REPRODUCTION 103 



Pier is in which the ovaries mature after emergence the corpora allata 

 are needed for yolk formation. In the male of Rhodnius and other 

 insects the corpus allatum secretion is necessary for the activity of 

 the accessory glands which produce the spermatophore. 



Like the other effects of the juvenile hormone, these responses can 

 be evoked by farnesol derivatives (p. 89). In some insects, notably in 

 the blowfly Calliphora and the locusts Schistocerca and Locusta, the 

 neurosecretory cells in the dorsum of the brain play an essential part 

 in stimulating the fat body and other tissues to produce the specific 

 protein required for incorporation in the yolk. 



Hibernation and diapause 



Reproduction, like growth, may suffer a periodic arrest; and as with 

 growth (p. 94) this arrest may be a direct effect of an adverse en- 

 vironment, or it may be a true 'diapause' which persists even under 

 favourable conditions. In Dytiscus and various other beetles, the 

 gonads revert to a resting state after the first reproductive period; 

 they show renewed activity about the same date the following year, 

 sometimes in a third or even a fourth year also. This seems to be a 

 deep-seated rhythm and not a simple effect of warmth following the 

 winter cold. Presumably the process is controlled ultimately by 

 seasonal changes in the environment acting by way of the central 

 nervous system. But the immediate cause of the arrest is the absence 

 of secretion from the corpus allatum : ripe eggs may be produced in 

 Dytiscus at any season of the year if corpora allata are implanted. 

 In Leptinotarsa, and probably in other insects, the arrest of repro- 

 duction is induced by the short day-length (photoperiod) in autumn. 

 But the immediate cause of the arrest is the absence of secretion from 

 the corpus allatum: ripe eggs may be produced in Dytiscus at any 

 season of the year if active corpora allata are implanted; and the 

 same applies to Leptinotarsa. 



Special modes of reproduction 



The eggs of some insects are retained within the genital tract of the 

 mother until development is well advanced. In Cimex, in which im- 

 pregnation takes the form of the introduction of sperm into the body 

 cavity by way of the so-called 'organ of Ribaga', fertilization takes 



