704 REPORT UNITED STATES GEOLOGICAL SURVEY. 



five worms hatch, probably only three or fbur perpetuate their kind ; 

 and so on throughout the insect-world. The struggle for existence is 

 so great, each species suffering from adverse climatic causes and insect- 

 enemies, that but a small proportion survive the perils of infancy and child- 

 hood, so to speak. Were it not so, the world would be overrun with 

 prepotent animals and i)lauts. The increase and great abundance of 

 the few species are an indication of the intense struggle for existence 

 by which the many alone maintain their livelihood. 



Remedies. — If lands are burned over in the dead of the year where 

 these eggs or j)upoe or moths abound, which is the best remedy we can 

 apply to keep off or kill off" this moth, the fire will certainly kill the 

 chrysalids just below the roots of the grass, as it surely will the eggs 

 on the stalks or the moths nestling among them. Tracts of land in 

 Maine thus burned over in the spring of 1861 escaped the army- worm in 

 the summer, while farms near by suffered from the incursions of worms 

 from the unburned grass-lands around. 



Ditching, or making a deep trench with steep or undermining sides, 

 especially efiScacious in sandy soils, will do much toward keeping them 

 out of fields of grain. People have also laid tar in the bottom of 

 ditches, laid trains of guano, and made bonfires in them. By turning 

 fowl and hogs into fields just as the caterpillar is going into the earth 

 to pupate, great numbers can be destroyed, and the hogs and hens will 

 grow fat on them. 



Enemies. — That birds of different kinds feed on these caterpillars has 

 been noticed. There are also night-birds that catch the moths as they 

 fly. Both the larva and moth are exposed on every hand to the attacks 

 of other insects, such as the dragon-flies, which are continually on the 

 wing, especially over low lands. A large purple beetle with rows of 

 golden spots on its wing-covers, the Calosoma calidum, which is very 

 common in grass-lands, either running about after their prey, or lying 

 on the watch in their holes among the grass, makes great havoc among 

 the army-wormi, and not only the beetle, but its larva, which is more 

 voracious, if possible. 



Ants are known to destroy the army-worm. I am indebted to Mr. H. 

 I. Hershs, of Richmond, Ind., for the following instance: *'Jn June, 

 1875, the army-worms took possession of a grass-plot near my study- 

 window, and for a time threatened to strip it of every vestige of green ; 

 but I noticed a few days after they made their appearance that a large 

 number of small- black ants were waging a war of extermination against 

 them, which, in conjunction with the unusually wet weather, soon put 

 a stop to their depredations." 



But undoubtedly the grand check that nature has imposed upon the 

 too great increase of caterpillars are their parasites, or those ichneu- 

 mon-flies belonging to the great order Hymenoi)tera, and two species 

 of Diptera, or true flies, which lay their eggs on the outside of the 

 caterpillar. The young hatching out feeds on the fatty tissues of the 

 caterpillar, w-hich lives just along time enough for the parasite 

 within to come to maturity. The larger ichneumons only live singly in 

 the body of the caterpillar, while as many as a hundred of the minute 

 species have been seen to emerge from the dead larva-skin, their cocoons 

 placed side by side within. 



We first notice a large species which Mr. Shurtleff raised from the 

 arnjy-worm between the first and middle of September. 



Opkion piirgatits. Say. This genus of ichneumons has a slender 

 body, with long filiform antennae. The thorax above oval, and as wide 

 as the head. The legs are long and slender j but the most apparent 



