394 



THE INVERTEBRATA 



tional spiracles but breathe by means of tracheal gills — expansions of 

 the body wall through whose thin walls respiratory exchange between 

 the animal and the water is effected (Fig. 312). They are usually 

 external but in certain dragonfly nymphs {Aeschna and Libelluld) 

 the rectal wall is raised into such gills and respiration is effected by 

 pumping water in and out through the anus. 



Reproduction. The sexes of insects are separate, with only one 

 known exception, viz. Icerya purchasi, a self-fertilizing hermaphro- 

 dite. The usual method of reproduction is by deposition of yolky 

 eggs following copulation. The egg, except in many parasitic Hymen- 

 optera, is richly supplied with yolk and invested with a vitelline 

 membrane and further protected by a hard shell or chorion. The 

 chorion exhibits different degrees of external sculpture and it is 

 perforated at some point or points to allow of sperm penetration. 



Fig. 295. Pupa of Anopheles maculipennis . After Nuttall and Shipley. 

 /. respiratory funnel. 



The spermatozoa, which are of the filiform type, may be transmitted 

 to the female in the form of a spermatophore. Though insects are 

 on the whole prolific creatures capable of producing large numbers 

 of eggs, a few cases are met with where females only lay a few eggs 

 in the course of their life. Thus, in the viviparous tsetse flies, a single 

 egg is passed to the uterus about every nine or ten days. The larva is 

 there nourished by special "milk" glands till it is fully fed when it 

 is passed out for immediate pupation. Viviparity and reduced egg 

 production are here obviously associated with one another. In a large 

 number of cases reproduction is effected without the intervention of 

 the male. This phenomenon of parthenogenesis is best seen in the 

 aphides or plant lice where several generations resulting in the pro- 

 duction of parthenogenetic females are passed through. The racial 



