6 GNATS OR MOSQUITOES — CHAPTER I supposed that all of them are Siviulia, and some of the most common and vicious of Indian species approach so closely in general structure, and even in the general appearance of the venation of the wing, to the anomalous gnats of the genus Corethra, that they are difficult to distinguish without close examination, as may be seen by comparing the figure given on previous page, of a species (probably new) of Phlebotomus, most pestilent in its attentions in Himalayan hill stations during the rains, with that given of the wing of Corethra in the systematic description of that genus, but it will be seen that what looks like a gnat's hinder fork cell is really not the fourth, but the fifth longitudinal vein, and that the sixth, in place of being simple, is also forked : distinctions that will be better appreciated after a perusal of the following pages on the terminology of the group. The insect, too, is clothed, not with scales but with hairs. A more darkly tinted species of the same genus is equally troublesome in the plains.
In almost all cases it is the female alone that attacks man and animals, an exception being noted in the case of C. salinus by Ficalbi in his description of that species ; but, however bloodthirsty they may be, it is obvious that they must mainly depend on other nourishment than blood, for these insects are sometimes found in just the places where air-breathing vertebrates are most uncommon. The males of some Indian species, notably of Stegomyia fasciatus (Fabr.), undoubtedly often settle on one, and place themselves in position as if to bite, but I have never seen one actually do so, although afforded every opportunity.
In England, and probably in other parts of the world, gnats may be seen feeding upon the nectar of flowers, and here it is certainly their usual food, a fact I have often verified by personal observation ; but Mr. Theobald has also seen C. ciliaris sucking the juices of small Diptera. The peculiarity of the females alone being addicted to bloodsucking is shared with the Tahanidce, or gad-flies, whose males, too, live entirely on the juices of flowers. Coming now to the question of terminology, it may first
POSITION AND TERMINOLOGY OF THE CULICID^ 7 be observed that the terms used by Dipterologists are broadly the same as those used by entomologists generally, with such modifications and additions as are required by the special pecuharities of the Order. Unfortunately, neither among the former nor the latter is there any agreement as to the terms employed. It would be impossible, in any moderate compass, to cover even the practice of the best known authorities, and without expressing any preference for Loew's terminology over that adopted by other authorities, it represents, I thmk, better than that of any other author what may be called the average practice in the matter, and on this account the definitions given below are, to a great extent, quoted verbatim from his " Monographs of the Diptera of North America," published by the Smithsonian Institution, Washington, 1862.
In the systematic portion of this book, except where the original descriptions have been transcribed or translated as they stand, Loew's terminology is adopted, except for the wing, for which Skuse's nomenclature is followed. As in all typical msects the body of the dipterous imago is divided into the three regions of the head, thorax and abdomen. In the first two of these the component somites are so soldered together that their limits can hardly be distinguished, but in the abdomen the segments are clearly separated.
The entire external surface is covered with a chitinous investment, which is in all sufficiently dense to afford efficient protection to the internal organs and a firm attachment to the muscles that move the various parts, and in many families forms a veritable coat of armour. This chitinous coating is always, to a greater or less extent, covered with an armature of chitinous processes in the form of hairs, spines or scales, and where these are so closely arranged as to form a more or less uniform covering, they are spoken of as a tomentum.
In the CulicidcB all three of these forms of armature are well represented, but its characteristic feature is the abundance of scales, which in most genera thickly cover the entire body, and in almost all cases the decorations of gnats
8 GNATS OR MOSQUITOES — CHAPTER I are due to variations in the coloration of this scaly coating ; and as these are very easily rubbed off, caution is necessary in comparing them with descriptions.
The tinting of the chitinous body wall itself is spoken of as " ground colouring," and in insects which are provided but scantily with hairs, this may be arranged in sharply defined and brilliant markings, as, for example, in many midges, but in the Culicidoi it is usually of a nearly uniform dull tint, and at most shares only in the production of their markings by showing through, between tracts of scales, in the form of symmetrical bare lines and patches. Fig. 3.— Head of Female Culex.
a, thorax ; b, neck ; c, uape ; d, occiput ; e, vertex ; /, eyes ; g, frc fe, clypeus ; i, antennae ; fc, palpi ; I, proboscis. In descriptions of Mosquitoes it is not uncommon to find the coloration of parts qualified as " when denuded," and where this is the case it must be understood that it is the ground colour that is referred to.
In Anophelea, as a rule, there are few or no scales on the abdomen, and its coloration is mainly ground colour. Their appendages, on the other hand, are exceptionally densely scaled. In Corethra, again, the appendages, though very hirsute, are so mainly through the abundance of hairs, but in all genera of gnats the armature of the wings is entirely composed of scales.
