Spiroacetals such as E-7-methyl-1,6-dioxaspiro[4,5]decane (trans-conophthorin; tC) and acetamides [predominantly N-(3-methylbutyl)acetamide; N3MBA], are two major groups of volatiles discovered in venoms of many Vespidae. In the course of testing the attractiveness of tC and N3MBA to Vespidae using Rescue® Wasp TrapStiks, a significant number of female milichiids, Desmometopa nearctica Sabrosky and D. sordida (Fallén) (Diptera: Milichiidae) were trapped as well. However, the attraction of vespid wasps was not significant at the dosages tested. We found a significant synergistic effect of tC and N3MBA in attracting Desmometopa flies. Both D. nearctica and D. sordida are kleptoparasitic species; and we conclude that females of these two milichiid flies use tC and N3MBA (and likely other volatiles) released from venom glands of the social vespids (yellowjackets, paper wasps and hornets) as kairomones to locate disturbed, injured, or freshly killed insects (vespids and/or their prey) as a protein-rich food source for egg development and production.