THE ANATOMY OF THE LARVA 53 shifts into the sixth segment. This last alteration is performed with remarkable rapidity during the few minutes the imago takes in emerging from the pupa case. Organs of Sense. — The eye in the encephalous larva is usually stated to be of the " simple type consisting of a group of ocelli with lens and retinal expansion." This sort of statement has been copied from one text-book to another till it has become stereotyped, but as a matter of fact, there is neither lens nor retinal expansion, properly so-called, and the eye appears to be rather a transition stage in the development of the compound eye of the imago, than a structure in any way like the ocelli of adult insects, or the eyes of spiders or molluscs.
In full-grown larvae of Ciilex pipieus, the eyes, although distinctly separated by an unpigmented gap, form for all practical purposes a single visual organ, the thick but perfectly transparent cuticle forming an unbroken spherical curve over both eyes, and although this is somewhat thicker in the middle line opposite the separation between the deeper parts of the eyes, there is nothing whatever in the form of a lens, at any rate in the optical sense of the word, as not only is its internal limit formed by the irregular surface of the pigment, but even taking this as a regular surface there is so little difference in the depths of the anterior and posterior curves that any image formed by it would fall nowhere near the visual nerve endings, but somewhere in the animal's thorax. If we examine a not too thin radial section of the organ it will be seen that it consists of a number of conical masses of pigment, the combined bases of which form the inner boundary of the transparent cuticular layer of the eye, and into the apex of each may be traced a fibre of the optic nerve springing from a bilobed mass of ganglion cells, almost in contact with, but yet distinct from, the large lobes of the cerebral mass.
Selecting a thinner section we find that each nerve fibril, shortly after starting from the optic ganglion, begins to acquire a rapidly thickening covering of pigment granules, and that as soon as this has become sufficiently bulky to merge with the neighbouring sheaths of pigment to form
54 GNATS OR MOSQUITOES — CHAPTER III the base of the great pigment mass of the eye, the fibre expands into a spindle-shaped body provided with a distinct nucleus. Beyond this the spindle-shaped body contracts into a rod-like structure, the actual ending of which I have not been able to trace, though I think it simply ends in the midst of the transparent contents of the tubular visual elements to be presently described. A further examination of radial sections shows that the pigment-covered sensory cones each consist of a deep portion enclosing the spindleshaped bodies, which may or may not be radial in direction, and may even be curved, and an outer portion containing the rod- like nerve end, which is always truly radial. If we now turn to the examination of tangential sections, we find that the superficial layer of the general pigmented mass consists of a number of cylindrical prolongations of the transparent superficial layer of the eye, each enclosed in a layer of pigment, which is very thick at the surface and becomes gradually thinner as the deeper layer containing the spindle-shaped bodies is approached.
In all sections that are sufficiently truly tangential to afford a clear image, the contained terminal rod can clearly be seen as a well-stained dot exactly in the middle of the transparent contents of the tube of pigment ; but whether it extends through the entire length of the tube, or ends somewhere during its course, is more than I can say ; in any case, however, it extends along it for some considerable portion of its length. From what has been said it is clear that the terminal rods can receive only rays the direction of which is parallel to the radius of the sphere of the eye coinciding with the transparent axis of the pigment-clothed visual element, and that such rays will reach this end rod and no other, so that although there be no dioptric apparatus, such as is found in each visual element of the facetted eye of the adult insect, for the purpose of concentrating on the contained rod the pencil of rays entering each transparent cyHnder, it is, nevertheless, like such eyes, suited only for mosaic vision, and is for such a purpose only less efficient in so far as in that a smaller portion of the entering pencil of rays will actually reach the end-rod placed in the axis of the
THE ANATOMY OF THE LARVA 55 transparent cjdinder. Indeed, to complete the development of the eye of the imago no great alteration of the deeper parts is required, but only the modification of the transparent superficial layer of the eye into the beautiful system of miniature dioptric systems, one for each end-rod, which forms the most striking characteristic of the facetted com- Fig. 15. — To Illustrate the Anatomy of the Eye op the Larva. a. Radial section through anterior part of the head of the larva of Culex pipiens, cutting through both eyes and the optic ganglia ; b, semi-diagrammaticrepresentation of three of the visual elements of the above more highlymagnified ; c, portion of tangential section of the outer layer of the pigmented portion of the eye.
pound eye. I have gone into somewhat more detail than usual in this point, because I have nowhere met with any account of the visual, organs of these larvae which appears to be based on actual observation of the family, most authors being apparently content to assume that their eyes must needs resemble the visual apparatus of other and often very distant groups of invertebrata. It is clear that
56 GNATS OE MOSQUITOES — CHAPTER III in such an eye the mosaic must be a very coarse one, and that, except in the case of very near objects, the impression gained of their form must be extremely fragmentary and ill-defined.
