The Host-Finding and Oviposition Behaviour of Mormoniella Vitripennis (Walker) (Hym., Pteromalidae), a Parasite of Muscoid Flies

Abstract

Mormoniella vitripennis is a chalcid wasp parasitic on pupae of Muscoid flies. In a current of clean air, the females walk upwind, but if the air is contaminated by the odour of liver on which blow-fly larvae have fed, they follow a twisting path and do not leave the contaminated area. This response does not occur if the liver has been decomposed by bacterial action, nor does it occur when the parasites are subjected to the odour of milk pads on which house-fly larvae have fed. The puparia of blow flies and house flies have no attractive odour of their own, but if contaminated with larval food they will elicit the turning response. The optical sense of the parasites is poorly developed and when once they are in the host environment they find the puparia by chance. A puparium is recognized by some intrinsic property which has not been determined. It is probably chemical, but attempts to transfer it to artificial hosts have failed. Roundness is an additional requirement, and large hosts are preferred to small ones. A parasite is unaware of the contents of a puparium until after it has drilled with its ovipositor and probed the interior. If the host is suitable, eggs are laid and a feeding tube is formed. More eggs are laid on large hosts than on small ones. Recognition of contents as unsuitable for oviposition is so rapid that it must be due to chemoreceptors on the ovipositor but oviposition can be induced on a number of abnormal hosts provided they are alive, and placed inside a fly puparium. Eggs are never laid on a dead host.