none Drosophila melanogaster; Drosophila simulans; HAEMOCYTES-; Concentration related to ability to encapsulate hymenopteran parasite; HYMENOPTERAN-PARASITES; Asobara tabida; Host haemocyte concentration related to ability to encapsulate parasite; IMMUNE-LIKE-REACTIONS; Hymenopteran parasite encapsulation; Ability related to haemocyte concentration Asobara tabida; DIPTERAN-HOSTS; Drosophila melanogaster & D. simulans; Host haemocyte concentration related to ability to encapsulate parasite Asobara tabida (Braconidae), a larval parasitoid of Drosophila species, successfully develops in a large proportion of Drosophila melanogaster, while most of larvae of D. simulans, a sibling of D. melanogaster, encapsulate the parasite. Study of the haemograms demonstrated that D. simulans larvae carried 4-5 times more cells in the haemolymph than larvae of D. melanogaster. The same difference in haemocyte number was observed between the reactive D. simulans hosts, which were able to encapsulate the parasitoid egg within 24 h after parasitization, and the unreactive ones, which did not form a capsule within the same period of time. Therefore, the success of the host defense reaction against A. tabida eggs was considered to relate, at least partially, to the number of haemocytes circulating in the haemolymph within a few hours following parasitization. It was also suggested that the encapsulation of A. tabida eggs may be the result of a physiological race between the ability of the host haemocytes to form the capsule, and the aptitude of the parasite to protect against encapsulation.