112 INSECTS 



by diseases and weather conditions, and yet, in spite 

 of all these factors, some species escape from their 

 checks almost annually to the greater or less detriment 

 of the plant hosts. And they always maintain them- 

 selves as against all these combined checks even when 

 each exercises its maximum influence. It is easily 

 appreciable, therefore, that when one or two of these 

 factors fail, e.g., weather and diseases, as not infre- 

 quently happens, it demands a great increase on the 

 part of the other checks to prevent the Aphids from 



Fig. 53.— a Syrphid fly. Mesograpta poiita. 



getting out of hand altogether. The Coccinellid beetles 

 have been already referred to in this connection and 

 the larvae of the Syrphid flies make a very good second. 

 The flies lay their eggs in the very midst of the lice and 

 the resulting larva, which is a slug-like creature with- 

 out legs and with a very extensile anterior portion 

 of the body, begins feeding at once on the specimen 

 nearest at hand. This feeding is interesting, for the 

 larva fixes the little hooks that serve it as jaws into 

 the body of the louse, lifts it high in the air, and holds 

 it thus helpless, until the juices have been completely 

 extracted. As each of these slugs feeds almost con- 

 tinuously from ten days to three weeks, it gains grad- 



