THE TWO-WINGED PARASITES 191 



should be welcomed as an opportunity for depositing an 

 egg and thus fulfilling the chief purpose of her existence. 

 Consequently the investigators were puzzled to find that 

 these flies not only did not so deposit their eggs, but were 

 actually alarmed by the presence of the caterpillars. 



It was easily learned through the study of dead flies that 

 the eggs were large elongated objects, too large to be swal- 

 lowed safely by the intended victims, — safely, that is, for 

 the parasite. Hence it could not be that they were laid 

 upon the leaves to be eaten. If they were not to be de- 

 posited on or in the caterpillars and were not to reach the 

 interior of their hosts by being eaten, how was it to be 

 done .' 



The final solution, may well be given in Dr. Townsend's 

 own words : " After much patient observation and experi- 

 ment this question was answered. The flies were found to 

 deposit living maggots, not on or in the caterpillars, but, 

 most remarkable to relate, on the green shoots, leaf stems, 

 leaf ribs, and even sometimes on the surface of the leaves." 

 This was not a matter of chance, however, for the flies 

 hovered in the air above the leaves where the caterpillars 

 were at work, evidently seeking for favorable places for 

 depositing the larvae. By some sense — perhaps sight, per- 

 haps smell, possibly both — they knew when the caterpil- 

 lars were present and could not be induced to deposit on 

 branches where they were not present or had not been 

 crawling. 



In crawling along, many of these caterpillars of the great 

 group of silk spinners have an interesting habit of marking 

 their trail by a silken thread which is continually spun from 

 the silk glands in the mouth as the larva marches to its 

 feeding grounds. This thread serves to guide it back to 

 its permanent or temporary quarters when its meal is fin- 



