ENTOMOLOGY 



framework (appearing in sections as a network) enclosing vacuoles of a 

 clear substance the secretion; the chitinous intima is penetrated by fine 

 pore canals through which the secretion .passes. In many insects, no- 

 tably the cockroach, the common duct is held distended by spiral threads 

 which give the duct much the appearance of a trachea. 



FIG. 153. Right salivary gland of cockroach, ventral 

 aspect, c, common duct; g, gland; h, hypopharynx; r, 

 reservoir. After MIALL and DENNY. 



.' b 



S 



FIG. 154. Histology of 

 salivary gland of Ccecilius, 

 radial section. b, basement 

 membrane; c, canal; g, glandu- 

 lar cell; i, intima; n, nucleus. 

 After KOLBE. 



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 facilitate 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 kid- 

 ney, or Malpighian, tubes, present in 



nearly all insects, are long, slender, blind tubes opening into the intestine 

 immediately behind the stomach as a rule (Figs. 145, 146), but always 

 into the intestine. The number of kidney tubes is very different in dif- 

 ferent insects; Collembola have none, while Odonata have fifty or more 

 and Acridiidae as many as one hundred and fifty; commonly, however, 

 there are four or six, as in Coleoptera, Lepidoptera and many other orders. 



FIG. 155. One of the three-lobed 

 salivary glands of a mosquito. The 

 mMdle lobe secretes the poison. After 

 MACLOSKIE, from the American Nat- 

 uralist. 



