Page:Popular Science Monthly Volume 79.djvu/444

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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 larvæ.

Another peculiarity of economic importance in the parasitism of Hymenopterous and Dipterous insects is its highly predatory character, for the voracious larvæ of these orders almost invariably kill their hosts.[1] 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 impossible 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 fullgrown or nearly full-grown caterpillars, and her larvæ 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 consider very briefly in the limited space at my disposal, are the following:

1. The occurrence of hypermetamorphosis which is frequently exhibited by parasitic insects often of the most remote taxonomic affinities, such as the Proctotrypids, and certain Chalcidids (Orasema and Perilampus) among the Hymenoptera, Mantispa among the Neuroptera, the whole order of Strepsiptera, and the Meloidæ and Ehipiphoridæ 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

  1. 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.