In Crabro cribrellifer (Packard), the outer edge of the male foretibia is greatly expanded into a thin, flexible plate or shield. Observations on the mating behavior of these unusual wasps describe the use of modified forelegs during mating. A male places his forelegs over the female’s eyes, strokes her antennae with his antennae, and then tries to copulate. Males in a flight cage frequently disrupted mating pairs by dislodging the resident males, showing that the plates are not effective "claspers," contrary to a hypothesis proposed by Darwin. Photographs of Crabro male foretibial plates show intra- and interspecific variation in patterns of light transmitted through the male plates. A tabulation of taxa (Apoidea, including Sphecoidea) with species having expanded forelegs is provided for comparative purposes. Information on emergence phenologies and secondary sex ratios over a 3-yr period is also presented.