This nymph (Pl. 52, Fig. 6) belongs to the only genus of burrowing mayflies found in New Zealand. The genus consists of two species, I. bicolor, discovered a few years ago by Tillyard (6), who described the imago and sub-imago, and I. hudsoni, described by McLachlan (11) in 1894, Eaton (5) in 1899, and Hudson (1) in 1904.
The nymph of the former species has not yet been described, that of the latter only briefly and with insufficient detail to distinguish it from nymphs of this family in other countries: for this reason, it is described more fully below.
Incidentally, the opportunity is taken of pointing out that gills are present on the first seven abdominal segments, not on the first six, previous describers having apparently mistaken the second abdominal segment for the first, which is fused with the metathorax and bears very minute, reduced gills.
Moreover, the writer would like to submit the opinion that Lestage (10), in describing the family Ephemeridae, was mistaken in regarding the mandibular tusk as a canine, for both outer and inner canines are present in addition to the tusk in our species, as they are indeed in American [see Murphy (12), Text-Fig. 1, p. 16] and European species [Eaton (4) Plates], and Needham (13) Plates.
This nymph burrows in the sandy or muddy banks of streams, below water level.
It swims with dorso-ventral, undulating movements of the body.
If disinterred from its tunnel, it will bury itself in the nearest sandy patch, burrowing with its forelegs and mandibular tusks and digging itself in with remarkable rapidity.
When about to transform, nymphs in the writer's aquarium left their burrows and floated for some time, unmoving on the surface of the water, rarely agitating their gills.
A quantity of air was noted under the skin of the thorax and of the anterior part of the abdomen.
The actual metamorphosis, which occurred as usual through a longitudinal rent in the dorsum of the thorax, was extraordinarily swift, in marked contrast to the long time (sometimes hours) spent on the surface waiting for the change.
This protracted wait may have been due to the unnaturally still condition of the water present in the aquarium.
The winged stages appear in December, January and February, but as nymphs of various sizes are seen at this time, it is likely that this fly has a two-year or even possibly a three-year life-cycle.
Description of Nymph:
Length (including setae).—About four centimetres.
Head (Text-Fig. 27).—Reddish-brown; Epicranium convexly arched. There are patches of hair at the postero-lateral angles and also—accompanied by areas of small spines—on the lateral edges between the eyes and the antennal pits and at the latero-anterior edges of the clypeus.
Eyes.—Dark brown, and of moderate size.
Antennae.—Long, filiform, becoming slenderer apically, with a whorl of fine short hairs at joints, emerging from a pair of truncated prominences.
Mouthparts—Labrum (Text-Fig. 31).—Length, about three-fifths of width. The anterior and lateral edges are heavily chitinised. The general shape of the anterior edge is convex, but there is a gentle median concavity. The anterior corners are rounded and the lateral
Fig. 17.—Ventral view of male appendages in the genus Ichthybotus Eaton: a, in I. bicolor n. sp.; b, in I. hudsoni McL × 14. 1, 2, 3, 4, the four segments of the forceps; 9, 10, the last two abdominal segments; ad, appendix dorsalis; c, cercus (the right cercus is omitted); fb, forceps basis; p, penis. Fig. 18.—Left lateral view of male appendages in the genus Ichthybotus Eaton: a, in I. bicolor n. sp.; b, in I. hudsoni McL. × 14. Right cercus and right forceps omitted. (For lettering see Fig. 3). Fig. 19.—Outline of ventral valve in the female of a, Ichthybotus bicolor n. sp., and b, I. hudsoni McL. × 14. (From Tillyard).
edges slope inwards posteriorly, more sharply so at a point about two-thirds of the way from the anterior edge. The postero-lateral corners are obtusely-angled. The surfaces and edges (except the posterior edge) are densely covered with long hair.
Hypopharynx (Text-Fig. 24).—Superlinguae rounded anteriorly and densely haired at the edges, except the exterior ones. Median lobe with concave anterior edge haired and a number of spines on anterior portion of surface.
Maxilla (Text-Fig. 30).—Palp three-jointed: the distal joint is the longest, the second one the shortest: all are covered with long hairs. The galea-lacinia is very small, being slightly shorter and much narrower than the basal joint of the palp. It is chitinised apically. On the interior edge is a fringe of spinose hairs, and anteriorly are a number of long spines of varying thickness. There is a row of long hairs, rooted medianly on the surface of the anterior portion and lying anteriorly-interiorly.
Fig. 20.—Details of the morphology of the female imago in the three New Zealand species of Ameletus: a, A. perscitus Eaton, humeral angle of hindwing; b, the same in A. ornatus Eaton; c, the same in A. flavitinctus n. sp.; d, A. perscitus Eaton, ventral valve; e, the same in A. ornatus Eaton; f, the same in A. flavitinctus n. sp.; g, A. flavitinctus n. sp.; hind tarsus; h, A. ornatus Eaton, hind tarsus. All figures × 14. (From Tillyard).
See Vol. 54, Trans. N.Z. Inst.
Mandible (Text-Fig. 29).—The mandible is prolonged anteriorly into an enormous tusk, which is curved slightly inwards and bluntly pointed at the apex; it is held by the nymph in front of the head. This tusk is densely haired along its exterior edge and along here, too, are a number of short, thick spines: it is also haired and spined upon its surface—mainly the exterior-anterior portion. At the posterior-interior corner of the tusk and at the base of the mandible are irregularly-shaped, chitinised prominences (Text-Fig. 29, p1, p2). Outer and inner canines (c), each with three crenate teeth (in some specimens the third tooth appears double, making 4 teeth). Prostheca (p) long, slender and lightly-chitinised. Prosthecal brush very small.
