Page:Popular Science Monthly Volume 79.djvu/445

Rh hence from the need of an active and inquisitive first larval stage to supply this defect.

2. The prevalence of hyperparasitism. We may distinguish primary, secondary, tertiary, quaternary and even quinary parasites among insects, according to the principle of the "little fleas ad infinitum." The numerical appellations in this series have been restricted to insects parasitic on other insects, although the primary parasites are really secondaries when they attack insects like caterpillars, since these are, of course, plant-parasites.

3. The absence in parasitic insects of hermaphroditism, a phenomenon so prevalent among vermian, crustacean and annelidan parasites. Only one small group of insects is known to be hermaphroditic, namely, the Termitoxeniidae, comprising a few genera of extraordinary flies that live in termite nests.

4. The rare occurrence of heterœcism, or change of host, a phenomenon very prevalent among tapeworms and flukes. It has been developed, however, within apparently very recent times in such groups as the plant-lice and in certain myrmecophilous beetles of the genera Atemeles in Eurasia and Xenodusa in North America.

5. The increasing development of viviparity as seen in such a series of parasites as Hemimerus, which, according to Hansen and Heymons, develops within the ovary of its mother, the larviparous Tachinidæ and Sarcophagidæ, the nymphiparous Hippoboseidæ, Nyeteribidæ and plant-lice, and the Termitoxeniidæ which, according to Wasmann, are born practically as adult insects.

6. The development of polyembryony among the Chalcidid and Proctotrypid Hymenoptera. Owing to the greatly increased reproductivity of these parasites through the formation of dozens or even