How they perform the primary functions 





/ 



^<^ 



/ / 



I \ 



Fig. 32. A caddis-fly. Order Trichoptera:, with hairy wings 



larvae may arise from one egg. A parasitic insect is handicapped by 

 having to find a suitable host in which to lay its eggs, and there must be 

 very many failures : a device such as polyembryony may perhaps help 

 to restore the balance of numbers. 



Nymphs and Larvae 



These names have been explained earlier in this work, along with 

 the process of metamorphosis by successive moults. Each of the periods 

 between moults is called a stadium, and the insect itself during a given 

 stadium is called an instar: ' ist instar nymph', '3rd instar larva' and so 

 on. 



The number of instars varies from two to more than fifty, and 

 there is no clear rule as to which insects need most. Generally all in- 

 sects of one species have the same number of instars, but even this can 

 vary, especially under bad living conditions. 



There is not much to say about nymphs, except in the aquatic groups 

 like mayflies (Ephemeroptera) and dragonflies (Odonata). Here 

 the problems of living under water have been met by a number of special 

 structures such as the gills of various kinds (Figs. 23, 24), which we have 

 discussed earlier in this chapter. 



53 



