PREDACEOUS AND PARASITIC INSECTS 325 



economic entomology, and the imi)ortiince of the subject 

 has been pretty generally recognized, though different 

 workers have assigned quite different vahie or importance to 

 to the subject. 



Predaceous insects are those which attack and devour 

 other insects, possibly quite different kinds of insects, and 

 do it without sacrificing their own activity or independence. 

 Parasitic insects, using this term as it applies more particu- 

 larly to economic entomology, are those which are dependent 

 for a larger or smaller part of their existence upon some 

 particular kind of insect host. Nearly all are internal 

 parasites and in a great majority of cases the entire develop- 

 ment of the individual from the egg to pupa or adult stage 

 is passed within the body of a single individual host. Some- 

 times a large number of individuals will develop within a 

 single host, as in case of the minute ichneumons which 

 parasitize the larger caterpillars. 



It must be noted here that a parasite itself may be para- 

 sitized by another species called a "secondary" parasite 

 which, by reducing the numbers of the parasitic species, 

 would become from the economic stand-point injurious. 

 This, again, in some instances may support still another 

 parasite, a so-called "tertiary" parasite, which by reducing 

 the numbers of the secondary parasite would be detrimental, 

 and a fourth, where such occurs, again assumes the opposite 

 role. It is evident where such a complex condition of para- 

 sites exists that it is practically impossible to adopt any 

 means of encouraging the beneficial or destroying the inju- 

 rious ones, and that this complex system of wheels within 

 wheels must be allowed to work out its own conclusion in the 

 balance of nature. 



For most species there is perhaps but little that can be 

 done in the way of preserving the beneficial parasites or of 

 facilitating their work. We simply allow them to go on un- 

 molested, serving so far as they may, as a natural check upon 

 the injurious species. Many of our destructive insects are 

 insects that without this check would be seriously destruc- 

 tive, the regular attacks of these ])arasites serving very 



