Contagions Diseases of Insects. 315 



as far as I am aware. The following observations on the history 

 of a tremendous outburst, in southern Illinois, of a species of 

 caterpillar, one of the most destructive insects known, and of 

 the means by which this irruption was apparently terminated, 

 will consequently be of considerable interest. 



In April and May, 1883, the extreme southern part of the 

 state, from Cobden southward, was the scene of one of the 

 periodical uprisings of the forest tent caterpillar (Clisiocampa 

 sylvatica), which have doubtless occurred at intervals in that 

 region from time immemorial. Vast numbers of forest trees 

 in the southern counties of Illinois and in the adjacent parts of 

 Missouri and Indiana were as completely defoliated as if mid- 

 winter had suddenly burst upon them in May, and whole 

 orchards of many acres of apple trees were left without a single 

 green leaf. Oak, hickory, the black and sweet gum, and dog- 

 wood were the trees especially selected for destruction in the 

 forest, and the apple on the fruit farms, the foliage of the 

 peach being scarcely touched, even when the trees were covered 

 with the caterpillars. Strawberry fields were likewise vigor- 

 ously attacked, young fields being occasionally nearly eaten 

 up. 



By the 18th May, when my visit there was made, the larvae 

 had nearly all attained their growth and were travelling rest- 

 lessly about by myriads, in every direction, in search of suit- 

 able places for pupation, a few having, in fact, already trans- 

 formed along the tops of fences and under rubbish on the 

 ground. As I walked along the road sides my attention was 

 immediately caught by the great numbers of dead larvae dried 

 against the boards of the fences, usually in a vertical position, 

 and the multitudes apparently in a diseased condition, traveling 

 more or less feebly, or resting motionless with the head down- 

 ward. These larvae were usually flaccid and shrunken pos- 

 teriorly, but not especially discolored. 



It was, unfortunately, impossible for me to make any care- 

 ful examination of the disease at this time, and no other oppor- 

 tunity offered during the season. 



Revisiting this region on the llth July, an assistant found 

 that the moths had all emerged sometime previously, but that 

 from one half to three fourths of the cocoons had never yielded 



