GO GNATS OR MOSQUITOES — CHAPTER IV flattened, and no distinction between its component segments is visible. The lateral tufts of balancing bristles which are so marked in Ciilex and Anopheles, are illdeveloped. The abdomen is cylindrical, slender, and of nearly uniform diameter as far as the seventh segment, the segments also progressively increasing in length up to this point, while the last two are of insignificant dimensions. The seventh, which contains the two large posterior air sacs, is of very exceptional size, being at least as long as any other two segments. The eighth segment, as in Culex, bears on its dorsum the respiratory process,, but this is quite small as compared with the organ in most members of that genus, and is of conical form. The ninth segment carries four small, slender anal tubercles, and a pair of swimming fans composed of closelyarranged, multifid bristles. The tracheal system is peculiar, combining many characters of these organs in Culex and Corethra. The main longitudinal trunks have the same course and general distribution as in the former genus, but are quite small, the hydrostatic function being fulfilled by two large pairs of dilatations or air sacs, which are situated in the thorax and seventh segment respectively. All four, and especially the thoracic sacs, are so large that they quite overshadow the rest of the tracheal system, the main trunks being reduced to the rank of mere communications between them.
The pupa closely resembles that of Culex, but the abdomen is relatively shorter and stouter. It may be most easily distinguished by the disproportionately large size of the seventh segment, which retains in the nymph the proportions it holds in the larva. The respiratory trumpets are small and somewhat olive-shaped. Genus Anopheles. — Meinert's description refers in particular to An. 7naculipennis, with shorter references to An. nigripes. The ground colour of the larva is a light yellowish green, with a dark brown stripe along the back, which, however, is whitish in the middle line. There are also four small dark spots on the hinder border of the anterior abdominal segment, and six small oblique bands
GENERIC CHARACTERS OF LARY.E OF CULICID.T: 61 on the sides of the other segments. In all the Indian species the larvae are of a dirty brown colour ; with a few rather ill-defined patches of pigment on the head and thorax. In Anojjheles Bossii mihi there are two patches of pigment on the dorsum of the head, which, combined with the eyes, give this region of the larva, when regarded from above, a quaint resemblance to a human skull. The head is less rounded than in Culex, and the constriction between it and the head is deeper but less obvious than in that genus, as it forms rather a truncated cone, with the broader part behind, than a sphere. The tergum of the third metamere is in the form of a lozenge, with the anterior corner cut off and the point behind, and is ornamented in front with six plumed bristles, the outer of which are the largest. That of the second metamere is broad and short, and carries at each of its outer corners a single plumose bristle. As in Culex, it is provided at its outer part with a dense whorl-organ. The labrum, which is tongue-shaped and crenated on either side, is small and hirsute. It is, moreover, overhung by the clypeus, so that it is little in evidence when viewed from above. The eyes form a band of pigment of somewhat pyriform outline, with the narrower end backwards, and on their outer sides are the small ocelli.
The antennoe generally resemble those of Culex, but are armed with a row of short, stout. spines along the inner border of the basal joint. They carry also certain fanshaped and other specialised bristles. The mouth parts closely resemble those of Culex, but the whorl-organs are larger. The lower lip forms an equilateral triangle, with a few strong dentations on its sides, and is prolonged into a peculiar dentated process, besides which, as seen from below, it is partly covered by two plates which appear to be connected with the ventral plate of the second metamere, which last structures do not appear to be represented in the other genera. The mandibles and maxillse also closely resemble those of Culex, but the latter have a straighter anterior edge, and are less cut off at the corners. They are fringed with bristles, some of which are of a specialised
<52 GNATS OR MOSQUITOES— CHAPTER IV character, and from their outer posterior corner there springs the maxillary palp, which is larger and more acutely conical, and also carries certain specialised bristles. Proportionately to the head the thorax is a good deal larger, and exhibits three rows of bristles besides the large lateral tufts, the posterior of which are the largest in the body, while those of the pro- and mesothorax are of insignificant size. The abdomen is cylindrical, and is very distinctly separated into nine segments, which increase in length while they diminish in breadth from before back. The lateral tufts of bristles of the three anterior segments are exceptionally large, but the hinder ones are very small. On the dorsal surface of the eighth segment are a pair of simply-formed spiracles, which can be withdrawn beneath a fold of skin when the insect requires to protect their openings. At the same time, if examined in profile, it is evident that the difference between the larvae of Gulex and Anopheles is really one of degree rather than of kind, and that although rudimentary in the latter, all the parts of the long syphon of Cidex are really present, and are especially evident in some species ; while, on the other hand, as already remarked, in some CiiUces the syphon is very short. The last segment carries the four anal tubercles, which, as well as the tail-fans, are rather less developed than in Ciilex. Although arranged on the same general plan within the body, the tracheal system is much less developed, the main longitudinal trunks being of very ordinary size, and quite without hydrostatic dilatations in any part of their course.
