162 



Elementary Principles of Agriculture 



other groups of insects by the way they get their food 

 from the plant or animal. Instead of having jaws with 

 which they may bite off and chew their food, their 

 mouth parts are shaped into a kind of tube which they 

 use to suck blood or sap, nectar or viscid matter. The 

 squash-bug (Fig. 99) and the chinch-bug get their food 

 by sucking. Plant lice, such as the green bug, and San 

 Jose scale (Fig. 100) are also sucking insects. 



m 



Fig. 100. San Jose scale on plum. A, natural size; 6, magnified; 

 c, greatly magnified. 



Insects should not be classed as "biting insects" and 

 "sucking insects" because some species have biting 

 mouth parts at one stage of their life cycle and sucking 

 mouth parts at another. The caterpillars gnaw or bite 

 their food, while the parent moths or butterflies have a 

 sucking tongue. Some kinds with sucking mouth parts 

 are comparatively free, their host and habitat being 

 often unknown. Many kinds, however, have developed 

 fixed parasitic habits. Most of the bloodthirsty pests 

 belong here, such as horse and cattle flies, the mosquitos 

 and the common bed-bug. The sucking insects are usu- 

 ally external feeders. Exceptions are noted in the case 

 of the horse bot and the cattle warble. 



230a. Structure of Insects. For this exercise the pupil should 

 secure good specimens of the grasshopper and butterfly, as these 



