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Volume 37, 1904
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Art. XXIII.—The Aquatic Larva of the Fly Ephydra.

By W. B. Benham, D.Sc., M.A., F.Z.S., &c,. Professor of Biology in the University of Otago.

[Read before the Otago Institute, 9th August, 1904.]

Plate XVII.

Amongst the organisms collected by Mr. J. A. Thomson in a saline pool at Barewood, Central Otago, which were submitted to me for identification by Mr. G. M. Thomson, were a number of small brown larvae, or rather “puparia,” having rather a peculiar form. This larva, which for reasons stated below I ascribe to the Dipteron genus Ephydra, resembles in general form and structure the “rat-tailed” larva of the hover-flies (Helophilus* and Eristalis †)—in the very reduced condition of the head, in possessing a series of paired groups of claw-shaped spines segmentally arranged along the ventral surface, and in the long posterior respiratory tube or “tail”; but in the two genera just named this tube is retractile, whereas in the insect herein described it is not retractile, and is, moreover, bifurcated.

In looking through the small amount of general literature on the Diptera available here, the only genus of flies that I was able to find in which the larva of this general form of structure has a bifurcated respiratory tube is Ephydra. No doubt a figure of this larva is to be found in one or more recent monographs, or in memoirs published in periodicals concerned with entomology; nevertheless, in the interests of those who are unable to consult these works, it seems to me worth while to present a brief illustrated account, so that naturalists in New Zealand may be able to identify the creature. My identification rests upon the brief accounts contained in Westwood's “Introduction to the Modern Classification of Insects” (1840), and in Packard's “Guide to the Study of Insects” (1872). The former author

[Footnote] * Hudson, N.Z. Entomology, p. 58, p1 vii., fig. 1.

[Footnote] †Miall's Nat. Hist. Aquatic Insects. p. 198 et sea.

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gives a poor figure (vol. ii., p. 569, group 132, fig. 11) which shows the bifurcated tail, following ten claw-bearing segments. He states (p. 574), “Many species of Ephydra frequent salt marshy situations. The larva is cylindrical, without feet, and the terminal segment of the body very long, and terminated by a long fork, the prongs of which support spiracles at the tip; the puparium scarcely differs from the larva.” Packard, on p. 414, also represents, in a slightly more detailed figure, the “puparium” of another species. I find no reference to the genus in Miall's interesting book, nor in the Cambridge Natural History.

Description of the Puparium.

Amongst the dark-brown puparia I find one grey larva, which differs in a few points from the puparia, to which I paid more special attention; and since the larva, by hardening of the stem, becomes the puparium, the description of the latter will serve, with slight modifications, for the former. The puparium is a dark-brown hard-skinned object, measuring 9 mm. in total length, with a greatest diameter of 2 mm. It is cylindrical, bluntly pointed anteriorly, and produced into a narrow cylindrical tail occupying about one-third of the total length—viz., 3.5 mm. This terminates in a pair of short, narrow tubes, paler than the body, but tipped with dark pigment; each branch or tube is 1 mm. in length, and bears a spiracle at its apex. The entire surface—except these terminal branches—is covered with closely set blackish spines (as in Eristalis), groups of which at certain segments become larger and claw-like.

The body consists of a small “head,”* carrying a mouth, followed by ten distinctly marked segments, the last bearing the anus, behind which is the long tail.

There are no definite feet on this puparium, but on the ventral face of each of the eight segments 3 to 10 are paired groups of black claw-like spines, those on the last segment being carried on a prominent median papilla. In the majority of the specimens the ventral surface of the 9th segment is sunk below the general level, and, the body being abruptly curved ventralwards in this region, the claws of segments 8 and 10 are brought close together, and even in contact in some specimens, thus giving rise to clasp-ing-organ.

Westwood's figure shows nothing of this kind, all the segments being represented alike, and he places claws on all ten segments. Packard's figure, however, shows a slight prominence behind the 7th, and another larger one on the 10th. These, however, are wide apart, and do not suggest a clasping-organ.

[Footnote] * Possibly the true head is retracted.

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The black claws are not arranged in circles, as they are in Eristalis, but in irregular groups, and the number differs in the different segments.

The first segment is armed on the ventral surface with a series of closely set black rectangular plates, each toothed along its hinder margin; these plates are in about half a dozen transverse rows, each row shorter than the preceding, giving rise to a somewhat triangular black patch. The anterior edge of this and of the second segment is also armed with a row of short black conical tooth-like spines.

The “head” bears on either side a small triramous organ, each branch of which is somewhat dilated at its extremity, and is apparently perforated by a minute aperture. Each is traversed by a brownish granular canal (in which I could not detect any rings), which at the base of the organ unites with the other two to form a single canal, which runs backwards for a short distance, then, near the hinder end of the head, dilates to form a hemispherical or conical saccule, which rests against and is partly embraced by a dilated tracheal tube, colourless and distinctly ringed as usual; from this a. trachea can be traced backwards into the body. This appears to be an “anterior spiracle” such as occurs in the pupa of several aquatic Diptera, such as the gnats and Eristalis, though in form it does not resemble the organ in the latter fly.