The pupa differs from that of Cidex only in the respiratory trumpets being shorter and more squarely cut at the end. It may be distinguished from the pupa of Mochlonyx by the fact that the eighth instead of the seventh abdominal segment is disproportionately long. In the particular species examined by Meinert the pupa, like the larva, is grass-green, but this coloration is not universal in the genus, and those of Indian species, like the larvae, are brown. Working in Italy on the same species, Grassi was unable to confirm Meinert's statement as to a green coloration, finding the larva, as we do in India, brown. As
GENERIC CHARACTERS OF LARV^ OF CULICID^ 63 a matter of fact, the green colour, when present, depends largely on the nature and stage of digestion of the intestinal contents. Hence, under certain conditions, Meinert's statement is correct, but it cannot be called coloration in the strict sense of the word.
Genus Corethra. — The species examined by Meinert were C. plumicornis, and C. pallida. The larvae of this genus differ markedly from those that have been already described, and resemble those of the GhironomidcB rather than any of the CidicidcB. These larvae are well known as favourite objects for microscopic demonstration of " pond organisms," and are generally known as "glass " or "crystalline" larvae on account of their extreme transparency, which is broken only by the four darker, but still transparent, air sacs placed in the mesothoracic and seventh abdominal segments respectively. The head is much smaller than in any of the preceding genera, being narrower than any other part of the body, except the last two abdominal segments. Its hinder half is bounded by straight parallel sides, as seen from above, but is contracted in front, so that as a whole it presents the outline of a broad-nibbed pen. The eyes are small and round, and are placed well back on the head, rather on the dorsal aspect of their sides, and behind each of the large eyes is a single separate ocellus.
The antennae are articulated, as it were, at the point of the pen and are relatively small. Each consists of a single joint with a constriction, followed by a small node just beyond the base, and is armed at the end with five large bristles, which spread out like the claws of a lizard. The tergum of the third metamere is rudimentary, but its ventral plate forms the greater portion of the under surface of the point of the pen, and is provided about its middle with a sort of tubercle, from which radiate five pairs of large bristles, while behind this are a pair of peculiar fanlike plates, the posterior border of which is fringed with fine hairs. The ventral plate of the second metamere is generally spoken of as the labrum. It forms a sort of ridge in the middle, and is provided with a number of flattened hairs ; on either side are a pair of projections
64 GNATS OR MOSQUITOES — CHAPTER IV carrying a number of strong radiating bristles, the analogue of the whorl-organ of the other genera. The mandibles are very large and trenchant, and being capable of very wide abduction are particularly well adapted for seizing the larva's prey. The maxillae are two small, very simple plates, and the lower lip can hardly be said to be represented as such.
The thorax, which is much the stoutest part of the body, is of fusiform outline, and shows little or no indications of its component segments ; both this and the abdomen are provided with only a few small compound bristles. The abdomen is composed of nine segments, which progressively increase in length to the seventh. These are of nearly equal width to the sixth, after which the body rapidly tapers off. The anal tubercles and swimming fans are small, but round the anus are four bristles of much greater size. Bound the anus also are several ranks of peculiarly-shaped hooks.
