ABSTRACTIn a previous paper was described the life-history of Cryptochaetum iceryae parasitic on Icerya purch as i, an Australian species introduced into California. It was shown that the genus is highly specialized for life as a parasite, and that it represents a separate and restricted line of evolution of parasitic habits among insects. The present study concerns Cryptochaetum grandicorne which is probably confined to the Mediterranean region, and which is the only species of the genus known to occur in Europe. The two species show notable differences in structure and life-history although both are highly adapted to a parasitic mode of life. The very minute eggs are laid in the haemocoel of the host. The egg hatches to form a short-lived ‘embryo-larva ‘at first atracheate and showing no trace of external segmentation. Mouth parts are present although the fore-gut is closed, food materials presumably being absorbed by diffusion from the blood of the host. The second stage larva is tracheate but apneustic. Segmentation is complete. The fore-gut is now open but the hindgut remains closed. The food consists of the blood and fat-body of the host. The third stage larva is omnivorous devouring the internal organs of the host indiscriminately and the hind-gut is open. The tracheal system is amphipneustic. A few days after the commencement of the instar the posterior spiracles pierce the skin of the host and establish connexion with the atmospheric air. The puparium is formed within the dead body of the host. Asin Cryptochaetum iceryae the larva is supplied with a pair of long tubular caudal filaments, lobes of the bodywall containing blood and tracheae, which arise from the posterior segment and ramify among the organs of the host. Experiments indicate that they serve to increase the surface area available for respiratory exchange between the larva and the blood of the host. They are also readily permeable to water. Although a large number of eggs may be placed within a single host, only one reaches maturity. The highly specialized ovipositor is described in detail since it appears to be a striking adaptation to a parasitic mode of life, and cannot be derived directly from the rasping ovipositor of the Agromyzidae. In South Italy as in South France there is one generation per year, the life-history of the parasite being closely correlated with that of the host. Special attention is paid to those features in which grandicorne differs from iceryae and the significance of these differences is discussed.