ORDER IIYMENOPTERA 301 



many other forms. They have broad liearls, rather thick 

 jaws and are quite generally of brilliant metallic colors. A 

 distinctive character is found in the reduction in the veins 

 in the wings so that only the costal vein borders the front 

 margin of the wing and is so developed as to be rigid. In 

 larger insects a great number of these parasites may develop 

 in a single individual. In the smaller species usually a 

 single parasite is found in the host. They are especially 



Fig. 244. — Isosomn triiici: adult of the joint worm — much enlarged. 

 Howard, Div. Ent., U. S. Dept. Ag.) 



effective in the destruction of scale insects, and some of the 

 species are parasitic in the eggs of other insects. In some 

 cases they become secondary or tertiary parasites in which 

 case their value is changed. 



Pteromalus puparum, one of the parasites of the cabbage 

 worm, is apparently distributed well over the area occupied 

 by this species and its attacks upon the host form serve as 

 one of the quite constant checks upon the abundance of that 

 species. The parasitized pupix? usually show a different 



