The imbibed mass of blood is surrounded in the crop with a "gelatinous" secretion from the chitin layer, so that the digestion of the blood and the absorption of nutriment take place through this as an intermediary agent. Shortly after the blood has reached the stomach the serum separates from the corpuscles and is found occupying the anterior portion of the cavity, while the corpuscles occupy the posterior dilated portion. The corpuscles are then acted upon. Hemoglobin dissolves out, and the serum becomes tinged with it, and finally the corpuscles also are completely broken down. The insoluble residue appears as a mass of brownish black, highly refractile granules; these are gradually passed on and become mixed with the excretory products of the Malpighian tubes, which consist of crystalline, colorless, refractile granules. As these mix with the undigested products of the blood the contents of the gut change from dark brown to grayish yellow, and when digestion is complete to white, so that by observing the color of the feces deposited by the mosquito, the state of the digestive process can be known; this can be, however, readily ascertained by observation of the stomach with the naked eye. When the excreta have been got rid of completely, the stomach is quite free from its central little black clump at the posterior end. The whole process of digestion is dependent upon the temperature. At a temperature of about 80° F. it is complete in about two days, while at 45° F. it may take as long as eight days. When digestion is complete, regeneration of the epithelium of the digestive tract proceeds, commencing in the fore gut and invaginated proventriculus. Here cells showing mitotic change can be found, even while in the end of the midgut the epithelium is still undergoing desquamation.
This is not well developed, and the elements which compose it are extremely delicate and difficult to define. The only vessels that exist as definite closed tubes are the large dorsal vessel or heart and its anterior prolongation, the aorta. This dorsal vessel is situated just below the abdominal tergal plates. It is suspended by a number of muscular fibers running in pairs from the side of each abdominal segment, giving the dorsal vessel a festooned appearance. Internally the heart is lined with a homogeneous layer in which a number of flat nuclei are distributed. The wall, which is extremely thin, consists of a few muscular threads. Externally there lie on it the so called pericardial cells. These cells, the largest in the body, are granular, opaque looking cells, containing some yellowish pigment and one or more nuclei. They can be followed back as far as the rectum, while the dorsal vessel appears to end about the level of the colon. Anteriorly the heart is continued forward as the aorta, passing beneath the mesophragma. In the thorax it gives off four branches, two of which supply the salivary glands, losing themselves in sinuses in that structure, while two other branches follow the course of the esophagus. The main aorta passes on through the neck and is distributed to the head. The aorta can easily be seen pulsating in the living mosquito. For this purpose it is best to use an old mosquito which has lost a good many of its scales. With a strong illumination under the microscope the heart can be seen to contract, according to Schaudinn, four or five times consecutively; then follows a pause of about, three seconds. It is during this interval that the respiratory process effected by the contraction of the abdomen goes on, and then follows the peristaltic wave, which can be observed passing from before backward over the ileum.
Fat is disposed in the body in two different ways: (1) As a general lining to the body wall lying immediately below the cuticle; (2) as isolated lobular masses in connection with the various organs and the muscles. Thus we find a lobe of the fat body in close association with the salivary glands.
The cells composing the fat body are of considerable size and contain numerous oil globules, with some granular material and also minute dark, refractile granules suggesting pigment. The cells contain deeply staining oval nuclei. The" pigment" granules are most abundant in old mosquitos.
The main mass of muscle in the mos- quito occupies the thorax. Here two large bands pass on either side dorsoventrally, while between these lateral masses bands running in an anteroposterior direction occur. Neither of these masses is inserted directly into the wings, the movement of the latter being effected by alterations in shape in the thorax due to contractions of the vertical and horizontal systems. We have already noticed the band of muscle which passes outside the salivary glands on either side, and which probably serves to exert pressure on these and so promote the flow of the fluid in the intra acinar lumen. In the head we have already described the muscles that serve to work the pharyngeal pump. We have, further, various longitudinal bands which serve to move the various portions of the proboscis.
(1) The mandibles are at their base attached to a fine tendon connected with a muscle arising from the ventral surface of the exo skeleton of the head, at the point where it is folded in beneath the eyes. The muscle runs forward and downward.
(2) The maxillce appear to arise from the under surface of the maxillary palpi, between them and the upper surface of the labrum. From this point, however, they are continued back as thick, chitinous apodemes or rods to the back of the head, where they end in stumpy processes apparently lying free in the cellular tissue; three sets of muscles act upon this intracranial apodeme of the maxilla?: (a) A horizontal muscle, attached along the outer margin for the greater part of its length: this muscle arises from the lower occipital region; .(b) a muscle inserted along the ventral side of the rod and taking its origin from the base of the labrum; (c) a muscle arising from the dorsal surface and inserted into the first joint of the maxillary palpi.