A full description is given of the life-history of Exenterus abruptorius, Thb., an important parasite of the pine sawfly, Diprion sertifer, Geoff.The insect is univoltine, like the host. It oviposits on the last stage larva or prepupa and only hatches after the host has spun its cocoon. Only one-quarter of the primary larvae develop immediately, the remainder resting up to two and a-half months during the summer before proceeding with their development. On the completion of feeding the parasite larva spins its cocoon within the host cocoon and enters the prepupal stage, which can be divided into two distinct phases, eonymphal and pronymphal. Hibernation is always in the eonymphal stage. In Sweden 37 per cent. of this species remained in hibernation for two winters, but in Hungary all emerged in the spring following the year of development. Further than this, in Sweden the sexes were in approximately equal proportions; in Hungary females exceeded males by four to one. This may indicate the existence of biological races.Other Ichneumonidae exhibit the prepupal stage, with eonymphal and pronymphal phases, in their development. The typical facies of this is analogous to that in the prepupal stage in Tenthredinidae, in which, however, ecdysis precedes this stage, which is then a distinct instar. It is suggested that an instar is lost in the development of the Ichneumonidae, though the facies of the prepupal stage is retained.
