THE HESSIAN FLY. 55 



the most remarkable feature in the economy of this insect. A 

 somewhat similar provision is noticed in one of its kindred, the 

 wheat midge, which will be referred to hereafter. We see at 

 the close of autumn the larva preparing for its long dormant 

 winter state, not by changing its position, and seeking security 

 from wet, or frost, or drought, (for dry air is common in the 

 winter months), nor by spinning a cocoon, in which similar pro- 

 tection may be secured, but by shrinking within itself, and allow- 

 ing its outer skin to form a hard and impervious protecting shield 

 to its tender body, which remains soft and pliant within, and, as 

 far as we know, safe from all ordinary atmospheric changes. 



90. The resistance of insects to the influence of intense cold 

 has long been known, but the source of the heat which enables 

 them to preserve their flexibility within their pupa cases during 

 the greatest extremes of temperature, still remains a mystery. 

 Dr. Wyman lately stated, at a meeting of the Boston Natural 

 History Society, that he had examined chrysalids of the com- 

 mon mud wasp, a species of pelopceus, and found that they were 

 not frozen during the coldest weather. 



91. On the morning of February 7th, 1855, when the ther- 

 mometer had fallen as low as 18 deg. Fah., or 50 degrees below 

 the freezing point, and had risen to 8 deg. Fah., the chrysalids 

 were still unfrozen, and when removed from their pupa cases 

 made obvious muscular motions. The pupa preserved its usual 

 transparency and flexibility ; but when crushed upon the surface 

 of the material on which they rested, the fluids of the body in- 

 stantly became opaque, and were congealed. Dr. Wyman has 

 also examined the eggs of the moth of the canker worm, and 

 found their contents unfrozen. 



SECOND GENERATION OF THE HESSIAN FLY. 



92. The following concise history of the second generation of 



