110 ZOOLOGY. 



After hatching from the egg, insects pass through a 

 series of changes of form called a metamorphosis. The 

 butterfly passes through four stages: 1, the egg; 2, the 

 caterpillar or larva; 3, the chrysalis or pupa; and, 4, the 

 imago or adult insect. In the grasshopper the perfect or 

 adult insect differs chiefly from the larva in having wings; 

 in such insects the metamorphosis is said to be incomplete; 

 while the butterfly and bee have a complete metamorphosis, 

 the larva or caterpillar being entirely unlike the imago or 

 perfect insect. 



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, inflicted 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 is about $100,000,000. On the other 

 hand, hosts of ichneumon flies and Tachina flies reduce 

 the numbers and usually 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 Hipne- 

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

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

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

 4600 species of A rack nida (spiders, etc.), and 800 species 

 of Myriopoda (millepedes, centipedes, etc.). 



Insects are distributed all over the surface of the earth. 



