Entomophagous carnivoroids, especially those in which the development of the individual (ontogeny) is effected by its subsistence on the body fluids and the flesh of one host individual, show an extreme diversity in form and habit whereby their embryonic and larval stages are adapted for a highly specialized mode of life.
One of the more interesting of these adaptations is exhibited by biparental species of Hymenoptera in the placement of their haploid and diploid eggs in a particular organ or tissue of the host species. This adaptation may or may not be correlated with sex differentiation in host relations, a reproductive habit characterized either by differing oviposition responses that segregate haploid (male) and diploid (female) eggs to different kinds of hosts or, lacking such responses, by differing developmental mechanisms that segregate first instar males from first instar females.
The available information regarding the occurrence of the obligatory associations between the hymenopterous egg and a specific organ or tissue of the host, their characteristics, and pertinent physiological prerequisites are summarized.
Basically, these associations appear to be dependent on (1) the gravid carnivoroid usually being free of any physiological pressure (or urge) to oviposit despite the presence of eggs ready for deposition in the ovary and/or stored in the oviduct and (2) the limitation of egg deposition to the moment when the tip of the female’s ovipositor makes contact with a host or the spoor of the host. Freedom from oviposition pressure derives from the female’s ability to dispose of her "ripe" ovarian eggs by methods other than deposition, that is, either by storage in enlarged oviducts or by resorption into the bloodstream.