10 THE BIOLOGY OF INSECTS 



over the receptive organ should be exceedingly thin or 

 perforated ; such sensory " pegs " or " pits " are found 

 in large numbers on feelers and palps. Thus an insect's 

 skeletal and protective cuticle is so modified as to ensure 

 all needful correspondence with the outer world, while the 

 high development of its central nervous system is correlated 

 with behaviour often apparently intelligent. 



One further aspect of the life of insects remains for 

 discussion in this brief preliminary sketch. All animals 

 reproduce their kind, so that any living creature that may 

 delight us as we watch its purposeful activity reaches always 

 its full development through a process of growth and change 

 from simple beginnings. The details of reproduction and 

 growth among insects are of exceptional interest to the 

 student. The two kinds of germ-cells or gametes — the 

 small, motile sperms and the large passive eggs (Fig. 3) are 

 developed, as is most often the case among animals, in two 

 sets of individuals known respectively as males and females, 

 which often present striking difference in their outward 

 aspect and mode of behaviour. Many male insects when 

 compared with their females afford examples of smaller 

 size together with more highly elaborated sense-organs, 

 brighter colours and greater activity ; it is not unusual, for 

 example, to find flying male insects that have wingless 

 mates, or chirping males that have silent females. The 

 eggs of insects are relatively large with a liberal supply of 

 food-material or yolk (Fig. 3, B, y). Fertilisation of the 

 eggs may not follow immediately on pairing, as the sperms 

 are received into a special female sperm-case (spermatheca) 

 to be discharged when required as the eggs are laid. Not 

 a few insects are derived from eggs never fertilised, pro- 

 viding the best known examples among animals of virgin- 

 reproduction (parthenogenesis). 



In the course of their growth, most insects undergo a 

 remarkable process of change. The cuticle wherewith 

 they are clothed, not being formed of living tissue, cannot 

 grow and possesses only a limited capability of stretching ; 

 hence it must, during the insect's growth, be periodically 



