GENERAL CONSIDERATIONS 5 



tubes and so smother the insect, while others, like the oils, 

 are said to penetrate the thin walls of the tracheae and thus 

 reach a vital part in the internal organs of the insect. 



The development of insects. 



Most insects, with the exception of some scale insects and 

 certain forms of plant lice, reproduce by means of eggs. The 

 newly hatched insect usually bears little resemblance to the 

 adult. As it increases in size its skin becomes too small and 

 a new skin is formed beneath the old one and the latter is dis- 

 carded; this is known as molting. The period between two 

 successive molts is called an instar. The number of instars 

 varies in different insects from three to six or seven ; five is 

 the more common number. In some insects the change from 

 the immature condition to the winged adult takes place with- 

 out any material change in form ; in others the transformation 

 is abrupt and striking. In the former case the insect is said 

 to have an incomplete metamorphosis; in the latter a complete 

 metamorphosis. 



Incomplete metamorphosis. 



In this type of development the immature stages resemble 

 the adult in form. The wings develop externally as pad-like 

 outgrowths of the thorax but do not become functional till the 

 adult stage is reached. The immature forms are known as 

 nymphs. In this type of development the life cycle of the 

 insect consists of three stages, viz. the egg, the nymph (3-5 

 instars) and the adult. The true bugs and grasshoppers have 

 incomplete metamorphosis. 



Complete metamorphosis. 



In this case the immature stages of the insect bear little or 

 no resemblance to the adult. The wings develop internally in 

 pockets formed by an infolding of the body wall of the thorax. 

 The immature stages are known as larvae. The larva molts 

 five or six times, and when full-grown transforms to an in- 

 active pupa, usually in a cocoon or earthen cell prepared by 



