How they perform the primary functions 



Fig. 29. Eggs of various insects: A, a butterfly; B, the House Fly; C, a 



chalcid; D, a butterfly; E, a gall-midge; F, G, plant-bugs; H, a small fruit 



fly. Greatly magnified. From Imms, 1957 



making it possible for eggs to be fertilised for a considerable time after 

 mating has taken place. The queens of bees and ants, for example, may 

 take part in a mating-flight only once, but go on laying fertile eggs 

 throughout their life. 



Reproduction from unfertiHsed eggs is called parthenogenesis, and 

 occurs now and then in most groups of insects. In the aphids and in the 

 gall- wasps (Hymenoptera; Cynipoidea) it is normal for one or more 

 generations each year to consist of females only, and to be partheno- 

 genetic. This occurs in the Spring, and may be one of the reasons why 

 these insects can increase so rapidly and disastrously in a favourable 

 year. Honey-bees produce females from fertilised eggs, and males 

 (drones) from unfertiUsed ones. 



Egg-laying 



The eggs are formed in ovarioles, or egg-tubes, of which there are 

 normally two bunches, each united at the base into an oviduct (Fig. 16). 

 Eggs form near the tip of each ovariole, and move down as they grow. 

 On the way down they receive their store of food from nutritive cells, 

 but the mechanism by which this occurs varies greatly in different 

 groups of insects, some of which have no nutritive cells at all. 



The eggs pass down the oviduct into a single cavity called the vagina, 

 and here they receive the sperm from the spermatheca, which we have 

 already mentioned. The fertilised egg then passes on and out. In insects 

 that lay eggs in a steady stream as soon as they are fertilised, the vagina 

 is a simple tube. The tsetse fly, which hatches the egg internally and 

 then feeds the larva until it is fully grown before releasing it, has the 

 vagina enlarged into a uterus. 



49 



