534 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 Orthoptera, 
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 
