No. 637] 



PABASITIS2I 



149 



mary or secondary parasites of the tent-caterpillar, at- 

 tacking a member of tlieir owti family in the latter case. 

 Since then other examples have come to light, but they 

 are by no means common. Another fact which is sig- 

 nificant in connection with secondary parasites is that 

 they are very generally much less particular than pri- 

 mary ones in restricting themselves to a small series of 

 hosts. 



In searching for the origin of secondary parasitism, 

 it is certain that it must be derived from the primary 

 form, since it is naturally dependent upon the latter for 

 its mere existence. The only other possibility appears 

 to be the assumption that the primary' parasites were 

 free-living forms when first parasitized, and that they 

 have since developed parasitic habits of their own. As 

 the secondaries are frequently structurally reduced such 

 a supposition appears still more improbable. 



If, then, secondary parasites are derived from primary 

 ones, what can have caused tliem to desert their free- 

 living hosts ? We have already seen how the restriction 

 of hosts among primary parasites seems to have a physi- 

 ological basis, in that the reaction of the tissues of the 

 host has been shown (Timberlake, '12) to eliminate para- 

 sites not adapted to it. In attacking insects very closely 

 related to themselves parasites should stand a much 

 better chance for successful growth, as the physiological 

 antagonism of all animals toward closely related forms 

 is much less than that toward very different ones. Young 

 larviE of parasitic spoeies should therefore meet with less 

 difficulty in de\-eloi)iiig in the bodies of related forms, 

 and secondary parasitism might arise with little diffi- 

 f'ulty wlion eggs were placed in another parasite rather 

 than ill the body cavity of the free-living host. This ex- 

 planation may account for the prevalent type of hyper- 

 parasitism, but not for cases like that of the Qialcidid 

 Dibrachijs which attacks Hymenoptera and Diptera alike. 

 This may simply be a case of great adaptability in cer- 

 tain species like some mentioned in connection with pri- 



