FOOD SX'PPLY 



47 



parasites which may frequently produce a death rate of from 

 50-90 per cent in a single season, the importance of this factor 

 in fostering imported pests is easily realized. 



A parasitic insect of this sort may be defined as one which 

 passes a i)art of its developmental stages (usually the larxal 



Fig. 22. Two species of Parasitic Hymenoptera. Above, Dino- 

 tomus, a common parasite of butterfly caterpillars. Below, Spilo- 

 cryptus; in this as in many other insects of this group, the female 

 has a needle-like ovipositor which she thrusts into her Mctim to 

 deposit her eggs within its body. 



and often the egg or pupal stage also) upon or within the 

 body of another insect or in its eggs, from which it derives 

 its entire food supply and which it almost invariably kills 

 before attaining its full growth. There are a great many 

 species of parasitic insects, and there are probably very few 

 or perhaps almost no predacious or phytophagous insects 

 which are not subject to their attack. Even parasitic insects 

 themselves are quite frequently destroyed by other species 



