He figures the maxillae as lying dorsal to the mandibles, and thicker and more triangular in cross section than the latter. The "saws" on the maxillae and mandibles have their points directed backward, so that they tend to fix the proboscis in position when inserted.

The Act Of Suction

The mosquito can be seen feeling with its labellae various portions of the skin, then, when one is selected, the labellae expand and the "tongue " is applied to the surface. By this time the palpi have moved out of the way. The point of the labrum now appears between the extended labellae and pierces the skin. At the same time the mandibles and maxillae can be seen working up and down, and the head of the mosquito rocks somewhat in its efforts to saw out the aperture. The proboscis then sinks in, up to half or even almost the whole of its length. During this time the proboscis is held at right angles to the body or directed slightly forward. The hind legs of the mosquito (C. pipiens) may have been, in the mean time, lifted off the skin and bent back above the abdomen, so that the mosquito rests on its front pair of legs only. The substance which causes the irritation accompanying the bite is injected at the beginning of the process, for if, as not uncommonly happens, the mosquito flies away before sucking blood, the irritation is felt notwithstanding. The whole process of feeding lasts two to three minutes.- It may be worth mentioning here, a fact well known to residents in the tropics, that a mosquito feeding on the back of the hand may be trapped by folding the fist and so tightening the skin.

Alimentary Canal

The constituents of the alimentary canal are: (1) The foregut, comprising-(a) The mouth; (b) the pharynx; (c) the esophagus. (2) The midgut, comprising-(a) The homologue of the proventriculus; (b) the so called stomach; (c) the pylorus. (3) The hind gut, comprising-{a) The pylorus; (6) the ileum; (c) the colon; (d) the rectum. We have, in addition, the following appendages of the alimentary tract: (1) The salivary glands; (2) the esophageal diverticula; (3) the Malpighian tubes.

The mouth parts we have already described; we shall now take in order the soft parts of the alimentary tract.

The pharynx extends from the base of the proboscis to the origin of the esophagus, at the point of junction of the head and neck. It is, in fact, the continuation backward of the upper lip (labrum) or epipharynx above and the hypopharynx below. It is lined throughout its extent with chitin. It can, for practical purposes, be divided into two parts: (1) A narrow portion, passing upward and backward and ending opposite the furrow which separates the clypeus from the head; (2) a horizontal portion, approximately at right angles to the former. This passes between the supra esophageal and infra esophageal ganglia and their commissures, and terminates in the esophagus, at the junction of the head and neck. In this second dilated portion the walls are modified so as to form a pumping organ. In the anterior portion specialized structures have been described by Dutton and Schaudinn, but there is some disagreement in the descriptions. Firstly, Dutton states that it is only the anterior and posterior portions of the upper wall of this portion that are chitinized, the remaining middle portion being covered with flat epithelial cells, whereas Schaudinn and Christophers do not mention this layer. Again, according to Dutton, there exists, on the anterior portion of the upper wall, a few low conic papillae (taste papilla?), while of these Schaudinn makes no mention; the latter, however, describes on the floor of the first portion of the pharynx, without more precisely defining the exact spot, a peculiar arrangement. The chitinous layer is, he says, traversed by fine canals arranged at right angles to the surface. In each of these canals there exists a cell, hair like in form, each connected with a nerve fiber of the inferior esophageal ganglion; from this we may infer, however, that the structures lie near the junction with the second portion of the canal, for this is the position of the ganglion. On reaching the horizontal portion it is only the free end of these cells that projects in the form of exceedingly fine papilla?. Schaudinn considers that this structure functions as a taste organ. Dutton describes this organ as situated at the point of junction of the first and second portions, where, as a matter of fact, as pointed out originally by Dimmock, the dorsal and ventral walls approximate, causing a constriction which probably serves as a valve to prevent the return of fluids to the mouth during the action of the pump. It is at this point that Dutton describes the existence of this organ. It is situated on the posterior end of the ventral chitinous plate. It consists of a ridge of stout, hair like processes, which curve forward so that their tips lie in the angle between the upper surface of the first and second parts of the pharynx. The hairs are of two kinds-an anterior large set, probably a single row, and a posterior small, fine set, situated in a clump immediately behind the former. The larger hairs consist of a short stout shaft firmly embedded in the chitinous pharyngeal wall; this shaft supports a cup with a free rim curved outward; within the cup lies the oval shaped bulbous extremity of the base of the hair. This bulbous extremity contains a single large cell. The remaining free portion of the hair curves forward and tapers to a fine point, and appears to have a central shaft inclosed within a chitinous cuticle, from which barb like processes project. The hairs of the posterior set are much finer and shorter and are more numerous; they appear to be simple in character. In transverse section this structure presents to some extent the appearance of "rods and cones." The subesophageal ganglion lies in close proximity to this structure, but no nerve fibers have been shown to communicate with these specialized hairs, although such probably exist (Schaudinn, we have seen above, describes them). That, in the first place, these hairs act in conjunction with the general conformity of this part of the pharynx as a valve to prevent the regurgitation of blood back into the mouth during the action of the pumping organ seems to admit of no doubt; on the other hand, such specialization in structure would lead one to suppose that they possess also a sensory function.