INSECTS 109 



the insect to crawl out, clothed in a new skin which stretches 

 for a short time. This new skin finally hardens again, and 

 the process must be repeated. After several such moults, 

 the insect reaches the adult stage and never passes through a 

 quiescent or a pupa stage, but always resembles the parent. 



Control Measures for Insects. — Insecticides are used for the 

 control or the prevention of insect injury. An insecticide 

 may be defined as a chemical, either liquid or powder, that 

 is used for the killing or the repelling of insects. An insecti- 

 cide should be applied to the plant before any great amount 

 of damage is done. 



Insecticides are divided into two classes, according to the 

 way in which they control the insect, and based upon the 

 manner in which the insect does its feeding upon the plant. 

 When the insect devours or eats up the plant it is called a 

 chewing or biting insect. It is apparent that if some poison 

 is deposited upon the plant where such insects are feeding, 

 it will kill the insect enemy. In such case the insect is 

 destroyed by the direct effect of the poison acting through 

 its digestive tract. The great majority of the insecticides 

 used for this purpose contain arsenic as the active poison, 

 and it is from this source that the name arsenical insecticide 

 has been derived. Paris green and arsenate of lead are the 

 most common arsenical poisons for the biting insects. Com- 

 mon examples of chewing insects are the cabbage worms, 

 the webworms, and all of the caterpillars. 



Besides the eating or the chewing insects there is another 

 large class, known as the sucking insects, that is exceptionally 

 injurious to growing plants. These are the most injurious 

 to the woody plants. Sucking insects cannot be killed by 

 any arsenical spray, because they do not eat the plant 

 tissue, and consequently it is a waste of time and a needless 

 expense to apply arsenical sprays of any kind to the plant. 

 Sucking insects, instead of devouring any part of the plant, 

 insert their sharp mouth parts, which are constructed in the 

 form of a long, narrow, cylindrical tube, through the plant 

 tissue and suck out the plant juices. An insect that feeds 

 in such a manner cannot be killed by coating the surface of 

 the leaves or the stems with any stomach poison, because 



