vir] Introduction to Insects 93 
In some insects such as aphides (green fly) and 
cockroaches, the young on hatching resemble the adult 
forms very closely—the difference being chiefly one of 
size. 
The first consideration in dealing with pests 
attacking crops is to find out what insect is causing us 
trouble. Our knowledge of the different stages should 
help us considerably in at least finding out whether 
it is being caused by a butterfly, a fly, or a beetle. 
The time taken by an insect to go through its 
different changes varies according to the species. In 
the case of wireworms and chafer beetles several years 
are required from the laying of an egg until the time 
when the insect formed from this egg commences egg 
laying. In some cases, such as the garden chafer, the 
insects have one brood every year, in others, such as 
the frit fly, several broods in the year, whilst green 
flies may have new broods every week. 
The length of life of any one stage varies according 
to the species as does also the stage in which the 
damage is done. Often the larval stage only is respon- 
sible for this: in other cases it is done both in the larval 
and the adult stage, and occasionally in the adult 
stage only. 
It is not necessary for the farmer to know what 
kind of insect is attacking his crop provided that he 
has sufficient knowledge to keep the pest under control. 
In order to deal successfully with an insect disease it 
is usually necessary for some one to study carefully the 
characteristic features of that insect throughout the 
whole of its life and very often throughout the life of 
some of its descendants. Such knowledge often enables 
us to attack the insect at a time when it is most 
