HYMENOPTERA 



253 



The chalcis-flies (Chalcididae) are even smaller, and are usually 

 blackish with strongly metallic reflections of bronze or green, and 

 are readily recognized by the stout bodies and the almost entire ab- 

 sence of wing veins. Some of them are parasitic on various cater- 

 pillars. One species (Pteromalus puparuui] attacks the common 



FIG. 401. Pteromalns pKparntii, a chalcis-fly which parasitizes the cabbage-worm 

 and many other injurious insects. (Greatly enlarged, hair line shows natural size) 



a, male ; , female. (After Chittenden, United States Department of Agriculture) 



cabbage-butterfly caterpillars, from one of whose chrysalids several 

 hundred of the parasitic flies may often be reared, and in some sec- 

 tions entirely prevents the increase of this troublesome garden pest. 

 Many of the species are parasitic in the eggs of insects, while others 

 are the most effective parasites of scale insects. Unfortunately one 

 or two species are injurious to crops, the best-known example being 

 the joint- worm of wheat 

 (Isosoma tritici], whose 

 larva w r orks in the lower 

 stems, causing gall-like 

 swellings of the joints 

 and weakening them so 

 that the grain is blown 

 over, much the same as 

 when affected by the Hes- 

 sian fly. 



Smallest of all the par- 

 asites are the little proc- 



tOtrypids (PrOCtotrypidae), FlG> 402< A c halcis parasite (Chiropachys colon} 



the largest of which are 



of the fruit-tree bark beetle 



