52 ENTOMOLOGY. 



expect invasions of the northern army-worm {Leucania 

 unipuncta). 



Periodicity in Insect-life.— As there may be a succession 

 of seasons favorable to the development of insect-life, so 

 there may be a corresponding increase in the numbers of 

 insects, until they abound to excess. In this way periodical 

 invasions of locusts happen the world over. A number of 

 successive favorable seasons may result in a greater number 

 of eggs of Leucania hatching, and the caterpillars nearly 

 all arriving at maturity, none dying from bad weather, they 

 abound in extraordinary numbers, and in great armies 

 march through grass-lands and wheat-fields in what seem 

 to us countless numbers. We thus realize how many vicis- 

 situdes await the caterpillars in ordinary seasons, and how 

 few pairs survive. Another striking case is that of the 

 spruce-bud Tortrix (T. fumiferana), which for a number of 

 years destroyed the spruce and firs on the coast of Maine, 

 this species being rare and seldom captured either in the 

 larva or imago stage in other years. 



Number of Species of Insects. — The insects number about 

 four-fifths of the animal kingdom, since it is estimated 

 that there are not less than from 200,000 to 250,000 species 

 in public and private collections. The Coleoptera are the 

 most numerous, there being 100,000 species known, 90,000 

 species at least existing in museums; of Hymenoptera and 

 Lepidoptera as well as Diptera there are not less than 

 25,000 species of each order; of Hemiptera about 27,000 

 species exist in museums, and Uhler supposes that the entire 

 number is nearly 50,000; the species of the smaller orders 

 would easily carry the total number of known species up to 

 200,000. As recently remarked by Dr. Sharp, probably only 

 from a fourth to a tenth of the existing species, of insects 

 are known; and as McLachlan has stated, it is not im- 

 probable that the number of species of insects now living 

 on the earth's surface will be found to be about 1,000,000. 



The number of described species of American insects 



