3G8 MANUAL OF VEGETABLE-GARDEN INSECTS 



are usually evident, closely applied to the body and covered 

 by the pupal skin. When the remarkable internal structural 

 changes in the pupa are complete, the adult winged insect 

 emerges. In this type of development there are four stages, 

 viz., egg, larva (five to six instars), pupa and adult. Butter- 

 flies, moths, beetles and flies have complete metamorphosis. 

 The four stages of an insect with complete metamorphosis 

 are illustrated in Figs. 93 to 97 of the tomato worm. 



The larvjfi of flies are commonly known as maggots ; those of 

 butterflies and moths as caterpillars, and the larvae of beetles 

 as grubs. The pupa of a butterfly or moth is often called a 

 chrysalis. 



Insecticides 



Injuries to vegetable crops by insects may be prevented by 

 various cultural practices, such as clean farming to reduce 

 hibernating shelter and to destroy the weeds and other wild 

 plants on which injurious species breed, and from which they 

 spread to cultivated crops, and by a proper system of rotation 

 in which the same crop is not planted on the same land for a 

 series of years and in which crops that are attacked by the same 

 insects do not succeed each other. In some cases, collecting 

 and destroying crop remnants is of great importance in pre- 

 venting injury in the same or near-by fields the following year. 

 Taken all in all, clean farming in combination with a proper 

 crop rotation is the most important and practical method of 

 preventing loss from insect attacks to vegetables. 



In the case of some crops, cultural practices can usually 

 be relied on to prevent serious insect injury, but with most 

 crops recourse must be had to special applications of materials 

 that either poison the insects or kill them by coming into con- 

 tact with their bodies. Such substances are known as insec- 

 ticides. 



Insecticides are usually divided into two classes, internal 



