532 



INSECTA. 



Amongst the secretory glands of insects the ylruirhdw odoriferce, the 

 wax-glands, spinning-glands and jioison (/lands are to be mentioned. 

 Of these, the first, to which belong the anal glands which we have 

 already mentioned (fig. 440), lie beneath the covering of the body 

 and secrete, usually between the articulations, strongly smelling 

 fluids. In the buys there is an unpaired piriform gland in the 

 metathorax, which pours out its secretion by an opening between the 

 hind legs and gives rise to the notorious smell. Unicellular cutaneous 

 glands have been shewn to exist in different parts of the body of 

 insects, and, like the sebacious. glands of vertebrates, seem to secrete 

 an oily liquid, which serves to lubricate the joints. Similar glandular 

 tubes of the integument, which may be called wax-glands, secrete 

 white threads and flakes, which cover the body as with a kind of 

 powder or wool (Plant lice, etc.. tig. 441). Spinning-glands occur 



exclusively in 

 larvae and serve 

 for the produc- 

 tion of webs and 

 cases. When 

 these glands 

 have the form 

 of two or more 

 less swollen and 

 elongated tubes 

 (sericter I a ) 

 opening behind 

 the mouth, they 

 may be com- 

 pared to a 

 special form of 

 salivary gland, 



which they also resemble in their structure. The larva of the ant- 

 lion has its spinning organs at the opposite end of the body ; the 

 wall of the rectum, which is shut off from the chylitic ventricle, 

 taking the place of the sericteria. 



The poison glands, which are present in the female Hymenoptera\ 

 consist of two simple or branched tubes, the common duct of which is 

 dilated to form a vesicular reservoir for the secreted fluid, which 

 consists of formic acid. The end of this reservoir is connected with 

 the poison spine. 



Vascular system. The blood, which is usually colourless but not 



a. 



FIG. 441. The wax glands and the prominences on which they open 

 of an Aphide (Schizoneitra Lonicerae). it, Pupa seen from dorsal 

 surface; Wh, prominences on which the wax glands open; 

 It, the unicellular wax glands (WD) beneath the cuticular facets 

 (Cf) of the skin. 



