BENEFICIAL INSECTS, PREDACEOUS AND PARASITIC 17 



white cocoons may frequently be seen almost covering one of our 

 large tomato- or tobacco-worms (see page 220 ), the pupae of which 

 are often known as "horn-blowers." Many mistake 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 abdomen the only evidence of where 

 one of these parasites has emerged. The Chalcis-flies, Chalcididcc, 

 which comprise another closely related family, are often exceed- 

 ingly minute insects, sometimes not over one one-hundredth of 

 an inch long. They are generally of a metallic black 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 larva) live and mature in the eggs of other insects. 



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

 plant-lice and insect eggs 

 are some even smaller 

 insects in fact the small- 

 est 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 cor- 

 respondingly tremendous 

 and unpronounceable name, 

 known to science as the Pro- 

 ctotrypidce. During the last 

 half century the American 

 farmer has been compelled 

 to contend with an increas- 

 ing number of insect pests. FlG - 13. -A plant-louse parade (AphUius 

 & *, ' avenaphis), showing above the parasitized 



which now and then have louse from which it has issued. (Copied 

 become veritable scourges. from J. B. Smith.) 

 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 



