After passing through the ganglionic ring formed by the supra esophageal and infra esophageal ganglia the pharynx widens into the large chamber, the pumping organ. In cross section here the lumen of the pharynx, in the position of rest, is triangular. This is caused by the deposition, in its walls, of three thick chitinous plates, one on each side and one superiorly. The plates are connected by a thinner (non chitinous) membrane, and their edges are inroiled, so that when extended by the muscles acting upon them they return with spring like force to their original position. Three sets of muscles are inserted into the sides of the pump: A dorsal pair of muscles, arising from the occiput, and two lateral masses attached to the lateral portions of the inner surface of the head. Beyond the pumping organ the cavity of the pharynx agains narrows, and there are here a distinct bend and thickening of the muscular wall, where the pharynx joins the esophagus. This portion functions as a valve, so that when the pump is in action, blood is drawn into its cavity by the vacuum formed, the posterior valve leading to the esophagus being then closed. When the pump contracts, this opens and blood flows into the esophagus, its forward motion being also promoted by peristaltic movements from before backward of this muscular portion and of the commencing portion of the esophagus. The return of blood into the mouth is prevented by the sharp angle (and the presence of the hairs, according to Dutton) which exists at the junction of the ascending and horizontal portions of the pharynx.
The esophagus begins immediately behind the posterior pharyngeal valve, and after a neck like portion, forms a sac or reservoir which, according to Christophers, does not possess a chitinous in tima, but does, according to Schaudinn. The epithelial lining of the first portion is subcylindric or cubical; that of the reservoir, subcubical, while in the diverticula the epithelium becomes quite flat. The muscular covering of these portions is also slight, while in the esophagus it is less than in the pharynx; in the diverticula there remain only a few scattered fibers. Besides the neck and main sac or reservoir of the esophagus we have as appendages of the esophagus the most interesting structures, known as diverticula (or sucking stomachs of some authors). They are readily seen in all dissections of mosquitos, and in hibernating mosquitos (in England) have a striking appearance, from the mass of bubbles of gas with which they are packed. These diverticula are usually three in number- a large ventral one and two smaller lateral ones. In the newly hatched mosquito the ventral diverticulum is small, but soon becomes expanded, and stretches into the abdomen as far back as the fifth abdominal segment. It not uncommonly happens, too, that a diverticulum may be wanting, being represented only by a slight pouching of the esophagus. The function of these diverticula is a somewhat disputed point. It can hardly be doubted, however, that it has something to do with the contained bubbles. Christophers has assigned to them mainly a physical function in supporting the pressure of the blood in the stomach. But Schaudinn's experiments throw new light on the subject. Any one can satisfy himself, moreover, that these diverticula often contain large masses of bacteria besides protozoa. The fact of the presence of these bacteria led Schaudinn to suppose that the gas was not air, as had been generally supposed, but carbonic acid gas. This was easily shown by adding baryta water to the preparation under the cover glass, when a turbidity followed. Schaudinn then supposes that the function of the gas is to prevent coagulation of the blood during the act of suction. But to this point we shall return. Another function that has been ascribed to these diverticula is that of blood reservoirs, but there is little to support this, as at the end of the sucking act the diverticula contain at most a few corpuscles. To proceed, however, with the anatomy of the gut. After the diverticula have been given off from the esophagus we reach the junction of the esophagus and midgut. This portion is considered by Christophers to be the homologue of the proventric ulus found in many insects.
This consists of a fold of the fore gut into the midgut, and presents some interesting features. The extent to which this invagination occurs differs according to whether or not the insect is feeding. In the fasting insect the infolding is considerable, and the infolded portion is thrown into a number of secondary folds. Moreover, at the point where the esophagus merges into the midgut there is a considerable development of muscle fibers, so that a sphincter is formed. This, together with the invagination, forms an efficient valve. This valve is opened when blood is being taken in; the invaginated portion is drawn out, the longitudinal muscle of the esophagus contracts, and the invaginated portion, now unfolded, is expanded by the entering blood and thus forms a "crop." The peristaltic movements of the esophagus then pass the blood on into the midgut. Another peculiarity of this "crop" described by Schaudinn is that the epithelium secretes a kind of jelly like layer which covers the mass of ingested blood, so that the food, as in other insects, is not in direct contact with the epithelium itself. While, then, in the fasting condition the beginning of the midgut and the necks of the diverticula are in apposition, in the feeding insect the necks of the diverticula are separated from the midgut by the region occupied by the drawn out invaginations, or, in other words, the crop.