The following morphological structures of the ovipositor of Homolobus truncator (Say) (Hymenoptera : Braconidae) are described and hypotheses of their functions are proposed: a series of sharp ridges on the distal surface of the notch helps maintain a grip on the inner surface of the host cuticle; the sperone directs eggs away from the inner surface of the ventral valves; a flap-like structure on each ventral valve covers the portal through which eggs pass; the valvillus maintains position of the egg within the ovipositor and acts against the egg to force it out; ctenidia on the inner surface of the ventral valves move eggs along the basal half of the egg canal; recurved barbs at the apex of each ventral valve hook into the inner surface of the host cuticle to maintain purchase while the thick dorsal valve is inserted into the host. The tribe Maxfischeriini (Hymenoptera : Braconidae) is emended to subfamily status based on morphological and biological evidence. A novel egg morphology is described for Maxfischeria, representing a new life history strategy among Braconidae. Based on egg and ovipositor morphology, I suggest that Maxfischeria is a proovigenic, koinobiont ectoparasitoid. Five new species of Maxfischeria are described (M. ameliae sp. nov., M. anic sp. nov. M. briggsi sp. nov., M. folkertsorum sp. nov., and M. ovumancora sp. nov.). A phylogenetic analysis of morphological and molecular characters for the braconid subfamily Euphorinae is presented. The results imply a revised classification that recognizes 9 tribes and 44 genera. Proposed changes include: Meteorus and Zele are recognized as Meteorinae. Planitorus and Mannokeraia are included among Euphorinae and comprise the tribe Planitorini. Cosmophorini, Euphorini, Helorimorphini, Perilitini, Leiophron, and Perilitus are redefined. The following synonyms are proposed: Cryptoxilonini and Dinocampini with Cosmophorini; Myiocephalini and Proclithrophorini with Perilitini; Myiocephalus with Microctonus; Bracteodes, Falcosyntretus, Sculptosyntretus, Syntretellus, Syntretomorpha, and Syntretoriana with Syntretus and are recognized as subgenera; Perilitus (Townesilitus) with Microctonus and are recognized as a subgenus. Transitions in host associations are examined with ancestral state reconstruction. Some ambiguous nodes in the reconstruction are reconciled by examining the overlap in host associations.

Parasitic wasps constitute one of the last remaining frontiers in the charting of animal diversity. The Braconidae is the second most species-rich family of parasitic wasps; the world fauna has been estimated at 40 000 species and the Swedish fauna is believed to include a little more than 2 000 species, 1 200 of which are currently documented. This thesis is a contribution to the rapidly increasing knowledge of braconid diversity. In paper I, a new gregarious parasitoid, Meteorus acerbiavorus sp. nov. (Braconidae: Eupohrinae), is described from specimens reared from the cocoons of the butterfly Acerbia alpina (Quensel) (Lepidoptera, Arctiidae) in northwestern Finnish Lapland. Based on a molecular phylogenetic analysis, the new species is shown to belong to the M. rubens species group. In the second paper, the Western Palearctic fauna of the tribe is revised, seven new species are described and a key to the Western Palearctic species is presented. Two molecular markers, 28S and COI, are used to study phylogenetic relationships in the tribe. The molecular results showed that the Meteorini fall into four well supported clades. The results also reveal a considerable cryptic species diversity. The third paper deals with distributional, phenological and in many cases rearing data from nearly 2 500 specimens (44 species) of the Meteorini in the collection of the National Museums of Scotland (NMS), Edinburgh. Patterns in the breadth of host ranges are discussed in relation to a reiterated speciation hypothesis. Paper IV examines the phylogenetic relationships of the entire subfamily Euphorinae based upon four gene regions (18S, CAD, 28S D2, and COI). A revised classification of the Euphorinae is proposed that recognizes 55 genera and 14 tribes. Our study shows that early members of the Euphorinae were parasitoids of coleopteran larvae, with a host shift to larval Lepidoptera, adult or immature hosts in the Hemiptera, Hymenoptera, Neuroptera, Orthoptera and Psocoptera. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: In press. Paper 4: Manuscript.</p>