14 ECONOMIC ENTOMOLOGY. 



ing of the difference between larva and adult, reference is particu- 

 larly made to those forms in which the two stages are totally 

 unlike each other in appearance. Some of them resemble each 

 other throughout their entire life, and here the term "nymph" 

 is often employed instead of larva. 



It remains to say a few words concerning the chitine, which 

 forms so large a part of the outer skeleton or crust. This sub- 

 stance is found in insects and in certain of 

 the crustaceans, and is a flaky, horny, or 

 shell-like material which may become very 

 hard, and is impenetrable to most Hquids 

 not absolutely corrosive in character. The 

 importance of this fact it is necessary to 

 recognize in our selection of insecticides, 

 because nothing that we can apply with any 

 safety to the plant is sufficiently active to eat 

 through or penetrate the outer skin into the 

 insect body except under peculiarly favorable circumstances. 

 None of the oils penetrate it readily, even gasoline, benzine, or 

 kerosene being resisted. Alcohol, chloroform, and ether are also 

 unable to make their way through any but a very thin coating. 



CHAPTER II. 



THE HEAD AND ITS APPENDAGES. 



The head of an insect bears structures of great interest from 

 the economic stand-point, and of these the mouth parts are the 

 most important. In most cases a glance at the structure of the 

 mouth gives a clue to the food habits of the species, and may 

 convict a culprit whose work has been discovered, or determine 

 the character of the remedy to be applied. 



Roughly speaking, two types of mouth structure are found in 

 insects, — the biting or mandibulate, and the sucking or haustel- 

 late ; though there are modifications of both which are somewhat 

 intermediate in character. 



In the typical "mandibulate" or biting mouth, the mandibles 



Fig. 2. 



Section through insect 

 crust, showing layers of 

 chitine at c, the cellular 

 layer or hypodermis at 

 }iy and basal membrane 

 at b. 



