ARTHROPOD A 



195 



mature life of some is merely a matter of hours, while they 

 have lived for two years or more as larvae. Queen bees are 

 reported to have attained an age of five years and queen ants 

 of thirteen, but these are of course exceptional. Many insects 

 live only a single season ; others hibernate, remaining quiescent 

 during the cold season and becoming active again as warmth 

 returns. Many insect larvae are aquatic ; in such cases, when 

 the larva lies entirely submerged, the tracheae, if present, are 



i. 14,13 



....17 



FlG. 192. Periplaneta: Female cockroach dissected, from dorsal side; enlarged about 

 two diameters. 1, head; 2, labrum ; 3, antenna, cut short; 4, eye; 5, crop; 6, nervous 

 system of crop; 7, gizzard; 8, hepatic caeca; 9, mid-gut or mesenteron ; 10, Malpighian 

 tubules; u, colon; 12, rectum; 13, salivary glands; 14, salivary receptacle 1,15, brain; 

 16, ventral nerve cord with ganglia; 17, ovary; 18, seminal receptacle; 19, oviduct; 20, gen- 

 ital pouch, in which the egg cocoon is formed; 21, colleterial glands; 22, anal cercus. 

 (After Shipley and MacBride.) 



non-functional, and there are sometimes little outgrowths from 

 the body wall called tracheal gills or branchiae, which function 

 as respiratory organs, — these disappear before adult life. There 

 is usually in insects more or less sexual dimorphism ; the males 

 as a rule are smaller than the females, more active, and more 

 brilliantly colored. In some cases there are two kinds of 