POSITION AND TERMINOLOGY OF THE CULICID^] 9 The head has a hinder plane, opposite the thorax, called the occiput (occiput) ; that region of it lying over the junction of the head is the nape (cervix). The part of the head which reaches from the antennae as far as the occiput, and is limited laterally by the compound eyes, is the front [frons), the upper part of which is the crown {vertex), the limit between the front and the occiput having the name of the vertical margin {margo vertical-is). The middle of the front, being often of a more membranous substance, and sometimes differing in colour from its borders, is called the frontal stripe (vitta frontalis). On the crown are the simple eyes {ocelli), which, however, are absent in the Gulicida', but in the majority of Diptera are usually three in number, sometimes on a sharply defined triangular space, the ocellar triangle {triangulum ocellce). Most of those Diptera which undergo their metamorphosis within the larval skin possess, immediately above antennae, an impressed arcuate line, which seems to separate from the front a small, usually crescentic piece, the frontal crescent {lunula frontalis). The impressed line itself, which continues over the face nearly as far as the border of the mouth, is called the frontal fissure (fissnra frontalis). It owes its origin to a large bladder-like expansion, which exists at this place in immature imagines, and which helps them in bursting the pupa case. The frontal fissure is, of course, the true anterior limit of the front, and the frontal crescent in fact belongs to the face ; but from its usual situation it is commonly considered as part of the front. In many genera the eyes of the males meet on the front, so as to divide it into two triangles, the upper of which is called the vertical triangle (triangulum verticale), the inferior, the anterior frontal triangle {triangulum frontale anterium), or simply the frontal triangle. The anterior portion of the head, reaching from the antennae to the aural margin {yeristomium) is called the iiiGe{ fades). In most Diptera it is divided into three parts adjoining each other, the limits of which depend on the situation which the frontal fissure, continued to the oral margin, occupies in the developed imago. The form and mutual
10 GNATS OR MOSQUITOES — CHAPTER I proportion in size of these parts are of the highest value in the classification and distinction of species. Beneath the antennae there are, in many Diptera, longitudinal grooves for their reception, the antennal furrows {fovecc antennales). The antennae lie in these while in the pupa case, and sometimes even after escape. That part of the head which lies on the side beneath the eyes is called the cheek (gena). The compound eyes are sometimes encompassed in a greater or less portion of their circumference by a ring, somewhat swollen, and separated, more or less distinctly, from the rest of the surface of the head, and named the orbit, the successive parts of which may be called the anterior (orbita anterior, sive facialis), the inferioi' {inferior s. genalis), the posterior {posterior s. occipitalis), the superior {superior s. verticalis) , and the frontal {frontalis) orbits. An orbit is also often spoken of when there is no distinct ring ; but in this case there is some difference of colour or structure to mark it off.
The oral parts of Diptera, destined for sucking, are called the sucker or proboscis. They are either inserted at the end of a more or less prolonged, cylindrical portion of the head called the snout {rostrum), or project from a wide aperture, often occupying a great part of the under surface of the head, called the mouth hole {cantus oris). The common, fleshy root of the oral parts is connected by a membrane with the border of the mouth. This membrane often has a fold of sometimes quite a horny consistence, and is then called the clypeus {clypeus s. prcelabrum). It is either entirely concealed by the anterior border of the mouth, and is then usually movable, or it projects over it as a ridge, and is then usually immovable, as in the Culicidce, in which it is usually bare, but is covered with crescentic scales in the genus Stegomyia, and is densely hairy in Trichoprosopon. The largest of the oral parts in most Diptera is the fleshy under lip {labium), consisting of the stem {stipes) and the knob {capituluin labii), formed by the two suctorial flaps {labella), and modified in the CidicidcB into the membranous, scaly sheath which encloses the other mouth parts. Besides
POSITION AND TERMINOLOGY OF THE CULICID^ 11 the under lip the palpi are most perceptible, and must be noticed in the description of species. The remaining oral parts are generally rather small and stunted, having the form of bristles or horny lancets. They are considered as being the tongue {lingua), under jaws {maxillcB), upper jaws {mandihulce) , and upper lip {labrum), the latter shutting the under lip from above, and certain prolongations of the chitinous pharynx such as an epi- or hypopharynx. These parts are not particularly valuable in the distinction of species.