The respiratory system is peculiar. Apparently the function must be entirely aquatic, as in the very young larvae there are no signs whatever of tracheae, and even in the fully-grown creature there are no stigmata or external breathing apertures whatever. In young larvae all that can be seen are the two pairs of air sacs, which are situated in the same positions as in Mochlonyx, but present a very different appearance as they contain no air, but are full of serum and are lined with a large-celled epithelium with prominent nuclei. Gradually with successive changes of skin the main longitudinal trunks and their branches appear piecemeal. At first they are full of serum, but as development proceeds they gradually fill with air, which, however, must be secreted from the blood, as there is at no period of larval life any direct communication with the exterior.
The pupa in this genus is distinguishable by the relatively large size of the abdomen and the small dimensions of the cephalo-thoracic inass. As far as its respiratory arrangements are concerned, however, it closely resembles the pupae of the other genera, the breathing trumpets being well developed, with very oblique mouths.
PLATE III. J^z.'a/ Fto^
PLATE III. — To Illustrate The Anatomy op the Pupa (after Hurst). Fig. 1. — Side view of the male pupa.
Fig. 2. — Ventral view of the female pupa, partly extended- Figs. 3-6. — Successive stages in the metamorphosis of the epithelium of the hinder part of the stomach.
Fig. 7. — Sagittal section of a very young female pupa. Ant. antennae ; Ao. aorta ; At. respiratory syphon ; B. buccal chamber ; CG. cerebral ganglion ; D. gastric pouch ; F. caudal fin ; FeK femur of first leg ; G. ganglia; Gn. outgrowth of "ninth" segment, within which the gonapophyses develop ; Hr. balancer ; H. head ; Ht. heart ; In. intestine ; Lb. labium; Lbr. labrum ; M. Malphigian tubule; M.Ap. its opening into the intestine ; MS. mesosternum ; Mt. metasternum ; Mx. maxilla (first) ; 3Jxp. its palp ; NC. nerve commisures and ventral cord ; Oc. ocellus ; Od. medium oviduct ; Op. compound eye ; P. prosternum ; R. rectum ; S. aperture of salivary duct ; SG. suboesophageal ganglion ; Si. larval respiratory syphon introverted into the eighth segment ; Sp. spermatheca ; St. stomach ; Ta\ To? proximal Joints of the tarsi ; K' — ■' tibiae ; Tr. trachea ; W. wing ; I. II. III. &c., first to eighth segments of the abdomen.
After the Plate in the late Mr. Hurst's Paper in the Mem. Owens Col. , somewhatreduced.
66 CHAPTEE V.
The Anatomy of the Pupa.
The following account is derived from the late Dr. C. Herbert Hurst's excellent paper on the subject, published in the " Studies from the Biolog. Lab , Owens College," II., 1890, pp. 47, et seq.
The paper is too full to be reproduced in full in an introduction like the present, but is characterised throughout by the most painstaking accuracy, and I have been able to verify most of the statements reproduced. During the latter stages of larval life, in addition to the visible head appendages, there appear eight other pairs beneath the larval cuticle. Of these six are thoracic and two abdominal. The thoracic pairs are three of them dorsal, the future pupal syphons, the wings, and the halteres ; and three pairs ventral, the future legs. The two abdominal pairs belong to the last two segments. Those of the eighth lie in the larval syphon, and are to form the fins of the pupa ; the hindmost pair form the outer gonapophyses of the adult, which are accessory organs of copulation.
All these eight pairs arise as foldings of the epidermis (" hypodermis ") outwards, and are quite hidden under the larval cuticle. The antennae, too, are much larger in an advanced larva than they appear to be externally, as the growing basal portion is folded, or even telescoped beneath the unyielding cuticle. Towards the end of larval life the animal becomes sluggish ; profound changes in the mouth parts deprive it of the power of eating, and it floats with its breathing tube at the surface. Shortly, the cuticle bursts in the thoracic region, the pupal respiratory