110 
ZOOLOGY. 
After hatching from the egg, insects pass through a 
series of changes of form called a metamorphosis. Tlie 
butterfly passes tlirough four stages: 1, the egg; 2, the 
caterpilhir or larva; 3, the chrysalis or pupa; and, 4, the 
imago or adult insect. In the grasshopper the perfect or 
iidult insect differs chiefly from the larva in having wings; 
in such insects tlio metamorphosis is said to be incomplete; 
while the butterfly and bee have a complete metamorphosis, 
the larva or cater})illar 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 Eocky Mountain locust, migrating 
eastward, inflicted a loss of $200,000,000 on the farmers of 
the West. In tlie 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 Hyme- 
noptera (bees, wasps, etc.); about 25,000 species of Lepi- 
doptera (butterflies s'.nd moths); about 26,000 Diptera (two- 
winged flies), and 90,000 ColeojJtera (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. 
