312 ENTOMOLOGY 



Hyperparasitism. Not only are primary parasites fre- 

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

 parasitism is 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 (Orgyia) 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 unusual 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 spe- 

 cies 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 un- 

 der certain conditions prove to be a quaternary 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 tertiary para- 

 sitism. Furthermore, it is quite possible that Dibrachys itself 

 is a tertiary parasite, in which event the Asecodes might be- 

 come a parasite of the quaternary order. 



Economic Importance of Parasitism. If a primary para- 

 site is beneficial, its own parasites are indirectly injurious, gen- 

 erally 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 fre- 



