98 GNATS OR MOSQUITOES — CHAPTER VI special set of muscles, by the action of which it can be actively dilated. Above, it is fixed by the connective tissue around it being intimately connected with that of the supra-oesophageal ganglia ; and on either side are inserted bands of muscular fibre, which pass obHquely forwards and outwards, to their origin from the chitinous wall of the ventral aspect of the occiput, and a glance at the figure above will make it clear that they must necessarily exercise a powerful suctorial action.
Behind this, the pharynx once more contracts to its previous diameter and just beyond the back of the head, where it enters the neck of the insect ; it ends in the soft oesophagus or true fore-gut, the walls of which are greatly thickened at this point so as to form a bulb, rather broader than that of the pharynx, and mainly composed of a muscular thickening, which probably acts by way of a valve, by means of which the contents of the mid gut are prevented from entering the pharyngeal bulb while the latter is exercising its suctorial function. It is on the ventral aspect of this junction, but mainly on the oesophageal bulb, that are placed the salivary glands which are therefore contained within the narrow neck connecting the head loith the thorax.
In the fresh insect, they extend but a httle way into the front of the thorax ; but in serial sections, as a result of the shrinking action of the various processes to which the preparation has been subjected, they are squeezed out of the soft and yielding neck, back into the cellular tissue filling the more rigid box of the thorax, and so are made to appear to extend backwards for a considerable distance into it ; and the erroneous representations that have found currency have no doubt resulted from a too complete reliance on the method of serial sections, unchecked by dissection of the fresh insect.
I can find no previous description of the salivary glands, including that quoted in my first edition, which is not, as I now find, full of inaccuracies. This has arisen from the fact that their extreme dehcacy renders their demonstration by far the most difficult piece of minute dissection that it has
THE ANATOMY OF THE ADULT MOSQUITO 99 fallen to my lot to attempt. It is comparatively easy to obtain a portion of them for microscopical examination, but to lay them bare intact, and still more to follow out the course of their ducts, is a task that will try the most patient. For often, as Dr. Daniels remarks, all one has to show for one's pains is the intra-glandular portion of a duct, denuded of its cells. As however, it appears probable that it is through the infection of these glands and their secretion that the germs of malaria are carried to man, their structure and relations require to be carefully studied. Although their duct passes forward to open into the fundus of the hypopharynx, these glands are rather closely bound down to the oesophageal bulb on which they lie by means of connective bands, and hence are occasionally brought away with the rest of the digestive viscera in the dissection of the stomach ; but such an occurrence is merely accidental and cannot be relied on ; and where the glands are required for investigation, the following is the best method I can suggest for exposing them :
— Place the insect in some salt solution on a slide, and then, with the point of an edged needle, lay open the narrow neck connecting the head and thorax, keeping well to the dorsal side. Next pass the needle into the midst of the thorax, so that it must divide the oesophagus about the middle ; then by a few gentle strokes, tear through the integuments of the neck here and there, where it joins with the thorax, still keeping away from the ventral side ; and finally, keeping the left hand needle entangled in the thorax, gently tear off the head with that held in the right and clear away the rest of the insect.
It is well at this stage to wash well with fresh salt solution from a pipette, and it facilitates the recognition of the various structures to just colour the solution with methylene blue.
If the separated head be now carefully examined there will be seen, protruding from the torn neck, the tag of oesophagus which has been drawn out from the thorax, and round it several minute glistening lobular bodies, which, under a sufficiently powerful lens, look much like tiny bunches of grapes.
100 GNATS OK MOSQUITOES — CHAPTER VI These are the blind ends of the lobes of the saHvar}' glands, of which there are six in all ; each gland consisting of three sausage-shaped lobes of nearly equal size ; but as a rule only two or three of their number will come into view at this stage and their subsequent separation from the structures surrounding them is purely a matter of manipulation, for which I can offer no better receipt than patience and steady hands. Once one has learned to recognise their very characteristic appearance, there is little difficulty in getting piecemeal preparations, but I was well nigh in despair ere I had cleared up my last doubts as to their exact form and connections.
