The Ichneumon Flies 



and eight-tenths mm. The duration of the egg state must be 

 very short and is probably only a matter of but a few hours. 

 After hatching the Pimpla larvae when at work upon a just spun 

 up tussock moth caterpillar feed externally upon the body of the 

 caterpillar, the mouth-parts closely applied to the skin and in fact 

 obviously sucking blood through a minute orifice. Their growth 

 is rapid and there is no perceptible casting of the skin. In mid- 

 summer the larvae will become full grown in four days, when they 

 are nearly ten mm. long, by three mm. in greatest diameter, fusi- 

 form in shape, and slightly curved. The color is yellowish white. 

 They soon begin spinning cocoons. These cocoons, at first white 

 and afterward turning gradually to a pale yellow brown, becoming 

 much the same color as the cocoon of the tussock moth, are 

 denser in structure and are composed of a considerably finer 

 quality of silk. They are long oval in shape and are closely 

 applied together adhering so firmly that it takes some little force 

 to separate them. They are applied side by side and so closely 

 that their oval outline becomes more or less angular at the point 

 of application. Two days after the spinning of the cocoon in 

 midsummer the larva changes to pupa and adults issue some- 

 times as early as six days later, making the entire life round of the 

 species about fifteen days. 



It sometimes happens that a tussock moth caterpillar is stung 

 when it is just on the point of transforming, but in such a case the 

 transformation to pupa is occasionally accomplished. The re- 

 cently formed pupa is also occasionally stung. Contrary to the 

 general rule holding when caterpillars are eaten out by the Pimpla 

 larvae the latter seem to feed within such pupae and evidently to 

 spin their cocoons within the caterpillar's pupa skin so that one 

 will frequently find an apparently perfect pupa of the tussock 

 moth within its cocoon which, however, contains four or five 

 cocoons of the Pimpla packed close together and completely 

 filling it. 



The number of Pimpla larvae nourished by a single tussock 

 moth caterpillar varies from one to ten, with perhaps an average 

 of three or four. This is the case in the summer time, but in 

 the autumn more are found. Ten or fifteen in a single cocoon 

 are not unusual at this time of the year, while in one case the 

 writer has seen twenty-three male Pimpla cocoons in a single 

 cocoon of the tussock moth. This particular cocoon mass was 



66 



