106 GNATS OK MOSQUITOES — CHAPTER VI extremely elastic and distensible tracheae, which swell out into bubble-containing dilatations, wherever their mutual pressure permits of their doing so. Apart from a few loose connective elements, the sacs consist of nothing else than these curiously modified tracheae.
The circulatory and nervous systems also present no peculiarities beyond the few points that will be found noted in the section on the anatomy of the larva and its metamorphoses. In the imago", however, the fused cephalic Fig 25.— Diagram of the Nervous System.
a n, antennal nerve ; o I, ophthalmic lobes ; s g, supra-oesophageal ganglion, or brain ; s b g, sub-cesophageal ganglion ; t g, mass consisting of the third thoracic and first dorsal ganglion ; a g, abdominal ganglia. ganglia, or brain, is very large, and although they do not appear to possess any specially great amount of insect intelligence, its relative weight as compared with that of the body, cannot fall far short of that of even the human subject. One striking point, however, is the manner in which almost the whole of the branches of the last abdominal ganglion are supplied to the internal generative organs. So much is this the case, that a successful preparation of either the ovaries or testes almost always carries away with it this ganglion.
THE ANATOMY OF THE ADULT MOSQUITO 107 The genital organs of the female consist of a pair of more or less spindle-shaped bodies, which, in the miimpregnated insect, are contained mainly in the last abdominal segment, and lie obliquely in it on either side of the middle line, their distal extremities being supported by a sort of ligament springing from the common fibrous lining of the segment. They contain a large number of soft closely packed ova in various stages of development, those nearest the opening of the oviduct being the most advanced. They <^#.
Fig. 26.— Generative Organs of a Gravid Female Mosquito. A. Generative organs of C. fatigans, ? x 18 diams. ; or, right ovary and oviduct full of mature ova ; ol, outline of left ovary ; g, last ganglion of abdominal chain ; d, ovarian ducts ; ocl, common oviduct ; st, spermothecse ; op, ovipositor ; B, spermatozoa from the spermotheca, x 450 diams. ; C, a mature ovum, x 45 diams. ; b, portion of coat of ovum proper, showingits spinous structure.
are richly supplied with tracheae, supphed from the stigmata of the penultimate segment, as well as with large nerve threads from the last abdominal ganglion.
When further advanced, the ovaries and their ducts take up a large share of the cavity of the abdomen ; forming a pair of large, lobulated masses. At this stage, very little of the true ovarian tissue can be distinguished, almost all that can be seen being the enormously dilated funnels of the oviducts. Each egg consists of the ovum proper, clothed in a chitinous envelope closely covered with minute
108 GNATS OK MOSQUITOES— CHAPTEli VI spines, within which may be distinguished an elongated embryo, and a pecuhar sac at one pole containing a single large polynucleated cell, which appears to be in some way concerned in the bursting of the egg membranes ; the whole bemg enclosed in a delicate, structureless, external membrane, the total length of the whole egg being 0'71 mm. Their width is but a fifth of their length, and they are much broader at the end provided with the polar capsule, the base of which is marked with fine radiating striae. In the oviduct this broad end always lies forward, so that the narrow pointed end is that which appears first in delivery. The ovaries communicate with the common oviduct by means of a short, transparent, funnel-shaped tube ; and this latter is a short transparent canal, which commencing in the junction of the funnels of the ovarian ducts, runs straight backwards without convolution or deviation from the middle line, to open between, and at the base of, the ovipositors. Just before its external termination, it receives on either side the ducts of three small glandular bodies, each consisting of a spherical glandular portion about 0-5 mm. in diameter and a short neck or duct. They are filled with an opaline white fluid, which consists of a material identical with that found in the receptaculce seminales of the male, the peculiar fibrous matrix as well as the spermatic elements being undoubtedly transferred en masse from the receptacles of the male to those of the female, their contents differing only in the fact that, whereas in the male only the earlier stages of spermatogenesis are represented, in the female, the contents consist of more advanced spermatic elements and of fully developed spermatozoa, so that the later stages of the process take place in the body, not of the male, but of the female.
The spermatozoa are minute comma-shaped bodies with a rather stiff one-sided tail, and a pear-shaped nucleus, the long diameter of the head being about 1 ^\{. I see no reason for thinking that these bodies serve any other function than that of spermothecae, and their dull colour is due, not to any opacity of their contents, but to the sooty pigmentation of the sac- wall. Thev contain absolutely no glandular
THE ANATOMY OF THE ADULT MOSQUITO 109 elements, and therefore cannot also as Arribalzaga supposed, secrete the gelatinous fluid that cements together the eggs in the " egg-boats."
The ducts of the spermotheca^ are strengthened by a spiral fibre of chitine, just as in those of the salivary glands ; and just behind their entry into the common oviduct will be seen that of a special accessory gland which secretes a mucilaginous fluid which can hardly have any other function than to furnish a protective coating to the eggs, and to cement them together in those genera which deposit them in boat-shaped masses.
Fig. 27. — Sagittal, Section op last Abdominal Segment op a 9 Culex, SHOWING THE EnTEY OP THE SpERMOTHECA AND CeMENT GlAND INTO THE Oviduct, (After Kulagnin.)
