INSECTS AND INSECTICIDES 



365 



carried through the trachese to the nervous system where 

 they cause death by paralysis. 



Fig. 230. 



A tarnished plant-bug feeding, showing the sucking type of 

 mouth-parts. 



The dcvcloyment 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, a 

 new skin is formed beneath the 

 old one and the latter is dis- 

 carded ; this is known as molt- 

 ing. The period between two 

 successive molts is called an 

 instar. The number of instars 

 varies in different insects from Fig. 231. 

 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 without any material change in form ; in 



Head of the onion mag- 

 got fly, showing the lapping type 

 of mouth-parts. 



