Description
Small to large-sized robber flies (10–40 mm) with distinctly different shape of the posterior end of the abdomen in males versus females. Females have a short or long ovipositor that is hairless and short conical to slender wedge-shaped, its color is usually glossy black. Males have a "helicopter tail" with glossy black claspers that are covered in hairs, forming part of a complex genital structure (the combined epandrium and hypandrium) that is clearly larger than the abdominal segments and is oriented diagonally to vertically upwards relative to the main body axis.
Abdominal coloration is usually greyish to brownish in females, versus more contrasting with silvery and/or black segments or patches in males. In both sexes the tip of the genitalia usually extends past the wing tips. Wings are clear or uniformly tinted, with tints varying from transparent brown to dark black. Venation includes a recurrent vein on R1, narrow cell r4, R4 extending roughly parallel to R5 or converging moderately with R5 towards the wing tip.
Biology
As is typical for robber flies, adult Efferia are ambush predators, taking off from a resting position on the ground or on a branch to intercept other flying insects in mid-air. Prey are taken from a wide variety of insect orders: Robert Lavigne's Predator-Prey Database for the family Asilidae [1] has 918 records for Efferia species feeding frequently on Diptera, Homoptera, Hymenoptera, Heteroptera, Coleoptera, Lepidoptera, Orthoptera, as well as a few reports of prey species belonging to Neuroptera, Odonata, Ephemeroptera - and even one record for Araneae (spiders). Other asilids are frequently preyed upon, and cannibalism is common as well.
Efferia species of deserts and grasslands can occur in high abundance, at times even exceeding one individual per square foot. They tend to perch close to the ground and often remain quite immobile - sometimes until they are about to get stepped on. Spotting them is often a matter of first hearing the typically short evasive flight, before seeing where one landed. This is especially true for the males, which emit a characteristic pulsing buzz during the few seconds in flight; this sound tends to be loud and noticeably higher pitched than that of a flying female. [2]
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