A detailed account of the biology and development of an important parasite of cockroaches, the Evaniid, Evania appendigaster (L.), is given. Adult and immature stages of the parasite were obtained and studied from collections of oöthecae of Periplaneta americana (L.) made in Saudi Arabia, some 25–29 per cent, of which were parasitised. There are at least three or four generations a year, a rate of multiplication which gives the parasite a very decided advantage over its much slower-breeding host (P. americana takes at least a year or more to pass through one generation).

The Evaniids are a neglected group of parasitic Hymenoptera and so far very little has been published about them.

Adult parasites emerged from the material collected in Saudi Arabia in or about February, May and October. Oviposition in this species is carried out in a rather peculiar manner. The female lies on her side and, with legs braced against the oötheca, penetrates the tough integument of the egg-capsule after about half-an-hour's hard labour. Only one egg (hymenopteriform type) is laid in an oötheca, and the larva which develops from it is solitary in habit, and completely devours all the eggs in the egg-capsule of its host. The number of larval instars was determined by an examination of the cast skins in the residual material of parasitised oöthecae, a task rendered easier by the very distinctive mandibles of the various stages. Altogether there are five separate larval instars, the first easily identified by the serried arrangement of small denticles on the mandibles. The mandibles of the next two stages are tridentate and shaped like a gauntlet glove, while those of the last two instars are sub-triangular and bidentate. The mature larva is described in detail, and recognition characters for eggs, larval instars and adults are provided.

An appraisement of the value of the two important parasites, Tetrastichus hagenowii (Ratz.) and E. appendigaster, in the biological control of cockroaches is made. It is pointed out that T. hagenowii in certain areas destroys from 15–57 per cent, of the eggs of its host and E. appendigaster 25–29 per cent. Both the former, with up to six generations a year, and the latter, with three or more, multiply much more rapidly than their host. Between them the two parasites appear to be capable of destroying up to 50 per cent, of the cockroach population.

The host records and general distribution of five species of Evania, one each of the related genera Prosevania, Zeuxevania and Brachygaster, and two of Hyptia, all of which parasitise the oöthecae of cockroaches, are fully documented. E. appendigaster itself has been recorded from the main species of cockroach, P. americana, P. australasiae (F.), Blatta orientalis L., Cutilia soror (Brunner) and Neostylopyga rhombifolia (Stoll), and its distribution ranges from Europe (Hungary) and the Middle East (Palestine) to the Pacific (Hawaii and Fiji), America (Gulf and Atlantic States), and the West Indies (Jamaica).