In structure these glands differ entirely from any with which the student of vertebrate histology is familiar, as, strictly speaking, each cell is a separate gland, there being no true duct formed by a lumen bounded by contiguous gland cells and supported by a fibrous layer, such as is typical of vertebrate glands, as well as those that appertain to the intestine proper in arthropods. Instead of this, the salivary cells are arranged, like the flowers of a " spike," on a chitinous tube prolonged from the lining of the buccal cavity, and the singularly sharp outline and appearance of rigidity of this tube, form a marked contrast with the indeterminate appearance of the lumen of an ordinary tubular gland. Apart from the fact that it is quite straight, the intraglandular portion of this chitinous duct resembles nothing so much as an elastic fibre from the ligamentuvi nuchce, but is of much less diameter. The external diameter of even the main duct is only 3 or 4 /u,, and the lumen of its intraglandular branches can hardly exceed 0'5/a. Each lobe of the gland consists of a single branch of the duct with the salivary cells closely clustered round it, but in no other way connected with each other, so that the least touch or the pressure of a cover glass, at once separates them from each other and from the duct, to which each cell is connected only by a delicate thread of cell substance prolonged from their smaller ends. Each is a typical gland cell of considerable size and pear-shaped form, with large nucleus and nucleolus, the smaller end tapering off to a delicate thread
THE ANATOMY OF THE ADULT MOSQUITO 101 which connects the broad-ended cell-mass with the duct ; and if the latter be examined under a good apochromatic lens, it can be distinctly made out that, minute though it be, this chitinous wall of the tube is pierced by spirally arranged minute perforations, each corresponding to the point of origin of a cell.
In connection with the search for parasitic elements it is well to remember that, there being no connective or other elements to confuse the issue, any other structure than these very characteristic gland cells must necessarily be abnormal. Each gland consists of three lobes about three times as long as they are wide, and the three ducts unite at the same point into the right and left salivary ducts ; but these have only a short separate course, and at a point near the base of the pharyngeal bulb, they, in their turn, unite into a common median duct which terminates in the manner already described. The six lobes lie close together on the oesophageal bulb, and together cover the whole of its ventral surface and the greater part of its sides. The only other point requiring notice is that the extraglandular portion of these ducts is strengthened by a spiral or annular thickening of the chitine, so that they present a close resemblance to a trachea emptied of air ; but there is this difference, that whereas in the trachea the thickening is on the inside of the tube, in the salivary ducts it is distinctly placed on its outer surface. There is no separation or distinction whatever between the salivary and poison glands, which hence are now commonly referred to as the " veneno- salivary " glands.
Behind the bubar thickening in which it commences,, the oesophagus is continued as a simple transparent tube, through the whole length of the thorax, and the only point that requires special notice is that it is not uncommon, in dissecting, to bring away attached to it a pair of delicate bags of air bubbles, the true nature of which will be referred to in the description of the respiratory organs. The dissection of the remainder of the digestive apparatus is a simple matter, and the structures that compose it may be easily demonstrated in the following manner : — Take a
102 GNATS OR MOSQUITOES — CHAPTER VI specimen, and place it on a slide with a little salt solution, and carefully remove the legs and wings. Next tear open the thorax, so as to loosen the anterior attachment of the tube, and then carefully partially detach the last two abdominal segments by separating the delicate intersegmental membrane. Now place one needle so as to fix the last two segments, and with the other entangled in the thorax pull steadily on the anterior portion, when the alimentary canal and its appendages will be drawn out intact attached to the hinder fragment. If the operation be successful even the cesophagus and salivary glands may be included in the preparation, but as a rule, the tube is torn just behind the latter, so that they and the so-called aspiratory vesicle are left behind.
Assuming, however, that, as is usually the case, only a portion of the oesophagus is brought away, we find that, immediately after entering the abdomen the digestive canal expands into the chylific ventricle or stomach, which forms by far the largest part of the intestinal canal. It is of considerable width throughout, but especially so at its hinder ■end. Throughout its entire length it is thrown into deep transverse folds, which recall somewhat the valvules conniventes of the higher animals, and probably serve the same purpose of increasing the secretory area. In insects that have but recently emerged from the pupal stage some remnants of the last meal of the larva may occasionally be seen, but in the well-established imago plant juice, pollen, or blood will alone be found. Lastly, succeeding the broad, hinder part of the stomach, comes the hind gut, and running into the junction of the two are seen opening five long, convoluted, dark-tinted glandular bodies, the Malphigian tubes. These are supposed to have an excretory function, and as uric acid and other renal products have been found in theui, the balance of opinion regards them as renal organs. They are lined with a series of large nucleated cells, the protoplasm of which is exceptionally rich in pigment and granules. These cells are arranged in a somewhat peculiar fashion, for as they are too large to admit of their forming a complete lining, the cell of one
THE ANATOMY OF THE ADULT MOSQUITO 103 side projects into the depression formed between two consecutive cells of the other, so that in a tube that has been a little flattened out by pressure, the lumen appears zigzag, and the individual cells roaghly triangular. The last part of the digestive canal is much narrower than the stomach, its hning epithelium smaller celled and of a less glandular character, and its wall thick and muscular, while around it may be distinguished some strands of voluntary muscular fibre springing from the wall of the penultimate segment.
