INFLUENCE OF TEMPERATURE ON INSECTS. 49 



"In Kruman, in the Bohemian forests, have been 

 counted, during a period of great increase of fir bark- 

 beetles in these forests from 1871 to 1875, in a portion of 

 bark a square meter in extent, from 1400 to 4800 larvae" 

 (Judeich and Nitsche). 



Influence of Changes of Temperature on Insect-life. 

 Perhaps changes of temperature and unfavorable seasons 

 have, next to the increasing competition or struggle for 

 existence among insects, and the attacks of parasites, the 

 greatest effect in maintaining the balance of nature, and 

 preventing the undue increase of destructive insects. Dr. 

 Shimer gives an account of an epidemic among the chinch- 

 bugs in Illinois, which " was at its maximum during the 

 moist warm weather that followed the cold rains of June 

 and the first part of July, 1865." Mr. C. Thomas claims 

 that the high temperature of 1854, 1871, and 1874, to- 

 gether with the diminished rainfall, furnishes the key to 

 the cause of the vast increase of chinch -bugs during those 

 years. * Wet weather is favorable and dry weather is un- 

 favorable to the increase of the cotton-worm. In times of 

 drought the eggs dry and fail to hatch, the worms are en- 

 feebled, " web up" prematurely, and die in attempting to 

 transform into the chrysalis state, and when they succeed 

 the chrysalides decay. " Nourishment and fecundity being 

 correlated, it is more than probable that the moths, poorly 

 nourished, will lay fewer eggs under such circumstances. 

 All the effects described are intensified and become most 

 marked during extreme drought, so that frequently at the 

 end of a dry spell, such as is not infrequent in July and 

 early August, not a worm can be found. A rainy season, 

 following such a spell, will produce a most noticeable 

 change."! 



The Hessian fly flourishes best in seasons when the 

 chinch-bug flourishes least. The hot, dry summer of 1881 



* Amer. Entomologist, iii. 241. 



f Riley, in Fourth Report U. S. Entomological Commission. 84 