The male generative organs consist of a pair of small yellowish bodies, the testicles, from which the sperm is carried by the vasa deferentia, which are simple straight tubes, to the ejaculatory duct which originates in their union. Just before the termination of the vasa deferentia, they receive the ducts of the receptacula seminales. The function of these vesicles requires further investigation, as their contents do not, as one would expect, consist of spermatozoa floating in a simple fluid, but of spermatic elements in various stages of the process of spermatogenesis, anterior to the appearance of completely developed spermatozoa ; and what is more remarkable, these are not floating in fluid, but are entangled in a fibrous
no GNATS OE MOSQUITOES— CHAPTER VI mesh. Presumably not only the spermatic elements, but detached portions of the fibrous basis of the testicle are carried down the vas deferens and stored in the receptacula, but the manner in which this is effected is difficult to understand. The ejaculatory duct is short, simple and straight, and ends in a short, fleshy penis, which is unprovided with any chitinous armature.
Fig. 28. — Male Generative Organs op C. fatigans. A, The male organs, the vasa deferenlia being represented as curved to save space (they actually pursue a perfectly straight course) x 18 diams. ; t, testicles ; v d, vasa deferenlia ; r s, receptaculcB seminales ; e d, ejaculatory duct; c, claspers or external gonapophyses ; ig, internal gonapophyses ; p, penis ; B, contents of receptacula seminales, x 450 diams. All these points can be made out, if not at the first attempt, at least after examining a few carefully made, teazed preparations, and the reader is strongly recommended to follow the preceding descriptions with such preparations before him rather than to trust to illustrations, which, owing to the extreme delicacy of the objects which it is attempted to represent, are at best mere diagrammatic representations, and can do no more than serve as a guide to the recognition of the parts sought for.
Apart from the fact that the ova differ markedly from
THE ANATOMY OF THE ADULT MOSQUITO 111 the above description, which applies only to Gulex, there is no noticeable difference between the internal anatomy of the different genera; and, putting aside of course the generative system, the two sexes closely resemble each other, so that I cannot confirm Prof. R. Blanchard's statement {Bull. Acad, de Medecine, 3e. Ser., t. xliv., p. 21.) that the buccal apparatus of the ? , is much the more developed ; for I find that, for all purposes of anatomical investigation, one may use either the 3 , or the ? , . as interchangeable material.
The above account will, I think, be found sufficiently detailed to serve as an introduction to those engaging in work on the subject, but is necessarily a mere sketch of the subject, for so many points force themselves on one's notice that it is difficult to avoid being tempted into too great detail. It would indeed be easy to fill as large a work as the late Prof. Huxley's wonderful work on the cray-fish if not to copy the luminousness and charm of the author, but anything beyond an outline of the subject would be out of place in a book like the present.
112 K CHAPTER VII.
On the Life History and Seasonal Prevalence of Mosquitoes. The life history of any species is necessarily mainly determined by the nature of its food ; and in these insects this differs entirely at different stages of their career ; for while the larva is a purely aquatic animal, feeding on pond organisms, vegetable and animal, the adult flying insect, from the structure of its mouth, can subsist only on fluid, or at most, emulsified forms of nourishment ; and as naturalists are generally agreed that gnats cannot travel to any considerable distance, it follow^s that food both for adult and larva must be obtainable within a limited area, for mosquitoes cannot and do not fly far. It is impossible to fix any absolute limit to their powers in this respect, but it may be safely asserted that few individuals ever stray much more than a quarter of a mile from the pool in which their larval youth was passed, and the great majority never travel further than the nearest shady spot. Nor, in spite of popular beliefs to the contrary, can they be carried far by the wind. Mosquitoes, indeed, exhibit a well-founded, instinctive dread of boisterous weather, and will not leave shelter in a high wind. Those accidentally carried away are, I am inclined to think, rapidly disabled. When serving as Naturalist to the Indian Marine Survey I was necessarily at sea for months together, and though butterflies, dragon flies, and other strong flying insects often boarded us far out at sea, I found no evidence of small and delicate species being transported in this way. A run up the Hooghly for coal would fill our cabins with these unwelcome shipmates ; but, once fairly at sea, their numbers rapidly diminished, though a certain number
LIFE HISTORY AND SEASONAL PREVALENCE 113 remained long enough to make it certain that the dissemination of species through the agency of steamships is by no means impossible, and may be of not infrequent occurrence. It was only, however, when the ship chanced to lie close to the shore that she was invaded in this way. The myriads of Mosquitoes that invade a ship moored alongside the bank in Garden Keach must be seen and felt to be believed, but when moored " in the stream " one is left comparatively at peace ; and in open ports, ships anchored at the usual distance from the land remain practically free from these pests.
Another reason that makes it impossible for Mosquitoes to be carried overseas any considerable distance by the wind is that, whatever ixiay be the rate of travel that they can bear without injury, the entire journey must be made at night, for in tropical regions shelter from the sun during the day is a matter of life and death to a Mosquito, as is proved by the following observation.
In Oudh, during April, it is difficult to sleep in comfort except in the open. My servant one night was careless in the tucking in of the mosquito net, and when I woke in the morning I found I should have done well to have imitated the Irishman of the story and " crept out under the bottom bar," for the net was aHve with happy, satiated dames of the gnat community.
My bed, as a matter of fact, had no bottom bar, but doing my best to imitate the astute Hibernian, I crept out with infinite precaution, tucked in the curtains, and gave directions that the bed should be left where it was. The day turned out somewhat cloudy, the temperature inside the net never exceeding 105° F., but by noon every Mosquito was not only dead, but bone-dry.
For these reasons, we may I think reject, as having no foundation in fact, such popular beliefs as that the swarms of Mosquitoes that sometimes appear on the Persian coast, have been carried by the wind 200 miles across the Gulf from the Arabian shore ; albeit you must be prepared to hear this belief quoted as an established fact, even by European residents.
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