By a little judicious teazing of the last two abdominal segments, there may be made out, lying on the ventral aspect of the hind part of the intestine, four pear-shaped follicular glands, richly supplied with a close net-work of air tubes, and connected with the intestine by short ducts. These are lined with large gland cells, and probably secrete some fluid accessory to digestion. In the natural position they are arranged not symmetrically in pairs, but are " echelonned " in obliquely placed couples. The respiratory system is, for the most part, quite of the usual type, the main trunks from the stigmata giving off communicating branches to those in front and behind them, and ending in a tuft of branches for the supply of the muscles and other organs contained within the segment. There are no distinct stigmata in the head or in the last abdominal segment, though I have observed certain tracheae undoubtedly communicating with the exterior near the base of the proboscis ; and the two anterior thoracic stigmata are by far the largest and most important in the body, the abdominal openings being individually small and difficult to make out, as they are completely hidden by the scales fringing the edges of the sterna and terga. They are placed rather nearer the front than the back of the segments, and are best demonstrated by first carefully removing all scales by brushing, and then crushing the insect as it lies on its side between cover and slide, so as to pinch the abdomen from side to side and bring the lateral surface into view beneath the microscope. There is, however, in this connection, one structure that
104 GNATS OR MOSQUITOES — CHAPTER VI calls for especial notice ; the " aspiratory vesicle," or, as I prefer to speak of them, the pneumatic sacs. This structm'e is not, as has been stated, in any way pecuKar to the gnats, but is, I find, often even better developed in the midges, and other allied insects ; moreover it is not a median, but a paired structure and I believe that its size, which has hitherto Pig. 24. — To Illustbate Certain Points in the Anatomy of the Respiratory System, and of the Pneumatic Vesicle. A, camera lucida tracing of the distribution of the main tracheae in the abdomen of C. fatigans (Wied.) x 15 diams ; B, one of the abdominal stigmata and the tracheae springing from it, x 75 diams ; C, dissection of a midge, to show the relations of the pneumatic vesicle; p, pneumatic vesicle; V, mid-gut, X 15 diams ; D, base of one of the pneumatic vesicles nearly emptied of air, x 200 diams ; E, air bubbles contained in fibres of the pneumatic vesicles as seen under -jVth apochromat. objective, x about 1,000 diams. It will be observed that the air bubbles are of irregular and varied forms, and not uniformly spherical, as they would be if simply suspended iu a mucilaginous fluid.
been absurdly understated, is inversely proportional to the size and power of the wings in the different species in which it is found.
The reason it has been hitherto mistaken for a single median sac is that, owing to the pressure of the contained air, the two sacs come to He one behind the other. They have no true organic connection with the CBsophagus and the only reason why they have been supposed to be diverticula of the digestive canal is that they are often
THE ANATOMY OF THE ADULT MOSQUITO 105 brought away attached to it, owing to the fact that the fibrous base of the sacs, which connects them together across the middle hne, is divided into two bands, leaving between them an opening through which passes the oesophagus, a little behind the valve in which the latter commences. To the naked eye they look like clusters of minute air-bubbles, and when intact, their walls rival in tenuity those of a soap-bubble. At the least touch they collapse into a bundle of fibres amongst which a few residual bubbles remain entangled.
Instead of being as they have usually been figured, barely larger than the salivary glands, they occupy, during life, as much or more space than the digestive canal when at its utmost distension. Moreover, the more gorged the insect or the heavier it be with eggs, the larger will these sacs be found, as together they occupy a large space and fill out the entire ventral portion of the body cavity from the front part of the thorax to the end of the fourth or fifth abdominal segment.
It is difficult to understand how any one could ascribe to such a structure as this any active suctorial function, and so describe it as " aspiratory," and there can be no doubt that its function is identical with that of the air spaces in the bones of birds, namely, to lighten the work of the wings by increasing the bulk of the body ; and further, I have now no hesitation in affirming that, as too, is the case in birds, these sacs are mere extensions of the respiratory passages. Into the base of the sacs may be traced large tracheae and these split up and become continuous with a brush of dichotomously dividing fibres of which the base of each sac is composed. These fibres, like the iutraglandular part of the salivary ducts, look much like those of elastic tissue, and are undoubtedly composed of chitine. Moreover, if we examine the collapsed " sac " under a sufficiently high power, a number of minute air bubbles will be found to remain, and close examination shows clearly that these bubbles are not entangled between the fibres, but contained in dilatations of their continuity. In other words, these fibres are neither more nor less than