As in other insects, the thorax of Diptera consists of three segments, the prothorax, the mesothorax, and the metathorax ; but in this Order the mesothorax is so much more developed than the other segments, that it forms by far the largest part of the thorax, and in the description of Diptera is exclusively designated by that name, while other names are given to the pro- and metathorax when some particular part of them has to be characterised. The prothorax, being generally very little developed, sometimes forms a neck-like prolongation which bears the head, and is then called the neck {collum). Sometimes the forecorners of the mesothorax or shoulders (humeri) are covered by a lobe of the prothorax (lobulus prothoracis humeralis), distinctly separated from the mesothorax ; but it is not uncommon for this to be soldered to the mesothorax so that no distinct limit is perceptible, except by differences of colour and armature, in which case it is called the shoulder callosity (callus humeralis). Sometimes also the prothorax is closely applied to the anterior border of the mesothorax, and has then the name of the collar {collare). The mesothorax frequently also has a transverse furrow {sutura transversalis) crossing the middle of its upper side, and ending on each side a little before the base of the wing ; its presence or absence, as well as its form, furnishes characters of importance in classification. On each side of the pleura or " breast-side " there is, beneath the shoulder, a spiracle belonging to the prothorax {stigma prothoracis). To the back of the mesothorax applies the scutcheon (scutellum), separated from it by a furrow.
1'2 GNATS OR MOSQUITOES — CHAPTER I Beneath the scutellum a part of the metathorax is to be seen called the metanotum, general!}^ descending obliquely, often very convex, and on each side a more or less inflated space, called the lateral callosity of the metanotum. In most mosquitoes this portion of the thorax is nude, but in Mr. Theobald's new genera Wyeomyia and Trichoprosopon, is provided with certain characteristically arranged groups of bristles ; and care should be taken not to confuse these metathoracic tufts with somewhat similar but much more uniformly present tufts on the scutellum, which overlie them in the usual position of the parts. The poisers (halteres) have their origin beneath this callosity, and before each is placed the stigma of the imetathorax. In many Diptera these are protected by membranous covers placed above them, and called covering scales {tegulce s. squamcB). The abdomen is the third of the principal parts of the body, but the word often is applied to the dorsal side only, the under side being called the belly {venter). Its segments, as in other Orders, are counted from before back ; but the anterior ones are often soldered together, while the posterior ones are stunted and concealed. Much caution therefore is desirable in counting them. The statements as to their number are often rather arbitrary and conventional, and must be taken to refer simply to such as are plainly obvious under a simple lens.
A careful observer will generally find they are really more numerous ; and it must also not be assumed that the entomologist's " segment " is necessaril}^ equivalent to the "somite" of the comparative anatomist. In many mosquitoes the scales clothing the abdommal segments are so coloured as to produce a conspicuous alternation of darker and lighter bands ; and the presence or absence and relative position of these bands is of the greatest importance in distinguishing the various species. This banding may be produced in three different ways. Either the lighter coloured scales may be confined to the front of the segment, in which case it is said to be " basally lighter banded"; or they may be located on its hinder portion, when the lighter banding is said to be "apical" ;
POSITION AND TERMINOLOGY OF THE CULICIDJE 13 or, lastly, the coutiguoas portions of consecutive segments may be lighter or darker, and the middle portion of each of the contrasting tint.
Another term, frequently recurring in entomological descriptions, especially m those of earlier date, is to speak of the fore parts of the segments wher6 they are connected with that before them as the incisurce abdominis ; and where, as in banded insects, this part of the segments is coloured in contrast, the term " with lighter incisurce " may be taken as equivalent to " basally lighter banded," and the converse ; though often the term is loosely employed, and it is therefore preferable to employ the modern equivalent. At the end of the abdomen we see in the male the claspers {hypopygiiLm), in the female the ovipositor. If the claspers have the form of pincers, and are not bent under the belly, they are called forceps. The Ovipositor may be either a borer (terehra) or a style (stylus), or, as in the family with which we are mainly concerned, may take the form of paired, lobular organs, w^iich, though wanting the hook of the male clasper, resemble it closely in general appearance. Both organs are very important in the distinction of species in many families, and their structure being generally very complicated and varying much in different families, deserves a most attentive study.
So far the terms employed are in such universal use among systematists that they may be considered as definite as those employed in human anatomy, but it is when we come to the wmg that the confounding of confusion commences ; for it may almost be said that the systems of nomenclature are as numerous as the authors who employ them.
This has come about from the natural desire to devise a terminology applicable to the entire Order, combined with the widest diversity of opinion as to the correspondence of parts in the various families ; so that probably a rigidly numerical system would be really the most convenient, in spite of the names affording no clue to the correspondence of the veins ; but an exclusively numerical nomenclature is, as a matter of fact, adopted by no entomologist who