From each of the posterior spiracles, situated at the tip of each of the forks of the tail, a trachea can be traced forwards into the body, and there is no dilatation except the small one just referred to.

The Larva.

Amongst the dark-brown “puparia” I find one individual which is grey in colour, with a softer and flatter body; the spines covering the body are much longer than in the puparium; each of the ventral paired groups of “claws” is on a distinct prominence or transverse ridge, which appears to represent the paired “legs” of Eristalis, the ridge of segment 10 being more prominent than the rest. Further, round each segment, as well as on the “head,” is a series of tufts of small pale hairs, no doubt sensory in function. Each tuft consists of three hairs rising apparently from a common base; they are colourless, and much longer than the spines. These sensory hairs are also observable on the puparium, but are less conspicuous. The anterior end bears a pair of dorsal “eye-spots.” The mouth is armed on each side with two or three rows of stout spines, and within the cavity is a pair of short antennae; each appears to be 3-jointed, the terminal joint being brown, and to spring from a circular

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convex disc, which I believe forms the anterior end of the real head, which is retracted, as has been observed to occur in Eristalis. The mouth leads into a short cylindrical tube, which soon dilates to form a pharynx, in the wall of which a number of sclerites are present, the most conspicuous of which is essentially a ring, with four backwardly directed processes, which no doubt serve for the attachment of muscles. Though built up on the same plan as that of Eristalis, it is very different in detail.

Some of the puparia were merely empty skins, and these were cleft horizontally on each side from the head backwards over two or three segments: no doubt the fly had emerged by this slit.

In one case the skin was ruptured somewhere near the middle of the body and exposed the pupa, which occupies only a small portion of this internal cavity; in length it extends through about four segments, and in breadth is only half the diameter of the puparium.

We have, then, the whole series of stages in the life-history of this creature, which I believe from the above—mentioned references to be Ephydra, both from its habitat in salt pools and from the resemblance in external form. Moreover, the genus has already been recorded by Captain Hutton as occurring in this country, for he described* E. aquaria from Christchurch, which is 5 mm. in length, and this agrees with the length of the pupa in the puparium above alluded to.

No doubt the general history agrees pretty well with that of Eristalis, and is as follows: The eggs are laid in the water of salt marshy ground; the larvae, issuing therefrom, burrow in the mud, where the spinous covering and the specialised claws serve as organs of locomotion, and the tail is directed upwards to the surface for respiration: this probably occurs only at intervals. The larva feeds on the organic matter in the mud, and as the time for pupation approaches it seems that it uses its hinder clawed segments 8 and 10 to cling to algae floating in the water, for in nearly all the individuals I find threads of algae held by these prominent segments; and at this time the anterior spiracles perhaps come into use. The skin now becomes hard and dark-coloured. The tissues, as in other cases of similar metamorphosis, become disintegrated, and later on become rearranged, if we may use the term, to give rise to the body of the pupa. The larval skin has become a cocoon or “puparium,” and it seems likely that the imago escapes through the anterior end—hence the object of the larva supporting itself by the algae, so as to allow the anterior end to emerge from the water.

[Footnote] * Trans. N.Z. Inst., vol. xxxiii., p. 90.

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Explanation of Plate XVII.
Fig. 1.

Puparium of Ephydra, viewed ventrally, slightly from the right side; claw-bearing segments numbered. b, patch of black plates on segments 1. The spines covering the body are omitted from these figures.
Fig. 2.

Side view of the hinder part of the body, showing the clasping-organ formed by the 8th and 10th segments.
Fig. 3.

Ventral view, in outline, with part of the body-wall cut away to expose the pupa within, a, anterior triramous spiracle-bearing outgrowth; d, cavity within puparial skin; e, pupal envelope; f, imago within the pupal envelope, diagrammatically represented; g, paired group of claw-like spines on 8th segment; i, anus; m, mouth; p, pharyngeal skeleton (seen through the skin).
Fig. 4.

Anterior end of a larva, showing the armature of the mouth and of the anterior margins of segments 1 and 2. a, anterior spiracular outgrowth; b, triangular group of spinose plates; m, mouth; n, the base of antennae (? = true head);'s, trifid sensory hairs.
Fig. 5.

The mouth enlarged. showing the probably “true head” retracted within. n, one of the antennae; h, “true head.”
Fig. 6.

Pharyngeal skeleton of the larva. m, mouth; p, pharynx.
Fig. 7.

Anterior spiracular outgrowth, with its air-canals passing back to the trachea (t) At edge of figure is seen a few of the plates (marked b in fig. 4).
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