The family Acroceridae (Insecta: Diptera; "Small Headed Flies") are a seldom seen yet cosmopolitan group of endoparasitoids of spiders. Recent host and distribution records are presented here for six species of acrocerids: Ogcodes borealis Cole, 1919; Ogcodes pallidipennis (Loew, 1866); Opcodes sp.; Acrocera bimaculata Loew, 1866; Turbopsebius sulphuripes (Loew, 1869); and Exetasis eickstedtae Schlinger, 1972. New hosts for each fly species are; O. borealis—Schizocosa rovneri Uetz and Dondale, 1979, Pardosa spp.; O. pallidipennis—Schizocosa rovneri, Schizocosa spp.; OgcodesSp.—Anyphaena californica (Banks, 1904); Acrocera bimaculata—Coras montanus (Emerton, 1890b); T. sulphuripes—C. montanus.Detailed field measurements and behavioral observations of host spiders and fly development are described and compared with known data. Examination of these comparisons suggests that host–parasitoid relationships follow spider guild associations (i.e. ground/surface dwelling hosts or those building webs in close contact with surfaces), especially with the spider family Agelenidae. These affiliations probably result from a combination of the spider’s web building, web maintenance, hunting behaviors, and fly oviposition activities, which dispose spiders exhibiting these behaviors to greater chances for parasitoidism. These factors act in concert to increase probabilities for host–parasitoid interactions. Compiled data indicate duration of pupation may be related to ambient temperature. Evidence is presented that acrocerid larvae may alter their hosts’ behavior to increase the parasitoids’ probability of survival.