INSECT PARASITISM 439 



lectual beings necessarily set great store, and the hypertrophy of the 

 alimentary and reproductive organs, which, notwithstanding their 

 immense biological significance, have nevertheless been assigned a very 

 inferior place in our scheme of ethical values. But parasites may 

 properly be regarded as more advanced organisms than the predators, 

 for they have not only had a more eventful phylogenetic career, but, 

 during their long history, have learned to use other organisms in a very 

 economical manner as instruments of nutrition. From a consistent 

 biological point of view, therefore, and from one embracing insect as 

 well as vermian and crustacean parasites, it is evident that the peculiar 

 convergent complexions of these organisms should be attributed to 

 specialization. " Degeneration " is properly a pathological term, and 

 parasites, however pathogenic they may be, are, of course, no more 

 pathological or diseased than predatory animals. There is^ome evi- 

 dence to show that the myzostomes have persisted in their modern form 

 since Silurian times, with a conservatism equalled only by that of their 

 Crinoid hosts. If all the generations of these peculiar annelids have 

 been pathological for millions of years, they should long since have 

 disappeared from the waters of the globe, but we find that though many 

 or all of the original species have doubtless become extinct, this was 

 probably due simply to the extinction of their hosts, for nearly every 

 extant species of Crinoid supports at least one species of Myzostoma. 

 Moreover, if we regard parasitic modification as an expression of degen- 

 eration, we must suppose that such forms as the adult Ichneumon are 

 produced by a post-larval regeneration. Apart from adding an unusual 

 meaning to the word "regeneration," this fails to express the actual 

 conditions correctly. The whole ontogeny of such insects is in reality 

 very highly specialized, the adult representing in many particulars as 

 great a departure from the primitive insect type as the larva, albeit in 

 a very different direction. In discussions of this subject I would there- 

 fore substitute the words " parasitic specialization " for such terms as 

 " degeneration " and " degradation/' Together with these, another 

 term, " retrogression," should be avoided, for the reason that the para- 

 sitic modifications of structure to which it is often applied can be more 

 properly attributed to " arrest of development." 



It will be seen from the foregoing discussion that the leading 

 peculiarity of insect parasitism, at least in such groups as the Hymen- 

 optera and Diptera, which are almost the only ones of value in con- 

 trolling noxious insects, is the restriction of the parasitic habit to the 

 sluggish larva and the specialization of the free adult for the purpose 

 of disseminating the species and of placing the coming generation in 

 intimate contact with the host. No one who observes one of our large, 

 graceful Ichneumonids, such as Thalessa lunator, alighting on a tree- 

 trunk and then conveying its greatly attenuated eggs by means of its 

 long hair-like ovipositor through some three inches of hard wood into 



