746 



THE HEX A POD A 



the intestine is defined by the openings of the caecal Malpighian tubules. 

 This is an important landmark, defining where the stomach ends and the 

 intestine begins. The intestine is subdivided into small intestine, colon or 

 large intestine, and rectum. The last-mentioned may possess rectal glands, 

 while anal glands may open into the rectum just in front of the anus, and are 

 useful to the insect by forming the secretion which gives rise to repulsive odours . 



With regard to the proventriculus, it should really be a powerful muscular 

 organ lined with chitin. and possessing teeth, bristles, etc., but in the Diptera 

 it will be found to be much degenerated. The Malpighian tubules, which 

 number from four to six, are excretory in function. The alimentary canal 

 possesses longitudinal and transverse muscular coats and a llining]of epithelial 

 cells. 



Fig. 365. — Phthirius pubis LiNNiEUS, 1758, to illustrate an Insect 

 WITHOUT Wings. 

 (From a photograph by J. J. Bell.) 



The stigmata or openings of the respiratory system are situate on the 

 membranes, on the side of the body, in the thorax and abdomen, and are very 

 varied in number. They are never to be found on the head or on the last 

 abdominal segment. From the stigmata the tracheae, or air-tubes, ramify all 

 over the body. These tracheae are kept open by spiral thickenings of the 

 chitinous lining membrane. 



The fat-bodies are lobulated masses packed beneath the skin and between 

 the organs, and are supposed not merely to represent reserve material, but to 

 be of great importance in metabolism. 



The circulatory system consists of a dorsal longitudinal vessel divided into 

 chambers, which pulsate, and an anterior aorta, from which blood flows freely 

 into the coelom or body cavity. From this it returns to the heart by two 

 lateral vessels and one dorsal and one ventral vessel. The blood is colourless, 

 and contains amoeboid cells. 



As already mentioned, some insects possess poison glands. 



The nervous system and sense organs will not be described here, as they are 

 not of importance in tropical medicine. 



The male reproductive organs consist of testes, vasa deferentia, ductus 

 ejaculatorius, and an external copulatory organ. The female reproductive 

 organs are two ovaries, which consist of a ^ries of egg-tubes, and oviducts, 

 Which are united together to form the single oviduct, the lower portion of 