Molar surface (m) with eight or nine parallel, serrated ridges.
Labium (Text-Fig. 28).—Palps two-jointed. Basal palp narrowing
somewhat anteriorly, with a fringe of long hair near the interior edge and at the base. (The left-hand palp, shown in figure, is distorted). Distal joint falcate with a fringe of long hair round the edges. Placed distally are a number of basiconic sensillae. Paraglossae triangular with rounded angles, the apex placed anteriorly: they are densely covered throughout with spinose hairs.
Glossae are somewhat similar in shape to the paraglossae; they are as long, but not so wide: they are also covered all over with spinose hairs. (The glossae are shown somewhat displaced—in particular the right-hand one—in the labium figured).
Thorax.—Dark-brown, with a thin light median line, running anterior-posteriorly: fringed with hair laterally: slightly wider than the abdomen.
Prothorax about two-thirds as wide as it is long: mesothorax as long as the prothorax: metathorax shorter than either of the other thoracic segments.
Wingpads.—Dark-brown, and of medium size.
Abdomen.—Dingy yellowish-white: narrowing posteriorly: first segment fused with metathorax: segments lengthening posteriorly as
far as the eighth segment: segment nine, though almost as long as segment 8, is noticeably narrower and the tenth segment is much narrower and only half as long as the ninth. Median tufts of hair occur dorsally and hairs project laterally from the pleura.
Setae.—Length, 6.5 to 7 mm.; median one a shade the longest. They are thickly fringed with long hairs on both sides. These hairs
become shorter and sparser distally till in the last few segments they are only represented by a whorl of short, fine hairs at the joints.
Gills.—There is a pair of gills present on each of the first seven abdominal segments. The gills of the first pair (Text-Fig. 25) are vestigial only and are each in the form of a bifid lash: careful scrutiny with a microscope is necessary in order to find them: they are near the latero-posterior edges. The six pairs present on the second
to seventh abdominal segments are held recumbent on the dorsum. Each gill (Text-Fig. 26) is double and resembles a pair of feathers with rhachides swelling out basally, and long, thin barbs, diminishing in length towards the apex. These ‘barbs,’ the gill filaments, are minutely serrated on each edge and from each serration, a small spine projects forwards and outwards.
Each filament contains a tracheal branch running through the middle of it.
Legs (Text-Fig. 34).—Dissimilar: in each leg the femur is shorter than the tibia and longer than the tarsus. The legs of the third pair are the longest; those of the middle pair are the shortest.
Anterior pair.—Femora greatly dilated, ovoid, covered with long hair and studded at the ventral and dorsal edges with a number of short, stout spines.
Tibiae dilated distally, with a V-shaped depression in the distal margin; the ventral arm of the V, which is equipped with an extra row of close-set spines at right angles to the margins, projects sharply and is probably an aid in digging. The greater part of the surfaces and the edges are thickly set with short, stout spines. Long hairs occur on the margins and a few on the surface; there are a few feathered hairs (Text-Fig. 32) on the distal margin in the bay of the V.
Tarsi slightly incurved: there is a row of short, stout spines near the ventral edge, and a fringe of long hairs on the dorsal edge, a few on the ventral one and some scattered over the surface: short feathered hairs occur on the basal part of the ventral edges.
Middle pair.—Femora ovoid, dilated, smaller than those of the first pair: covered with long hair, but without spines.
Tibiae becoming wider distally, but not markedly dilated as in those of the anterior pair: distal margin of normal contour: covered
with long hairs; there are a number of short, thick spines on the distal half of the surface and along the dorsal and distal edges. The ventral part of the distal margin is—as in the first pair of legs—a veritable ‘chevaux-de-frise’-like structure.
Tarsi very slightly incurved; fringes of hair arise on the dorsal and ventral edges and a few shorter hairs on the anterior surface. A few short feathered hairs occur distally-ventrally on the tibiae and tarsi.
Posterior pair.—Femora ovoid, dilated, longer than those of the other two pairs. They are covered with long hair: along the ventral margin are a number of rows of short feathered hairs.
Tibiae similar in shape to those of the second pair, but longer: covered with long hairs on the edges, and in among these, on the ven-
tral margin, are a number of the short feathered hairs. On the distal margin, these peculiar hairs replace, for the most part, the spines which are found in the same position on the other legs; there are, however, a few stout spines, notably one at each end of the distal margin, the one at the ventral end being especially prominent and projecting forwards and outwards.
Tarsi slightly incurved, with a fringe of long hairs on the ventral and dorsal margins and a few on the surface. Feathered hairs occur on the distal-ventral portion near the edges; from the ventral edge, near the apex, a thick formidable spine projects forwards and outwards.
Claws (Text-Fig. 33) alike in each leg: curved and hooked at tips: not toothed underneath: dark brown. They become progressively more sharply-pointed and more acuminate posteriorly, those of the last pair (illustrated) being very sharply-pointed, the anterior ones blunt—possibly through being used for digging.
N.B.—The feathered hairs (one is figured Text-Fig. 32) which occur on certain parts of the legs of this nymph are extremely curious.
They have been noticed by the writer on only one other mayfly nymph, Atalophlebia cruentata (Hudson), an entirely unrelated species, which is described later.
What is still more curious is the fact that in the case of A. cruentata, these hairs appear on the anterior pair of legs only.