ANATOMY AND PHYSIOLOGY 



123 



Histology of salivary gland 

 of Cacilius, radial section. b, 

 basement membrane; c, canal; 

 g, glandular cell; i, intima; n, 

 nucleus. After KOLBE. 



intima is penetrated by fine pore canals through which the 

 secretion passes. In many insects, notably the cockroach, the 

 common duct is held distended by 

 spiral threads which give the duct 

 much the appearance of a tra- 

 chea. 



In herbivorous insects the saliva 

 changes starch into glucose, as in 

 vertebrates; in carnivorous forms it 

 acts on proteids and is often used 

 to poison the prey, as in the larva 

 of Dytiscus. In the mosquito each 

 gland is three-lobed (Fig. 155) ; the 

 middle lobe is different in appearance 

 from the two others and secretes 

 a poisonous fluid which is carried out 

 along the hypopharynx. Though this poison is said to facili- 

 tate the process of blood-sucking by preventing the coagulation 

 of the blood, its primary use was perhaps to act upon proteids 

 in the juices of plants. 



Malpighian Tubes. The kidney, or Malpighian, tubes, 

 present in nearly all insects, are long, slender, blind tubes open- 

 ing into the intestine imme- 

 diately behind the stomach 

 as a rule (Figs. 145, 146), 

 but always into the intestine. 



One of the three-lobed salivary glands The number of kidney tubes is 

 of a mosquito. The middle lobe secretes 1 . ,,.. . < /v 



the poison.-After MACLOSKIE, from the very different in different m- 



American Naturalist. gects . Collembola have none, 



while Odonata have fifty or more and Acridiidge as many as 

 one hundred and fifty; commonly, however, there are four or 

 six, as in Coleoptera, Lepidoptera and many other orders. 

 Not more than six and frequently only four occur in the em- 

 bryo (Wheeler), though these few embryonic tubes may sub- 

 sequently branch into many. 



FIG. 155. 