Hearing. — As regards the organ of hearing it happens that in one member of the family, viz., in the larva of Corethra, an organ as to the auditory nature of which there can be little doubt has been studied in some detail. In this genus the larvae are so transparent that they are known as glass larvaB, and hence can be studied with exceptional advantage, and the organ which is situated in the eighth segment is described and figured by Professor Lubbock in his popular work on the " Senses of Animals." In this form the ganglion which is placed in the anterior part of the segment gives off a branch, the auditory nerve, which after a short course outward expands into a small auditory ganglion from which a sheath containing two or three auditory rods passes outwards and backwards to the skin. The auditory ganglion and end-organ are further supported and kept in a state of uniform tension by a ligament which runs in an opposite direction from the ganglion to the skin at the anterior part of the segment.
The organs of smell and taste are probably situated in the antennae and maxillary palpi and other parts of the mouth respectively, but I am not aware of any work on this subject bearing on the CuUcidcc in particular.
PLATE 11.
PLATE II. — To Show the Characteristics of the Larv.e op Different Genera.
Fig. 1. — The larva of Anopheles maciilipennis. Pig. 2. — Hinder extremity of the same, more enlarged. Fig. .3. — The larva of Anopheles nirjripes, to show the arrangement of the respiratory system.
Fig. 4. — Pupa of Anopheles macidipennis. Ffg. 5. — One of the natatory bristles. Fig. 6. — The larva of Corethra pliimicomis. Fig. 7. — The pupa of Corethra plumicornis. Fig. 8. —Embryo of Corethra plumicornis within the ovum. Fig. 9.— The larva of JJochlonyx culiciformis. Fig. 10. — The pupa of Mochlonyx culiciformis.
58 CHAPTEE IV.
The Generic Characters of the Larvae of the Culicidae. The number of species which have been followed through their complete metamorphoses is, as already remarked, very small. In the case of the genus Megarlmia, I can find no record of any observation whatever on this subject, while in ^des the only note met with is one by Osten-Sacken, on jS^des fascus, an American species, and this is of the most cursory description, all that is said being that they exactly resemble the larvae of Culex, except that they are smaller. Putting then aside Megarhina, the larvae of the various genera may be divided into two categories, according as to whether they possess a dorsal respiratory process to the eighth abdominal segment or not. In the first category are Culex, J^des, and Mochlomjx ; in the second Anopheles, and Corethra. Amongst those possessing the respiratory dorsal process, the larva of Culex has already been sufficiently described : of ^des nothing more can be said, and of those sufficiently described, only the larvae of Mochlonyx remain to be described. I have actually handled the larvae of Culex, Stegomyia, and Anopheles only, and the notes given below are almost entirely a precis of Fr. Meinert's paper, " De Encephale Myggelarver " (Vidensk Selsk., 6, Raekke, Naturvidensk, og mathem. Afd. iii., 4). Genus Mochlomjx. — Meinert's remarks apply in especial to M. culiciformis, which formed the subject of his investigations. The full-grown larva is of a light brown colour, the tracheae and air sacs often showing through the skin with a golden lustre. On the dorsum of the thorax are several small, dull white spots, and the pleurae and venter are whitish. After each change of skin the colour
GENERIC CHARACTERS OF LARVAE OF CULICID.E b'i) is almost white, and this hghter coloration persists longer on the head and anal tube than elsewhere, so that the newly-dressed larva presents quite a distinctive appearance. In many points these larvae are intermediate in form between those of Culex and Corethra, but the head is most like that of Anopheles, forming like it a truncated cone, but differing in being pinched in m front of the eyes, so as to present a pyriform outline ; and seen from the side the difference is even greater, as the tergal plate of the second metamere is bent downwards and then backwards, so that the anterior part of the clypeus overhangs the mouth. The tergum of the third metamere is cordate, being deeply incurved in front for the reception of the hinder part of the clypeus. The eyes are placed well back on the broadest part of the head, and are of oval form, with the long diameter transverse. The antennae consist of a single joint, are moderately long and thick, and when at rest are directed downwards so as to be scarcely visible from above : they are provided with several jointed bristles, some of which are longer than the antenna itself. The labrum is almost rudimentary.
The mandibles are strong and trenchant, and in addition to their teeth, carry a variety of simple and compound hairs, some of which are of very specialised forms. The maxillae are short broad plates, with a sinuous anterior border and stumpy, conical palps. They and the lower lip, in addition to ordinary hairs, are provided with several ranks of peculiar flattened bristles with truncated dentate ends.
On the maxilla, the inner face of the external sinuosity is provided with a single row of these peculiar plates, but on the labrum, which is transversely oval, there are three, and on the lateral plate of the metamere, with which the maxilla articulates, no less than five ranks of different lengths. They look as if they were designed to act as a sort of sieve to exclude too coarse materials from the mouth, but, as the larva is predatory and subsists mainly on small crustaceans, they are more probably retentive organs. The thorax is relatively very large, somewhat