This family is distinct among the Chalcidoidea both morphologically and biologically, and holds the unique position of being the only chalcidoid family monographed at the world level. Boucek (1974a) treated 130 species in 4 genera, giving keys, hosts, and distributions for all taxa. He stated that leucospids are closely related to Chalcididae and have been placed therein in past times as a subfamily. Most of the information discussed under this section is cited from Boucek. Nearctic members of the family were treated most recently by Boucek (in Gibson, et al. 1997).

STATISTICS: Number of world species: about 150 (6 Nearctic); number of world genera: 4 (1 Nearctic).

BIOLOGY: Of the 134 known species, host records have been reported only for 33. Much remains to be learned about the biology of this group. Where biologies are known, all species are ectoparasites of larval aculeate Hymenoptera. While solitary bees form the major group of hosts, leucospids have also been reared from Eumenidae and Sphecidae. A single record is questionably given for Vespula. Females, by virtue of the oddly modified abdomen, would appear to have quite a problem ovipositing. Because the host larva is within a wood or mud cavity, a female may spend anywhere from 10 minutes to 3 hours drilling to reach it (as reported by Clausen 1940). But not until the host cavity is reached, can the leucospid female tell if there is a suitable larva present. The cell may be empty or the host dead. Apparently females will oviposit into cells with dead larvae. If a parasitic bee larva is present (e.g. Stelis or Coelioxys), the leucospid does not discriminate and so may be considered a facultative hyperparasite. One or more eggs are deposited per host cell, but only one larva ultimately survives. The egg is attached to the cell wall of the host cell, or in the host cocoon if one is spun. Of interest is the observation by Boucek (1974a) that most leucospids mimic species of wasps or bees but not the species they parasitize.

DISTINGUISHING CHARACTERS: The species of leucospids are robust, usually wasp-like, and often patterned in colors of red and/or yellow on black. The enlarged, toothed hindfemora and arched hindtibiae readily place this family near the Chalcididae. Females are most easily distinguished by the ovipositor which curves upward and may extend over the dorsum of the metasoma. Males have tergum 2 shorter than tergum 1 and the remaining terga are more or less fused into a shell or carapace (as in the braconid genus Chelonus). The tegula is narrowly elongate and essentially reaches the pronotum. The forewings are densely setose, having the postmarginal vein many times longer than the stigmal, which in turn is at least slightly longer than the marginal, and the forewings are usually folded lengthwise as in vespid wasps.

COLLECTING: Leucospids may be reared by collecting dead woody plant stems with evidence of nesting behavior (i.e. holes, sawdust; see Krombein 1967 for aspects of trap-nesting bees and wasps and their parasites). Adults are most often swept from flowers (especially umbellifers and small flowered composites), where they could be mistaken for eumenid wasps. Leucospids are relatively uncommon, but a few specimens may generally be taken during a day's collecting if it is warm and flowers are in bloom.

DISTRIBUTION: Mostly confined to warm areas of the world.