44Q THE POPULAR SCIENCE MONTHLY 



the burrow of a Tremex larva, the presence of which it has been able 

 to detect by means of its marvelously acute sense-organs, can fail to 

 appreciate the advantages of such a method of bringing a parasite to 

 its host, rather than by the tape-worm's shot-gun method of scattering 

 minute eggs about promiscuously, or by the Sacculina's almost equally 

 haphazard method of employing minute, feeble, aquatic larvae. 



Another peculiarity of economic importance in the parasitism of 

 Hymenopterous and Dipterous insects is its highly predatory character, 

 for the voracious larva? of these orders almost invariably kill their hosts. 9 

 Other forms, like the Strepsiptera, which permit their hosts to reach 

 the adult stage, nevertheless destroy their gonads and thus decrease the 

 reproductivity of the host species. In some cases, indeed, it is impos- 

 sible to decide whether we are dealing with parasitism or predatism. 

 The Sphex, that lays her eggs on caterpillars which she has carefully 

 paralyzed, is commonly regarded as a predatory insect, but she is from 

 another point of view, an even more specialized parasite than the 

 Ichneumon. Her sting immobilizes but does not kill the active full- 

 grown or nearly full-grown caterpillars, and her larvae are careful to 

 feed in such a manner as to spare as long as possible the life of their 

 victims. We have here merely a further extension of the maternal 

 instincts primarily devoted exclusively to bringing about the union of 

 the parasite with the host, to a unique and effective preparation of the 

 host's body for easier exploitation by the parasite. 



A third peculiarity of economic importance in the Hymenopterous 

 and Dipterous insects is their pronounced tendency to confine their 

 attacks to species of large, recently developed and eminently noxious 

 groups, such as the Lepidoptera, Coleoptera, Homoptera and other 

 plant-destroying insects. 



There are also a number of peculiarities some of which are of less 

 practical but of no less theoretical interest. These, which I must con- 

 sider very briefly in the limited space at my disposal, are the following : 



1. The occurrence of hypermetamorphosis which is frequently ex- 

 hibited by parasitic insects often of the most remote taxonomic affini- 

 ties, such as the Proctotrypids, and certain Chalcidids (Orasema and 

 Perilampus) among the Hymenoptera, Mantispa among the Neurop- 

 tera, the whole order of Strepsiptera, and the Meloidae and Ehipi- 

 phoridae among Coleoptera. The complication of development arises 

 in all of these cases from an inability of the mother insect to find the 

 host or at any rate to reach it during the proper ontogenetic stage, and 



9 It would seem that the death of the insect host is necessitated either by 

 the relatively very large size of its insect parasite at maturity, when acting 

 alone, or (in cases of polyembryony and simultaneous infestation by several 

 individuals of the same species) to the equally considerable bulk of a number 

 of small parasites acting together. The comparatively slight difference in 

 stature between host and parasite is certainly one of the most remarkable 

 peculiarities of insect parasitism. 



