182 FARM FRIENDS AND FARM FOES 



also, because they can multiply rapidly when their caterpil- 

 lar hosts are numerous. And so they tend to check the 

 caterpillar outbreak. But when the primary parasite is 

 abundant, the secondary parasite can also multiply rapidly. 

 So these will tend to check the numbers of the primary 

 parasite and thus relieve, so to speak, the pressure upon the 

 caterpillars. The case will also be complicated by the 

 many examples of superparasitism. Consequently there is 

 a constant fluctuation in the numbers of these different in- 

 sects each one now abundant, then scarce with a ten- 

 dency toward a normal level. This complicated condition 

 is often spoken of as the Balance of Nature. 



THE CHALCID FLIES 



The ichneumon flies are not the only important group of 

 four-winged parasites. From an economic point of view 

 the great family of Chalcid Flies is perhaps of as great 

 value, although on account of their smaller size they are 

 not so well known. It is believed, however, by Dr. L. O. 

 Howard, our foremost authority upon these insects, that 

 there are more species in this family than in any other of 

 the great order of four-winged flies. 



The Chalcids are very small four-winged flies that develop 

 in the earlier stages of other insects. They are especially 

 likely to infest the eggs and larvae of butterflies, moths, 

 beetles, and sawflies, as well as the larvae of great numbers 

 of two and four-winged gallflies, ichneumon flies, wasps, bees, 

 and two-winged flies. They are also very destructive to 

 scale insects and sometimes develop in plant lice and other 

 families of true bugs, as well as in the eggs of tree crickets 

 and other members of the order Orthoptera. 



In the case of many Chalcid Flies the life history seems 

 to be as simple as in that of the more familiar ichneumons. 



