DEVELOPMENT. 



639 



(Id 



FIG. 398. Terminal region of 

 the female generative organs 

 of Musca domestica (after 

 Stein). Od oviduct; Re ; the 

 three receptacula seminis ; 

 Dr glandular appendages of 

 the vagina ; Bl blind sac-like 

 appendage. 



vitality in the spermatheca of the queen-bee for over a year. 



The ova are fertilized internally, and are laid singly or in clusters, 



or in a cocoon (ootheca) as in the cockroach. 

 The testes are less conspicuous than 



the ovaries. Each may consist of a 



long tubular gland, but more usually 



they are tufted and provided with a 



number of follicles. In many Ortho- 



ptera, Coleoptera, Lepidoptera and 



Hymenoptera the right and left glands 



may be closely united and appear as 



one. Each opens into a vas deferens, 



which may be short or long. At the 



outer end each vas deferens usually 



swells to form a vesicula seminalis in 



which the spermatozoa accumulate ; 



they then unite into an ejaculatory 



duct which is protrusible. As in the 



corresponding parts of the female the 



ducts of the male usually bear certain accessory glands of 



often doubtful function, though in some species they supply a 



secretion which forms the packets containing the spermatozoa 



(spermatophores). The spermatozoa of insects, unlike those 



of most Arthropods, are pro- 

 vided with vibratile flagella. 



Development. In the course 

 of their life many insects un- 

 dergo profound changes in 

 their constitution and in their 

 form. The egg as a rule gives 

 rise to a larva which forms the 

 predominantly feeding and 

 assimilating stage. The larva 

 is succeeded by a pupa or 

 chrysalis stage, a period of 

 quiescence during which many 



of the tissues are disintegrated and rebuilt. This in turn gives 



rise to the imago, during whose life as a rule little growth 



occurs, but the animal is given over almost entirely to reproduction. 



A few insects produce their young already freed from the egg- 



FIQ. 399. Male generative organs of the 

 Cockchafer (after Gegenbaur). T testes ; 

 Vd dilated portion of the seminal duct ; 

 Dr coiled accessory gland. 



