PARASITISM. 



Charles H. Gable, Lincoln. 



The cultural methods and the employment of poisonous sprays aa 

 means of fighting injurious insects are well and generally understood, 

 but few appreciate, however, the role which natural enemies play in 

 holding in check the increase of our insect pests. It is the object of 

 this paper not only to give some of the principles of parasitism but to 

 give a discussion of its extent and efficiency as well as a brief description 

 of the groups of insects involved. 



We will not consider predaceous insects, parasites of mammals, nor 

 the external or exo-parasites which the more popular definition might ex- 

 clude. But as here used, "parasitism" denotes the relationship which 

 exists between certain insects, called "parasites," which live, during 

 some stage of their developement, within the bodies or eggs of other 

 insects or "hosts." 



The lives of most insects are divided Into four distinct stages or 

 periods: the egg stage; the larval or "worm" stage during which they 

 do all of their growing; the pupa or resting stage during which the in- 

 sects remain inactive, take no food, and are undergoing the transforma- 

 tion from the larval to the last or adult stage. The grasshoppers, katy- 

 dids, the true bugs such as "stink-bugs," and some other groups do not 

 have the pupal stage but remain active throughout life, reaching maturity 

 by a steady process of growth. Parasitism is not confined to any one of 

 these stages. Some parasites attack the egg, others the larva, while 

 still others choose the pupa or adult as hosts. It is to be noted, however, 

 that parasites which attack the eggs never molest the larvae. With few 

 exceptions the work of each kind of parasite is confined to a particular 

 stage of the host. 



The family of insects called Proctotruyidae contains the largest ma- 

 jority of the egg parasites. They are tiny creatures, and of some it would 

 take 160 placed end to end to make an inc'h in length. They may be 

 funished with four wings though some of them have no wings at all. 

 Their color is usually a dead black, not at all shining. The most con- 

 spicuous character by which the females may be known is their oviposi- 

 tor. This is a sting-like projection which extends from the very tip of 

 the abdomen instead of the back a little on the under side, as is the case 

 in other groups. With this ovipositor the parasite drills through the 

 shell of the egg and deposits its own egg in the midst of the rich food 

 supply. The egg of the patasite soon hatches and the little worm feasts 

 on the contents of the egg. By the time the food is exhausted the worm 

 is full grown and enters the pupa stage within the egg-shell. At the com- 



