96 ASSOCIATION OF ECONOMIC ENTOMOLOGISTS. 



though all of my assistants haA T e repeatedly observed the female 

 Polygnotus ovipositing in the eggs of the Hessian fly, and Mr. G. I. 

 Reeves has found larvae to the number of 28 in the matured maggot. 

 We have thus observed the oviposition in eggs of the fly and reared 

 adults from the " flaxseeds." If Doctor Marchal's suppositions are 

 correct, and, as is very probable, our American species have similar 

 habits, their economic importance will be immense, for it will be pos- 

 sible for a single female Polygnotus to place her supply of eggs 

 singly in those of the Hessian fly, and the number of her progeny 

 would be limited only by the sustaining capacity of the host larva?. 



Another parasite with polyembryonic development is Platyg aster 

 herrickii Pack. Mr. Reeves has observed this also, ovipositing in the 

 eggs of the Hessian fly, and at another time, in a locality several 

 hundred miles distant, he counted over 10 larva? in a single maggot. 



Whether or not there are other species of Platygaster infesting the 

 Hessian fly it is just now impossible to state, but there are certainly 

 more than one species of Polygnotus engaged in this efficient work, 

 and the saving to the wheat growers of North America through their 

 influences is immense. 



The whole problem of polyembryony among parasitic insects is 

 exceedingly interesting and of the utmost economic importance. 

 When Doctor Howard called attention to the enormous numbers oi 

 Copidosoma that he had reared from a larva of Plusia brassicce h 

 1882, the number from a single individual, of 19 larva?, being 2,528, 

 he was unable to learn by dissection that the female Copidosoma was 

 capable of laying more than 160 eggs. Later, in Ohio, I reared froi 

 a lot of 5 larva? of this species the enormous number of over 4,80( 

 and since that time Mr. C. W. Mally, in South Africa, reared 2,11' 

 from a Plusia larva, while Monsieur E. Bugnion has reared over 3,00( 

 individuals from a single host larva. Doctor Howard first thought 

 that more than one female Copidosoma had oviposited in a single 

 host larva, but this has since been found to be exceedingly unlikely. 

 In the case of some of our American species of Polygnotus an< 

 Platygaster herrickii the female parasite positively refuses to ovi- 

 posit in eggs that have already been visited by another female. 

 Attention has also been called to the almost general phenomenon oi 

 all individuals from a single larva being of the same sex, all of whicl 

 goes to imply that but a single ovum is placed in a single host, am 

 the extent to which the parasitic larva? will multiply from this is 

 only limited by the size of the host larva. Thus we have an expla- 

 nation of what has been a puzzle to entomologists, viz, the suddei 

 and almost complete disappearance of the Hessian fly between broods 

 It also indicates the great value of these insects for use in introducing 

 parasites in sections where an outbreak of the host insect is seem- 

 ingly impending. How many of our parasites have this polyembry- 





