334 ZOOLOGY. 



seen, among the lower branches of the animal kingdom. 

 The object or design in nature, at least in the case of the 

 plant-lice and bark-lice, as well as the gall-flies, is the pro- 

 duction of large numbers of individuals, by which the per- 

 petuity of the species is maintained. 



Insects are both useful and injurious to vegetation. "Were 

 it not for certain bees and moths, orchids and many other 

 plants would not be fertilized ; insects also assist in the 

 cross-fertilization of plants. For full crops of many of our 

 fruits and vegetables, we are largely indebted to bees, flies, 

 moths, and beetles, which, conveying pollen from flower to 

 flower, ensure the production of abundant seeds and fruits. 

 Mankind, on the other hand, suffers enormous losses from 

 the attacks of injurious insects. Within a period of four 

 years, the Rocky Mountain locust, migrating eastward, in- 

 flicted a loss of $200,000,000 on the farmers of the West. 

 In the year 1864, the losses occasioned by the chinch-bug in 

 the corn and wheat crop of the valley of the Mississippi 

 amounted to upward of $100,000,000. It is estimated that 

 the average annual losses in the United States from insects- 

 are about $100,000,000. On the other hand, hosts of 

 ichneumon flies and Tachina flies reduce the numbers and 

 prevent undue increase in the numbers of injurious insects. 



The number of species of insects in collections is about 

 200,000. Of these there are about 25,000 species of Hyme- 

 noptera (bees, wasps, etc.) ; about 25,000 species of Lepi- 

 doptera (butterflies and moths); about 25,000 Diptera (two- 

 winged flies), and 90,000 Coleoptera (beetles) ; with about 

 4600 species of Arachnida (spiders, etc.), and 800 species 

 of Myriopoda (millepedes, centipedes, etc.) 



Insects are distributed all over the surface of the earth. 

 Most of the species are confined to the warmer portions of 

 the globe, becoming fewer in the number of species as we 

 approach the North Polar regions. Many are inhabitants 

 of fresh water ; a very few inhabit the sea. 



Insects, except a Silurian Hemipter and a Blattid, appear in 

 the Devonian rocks ; these were Neuroptera and Ortlioptera, 

 with representatives of other groups which seem generalized 

 in their structure. But if highly developed flying insects, 

 belonging, at least the May-fly, to existing families, appeared 



