250 



INVERTEBRATE ZOOLOGY 



In the classification of Linnaeus, there were but seven order 

 of insects recognized. These have, in recent times, been sub- 

 divided and new orders have been added until today there are 

 more than twenty well-defined orders of the Insecta. In the 

 accompanying table, these orders are listed and some of the 

 characters which are usually considered of importance are 

 furnished for each. 



The Importance of Insects. — Insects have such significance 

 in their relations to human welfare that it is difficult to give even 

 a faint idea of their importance without going into extensive 

 details, for volumes have been written upon this phase of the 

 subject. The honeybees, the silk worms, and the various insects 



Rudiments 

 of legs v,_ 



A B C D 



Fig. 126. — Development and transformation of imaginal discs in head and 

 thorax of A, larva; and B-D, pupa of a fly (Musca). Wing rudiments are omit- 

 ted. {From Korschclt and Heider after VanRees, in part after Kowalevsky) . 



which pollinate flowers are so widely and so popularly known that 

 their importance need not be discussed here. As pests, the 

 Hemiptera occupy a place of extreme significance, with the 

 chinch bug {Blissus leucopterus) as a menace to grain farming 

 and the hundreds of species of scale insects which attack trees 

 and shrubs as outstanding examples. Among the Dipt era, 

 numerous disease-carrying flies and mosquitoes, the Hessian 

 fly which destroys growing grain, and warbles and bots which 

 attack domestic animals demand attention. The gipsy moth 

 in its ravages upon shade trees, the coddling moth in its molesta- 

 tion of the apple industry, and the numerous cutworms are 

 examples of important Lepidoptera. Extensive damage to cotton 

 crops through the boll weevil and much of the injury by wood- 

 boring and leaf-eating insects are attributable to species of the 

 Coleoptera. 



Predaceous insects such as the coccinelid beetles, the dragon 

 flies, and the Neuroptera aid materially in checking the damage 



