Page:Tropical Diseases.djvu/177

IX repeated inoculation tends to produce tolerance, as in the case of many other organic poisons.

A buccal tube is formed by the apposition of the upper surface of the hypopharynx to the under surface of the labrum (Fig. 33). Along the tube so formed the blood is aspirated by the expansion of the gizzard-like organ (Fig. 11, b), and then driven by the contraction of the same into the stomach (Fig. 11), or middle intestine, as it is called. A mosquito will fill herself in a minute or thereabouts. She then withdraws her proboscis and flies heavily away to some sheltered spot to digest the meal. Apparently the first step in digestion is the concentration of the blood she has imbibed; this is effected by excretion

of the watery portion of the the liquor sanguinis. Often while this process of dehydration is proceeding, even while she is sucking, droplets of clear fluid may be seen ejected at her anus. The concentrated blood becomes in this way a viscid tarry mass, which is gradually, in the course of three or four days, partly absorbed, and partly voided as gamboge-coloured fæces. The mosquito is now ready for another meal.

The rich pabulum supplied by blood seems to favour ovulation.

Diagnosis.—Many kinds of insects possess blood-sucking propensities. As a rule there is little difficulty in distinguishing most of these from the mosquito. There are certain Diptera, however, which closely resemble the latter in appearance as well