BENEFICIAL INSECTS, PREDACEOUS AND PARASITIC 19 



these cocoons for the eggs of the worms, and therefore destroy 

 some of their best friends. Though some thus spin their cocoons 

 on the outside of the host, others remain inside of the parasitized 

 insect until the adult fly 

 emerges. Thus dead plant- 

 lice may often be found with 

 a large round hole in the ab- 

 domen the only evidence 

 of where one of these para- 

 sites has emerged. For this 

 reason dry, shrunken plant- 

 lice should never be de- 

 stroyed. 



The Chalcis-flies, which 

 comprise another closely re- 

 lated family, are exceedingly 

 minute insects, sometimes 

 not over one one-hundredth FIG. 13. A plant-louse parasite (Aphidius 

 of an inch lontr They are avenaphis), showing above the parasitized 



louse from which it has issued. (Copied 

 generally of a metallic black f rom j B. Smith.) 



color, and the usual veins of 



the wings are almost entirely absent. Many of these flies are 

 parasitic upon plant-lice, while a large number of their larvae live 

 and mature in the eggs of other insects. 



Very similar to the chalcis-flies in their habits of infesting 

 plant-lice and insect eggs are some even smaller insects in fact 

 the smallest known, the largest being rarely over one-twenty-fifth 

 and the smallest only six- or seven- one-thousandths of an inch in 

 length with a correspondingly tremendous .and unpronounceable 

 name, known to science as the Proctotrypidce. 



During the last half century the American farmer has been 

 compelled to contend with an increasing number of insect pests, 

 which now and then have become veritable scourges. Every now 

 and then we hear of communities assembling for prayer and fasting 

 to appease the Almighty, whose wrath has hurled a new insect 

 plague against them, but such a procedure is by no means as com- 



