Fig. 3, CC) on either side of the body (Text-fig. V, Fig. 1, E). They are not joined, but there is a single optic nerve on each side which divides only just before reaching the eyes (Fig. 3, NE). On each side the nerve then broadens out towards the circular retinal area (RE), which is rich in pigment. Above the retinal area lie the rhabdomes (RH) which are short, broad and rod-like. The almost spherical lens (LS) is situated inside the cup formed by the rhabdomes and beneath a thin, trausparent layer of the chitinous exoskeleton. In Hydracarina the retina is the only site of pigment, and all retinal cells contain pigment. Vitzthum refers to Croneberg (1878) for a description of the eyes in Eylais. The eye differs from Vitzthum's figures of Piona carnea.
The most noticeable feature of the anatomy of the female Eylais waikawae is the great modification that it has undergone, probably in relation to the freshwater life and the consumption of liquid food. In the mature individual most of the body is taken over by the reproductive organs, which branch prolifically. The area of absorptive surface of the ventriculus is large and is increased by the presence of the diverticula. The excretory organ has also become greatly branched, perhaps in relation both to the large area of the ventriculus and to the branching of the reproductive organs. Other systems, such as the nervous system, have been very much condensed. The anatomy of Eylais is very different from that of Thyas where the alimentary system takes the form of a ring and the excretory system is a single canal lying in the middle of this ring and leading back to open near to the posterior end of the ventral surface. Differences from Croneberg's description, in so far as it is known, lie in the mouth glands, the excretory system, and the nervous system.
Eylais waikawae is closely related to the previously described New Zealand hydrachnid, Eylais schauinslandii Koenike, and it is possible that both radiated from a single species which resembled the widespread E. infundibulifera. The life-history of E. waikawae agrees with observations made on species of this genus overseas, but the parasitic stage is passed on different hosts. The native hosts include both Corixidae and Notonectidae as are attacked overseas, but apparently in New Zealand Dytiscidae and Odonata are not parasitised, although these families are especially favoured in other countries. The plants used for egglaying and the metamorphosis from nymph to adult, are regarded as introduced species. Species of Anacharis are abundantly used elsewhere, as in New Zealand.
The internal anatomy of E. waikawae differs from that of the one overseas species described, E. extendens, in the more extensive branchings of the excretory system, in the shape and structure of the salivary glands and in the presence of a median unpaired nerve innervating the pharynx. It is possible that further work on the anatomy of E. extendens may reveal an excretory system more like that of Eylais waikawae than Croneberg is reported to have described. This study has yielded, besides the description of a new species and its complete life history including several processes not previously described for any hydrachnid, the first account of the anatomy of a female Eylais species, since Croneberg's description in 1878.
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