274 



ENTOMOLOGY 



of the Proctotrypidae are parasitic in the eggs of other insects or oi 

 spiders, several sometimes developing in the same egg; others affect 

 odonate nymphs and coleopterous or dipterous larvae, while seven 

 species have been reared from itonidid and cynipid galls, am 

 many proctotrypids are parasites of other parasitic insects in othe] 

 words, are hyper parasites. 



Hyperparasitism. Not only are primary parasites frequently 

 attacked by other, or secondary, parasites, but tertiary parasitism 

 known to occur in a few instances, and there is some reason to believe 

 that even the quaternary type exists among insects, as in the following 

 case. 



The caterpillar of Hemerocampa leucostigma defoliates shade trees 

 in the northeastern United States. An enormous increase of this 

 species in the city of Washington in 1895 was attended by a 

 corresponding increase of parasitic and predaceous species, and this unu- 

 sual opportunity for the study of parasitism was made the most of by 

 Dr. Howard, from whose admirable paper these facts are taken. 



The primary parasites of H. leucostigma numbered 23 species 17 

 Hymenoptera and 6 Diptera; of the hyperparasites (all hymenopterous) 

 13 were secondary, 2 and probably 5 were tertiary, and one of these 

 (Asecodes albitarsis) may under certain conditions prove to be a quater- 

 nary parasite. To illustrate The ichneumon Pimpla inquisitor, an 

 important primary parasite of lepidopterous larvae, lays its eggs in cater- 

 pillars of H. leucostigma; its larvae suck the blood of their host and at 

 length spin their cocoons within the loose cocoon of the Hemerocampa. 

 These cocoons have yielded a well-known secondary parasite, the chalcid 

 Dibrachys boucheanus. Now another chalcid, Asecodes albitarsis, has 

 been seen to issue from a pupa of this Dibrachys, thus establishing terti- 

 ary parasitism. Furthermore, it is quite possible that Dibrachys 

 itself is a tertiary parasite, in which event the Asecodes might become 

 a parasite of the quaternary order. 



Economic Importance of Parasitism. If a primary parasite is 

 beneficial, its own parasites are indirectly injurious, generally speaking; 

 while those of the third and the fourth order are respectively beneficial 

 and injurious. The last two kinds are so rare, however, as to be of no 

 practical importance from an economic standpoint. The first two kinds 

 are of immense economic importance, particularly the primary parasites. 

 " Outbreaks of injurious insects," says Howard, "are frequently stopped 

 as though by magic by the work of insect enemies of the species. Hub- 

 bard found, in 1880, that a minute parasite, Trichogrammapretiosa, alone 



