The egg-parasite ofSialis lutaria: a study of the influence of the host upon a dimorphic parasite

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Abstract
1. Nearly a quarter of a million eggs ofSialis lutariawere collected at Cambridge in 1936. About 0·6 per cent of them were attacked by a parasite.2. The egg-parasite ofSialisis distinct fromTrichogramma evanescens, and is to be calledT. semblidis(Aurivillius).3. The male ofTrichogramma semblidisoccurs in two forms. Neither consists merely of imperfect or degenerate individuals of the other, for the two forms are equally large and differ constantly and fundamentally in several characters. The species, therefore, exhibits true dimorphism.4. Rearing experiments involving isolated pure lines show that it is principally the host that determines which form of the parasite shall emerge. Males reared onSialisare of the apterous form; those reared on three species of Lepidoptera are of the winged form.5. The dimorphism ofT. semblidisis discussed in relation to other examples of dimorphism in the Hymenoptera. It is shown to have several features of special interest.

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