|A female velvet ant.|
The Mutillidae are a family of more than 3,000 species of wasps (despite the names) whose wingless females resemble large, hairy ants. Their common name velvet ant refers to their dense pile of hair which most often is bright scarlet or orange, but may also be black, white, silver, or gold. Their bright colours serve as aposematic signals. They are known for their extremely painful stings, hence the common name cow killer or cow ant. Unlike a real ant, they do not have drones, workers, and queens. However, velvet ants do exhibit haplodiploid sex determination similar to other members of Vespoidea.
The exoskeleton of all velvet ants is unusually tough (to the point that some entomologists have reported difficulty piercing them with steel pins when attempting to mount them for display in cabinets). This characteristic allows them to successfully invade the nests of their prey and also helps them retain moisture. Like related families in the Vespoidea, males have wings but females uniformly are wingless. They exhibit extreme sexual dimorphism; the males and females are so different, it is almost impossible to associate the two sexes of a species unless they are captured while mating. In a few species, the male is so much larger than the female, he carries her aloft while mating, which is also seen in the related family Tiphiidae.
In mutillids, as in all Hymenoptera, only the female is capable of inflicting a sting because the stinger itself is a modified female organ called an ovipositor; female mutillids have unusually long and maneuverable stingers. In both sexes, a structure called a stridulitrum on the metasoma is used to produce a squeaking or chirping sound when alarmed. Both sexes of mutillids also bear hair-lined grooves on the side of the metasoma called felt lines. Only one other vespoid family, the Bradynobaenidae, has felt lines, but the females have a distinct pronotum and an elongated ant-like petiole.
Mature mutillids feed on nectar. Although some species are strictly nocturnal, females are often active during the day. Females of Tricholabiodes thisbe are sometimes active up to two hours before sunset. Guido Nonveiller (1963) hypothesized the Mutillidae are generally stenothermic and thermophilic; they may not avoid light, but rather are active during temperatures which usually occur only after sunset.
Life cycle 
The male locates a female on the wing and mates. The female then enters an insect nest, typically a ground-nesting bee such as a bumblebee or wasp nest, and deposits one egg near each larva or pupa. Her young then develop as idiobiont ectoparasitoids, eventually killing their immobile larval/pupal hosts within a matter of days.
The 3,000-5,000 species of Mutillidae occur worldwide, mainly in the dry tropics. They are especially common, however, in desert and sandy areas, with most of the over 400 North American species found in the southwestern United States and adjacent parts of Mexico, with others found in generally sandy regions throughout the United States and Canada.
- A. S. Lelej Catalogue of the Mutillidae (Hymenoptera) of the Palaearctic Region (pdf)
- J.H. Hunt. 1999. Trait mapping and salience in the evolution of eusocial vespid wasps. Evolution 53: 225-237 (pdf)
- Lorus J. Milne, National Audubon Society Field Guide to North American Insects and Spiders (Audubon Society Field Guide) (Turtleback)(1980) Knopf. ISBN 0-394-50763-0.
- Nonveiller, G. Catalogue of the Mutillidae, Myrmosidae and Bradynobaenidae of the Neotropical Region including Mexico (Insecta: Hymenoptera). SPB Academic Publishing bv, the Netherlands, pp. 1–150.
|Wikimedia Commons has media related to: Mutillidae|
- Velvet Ant (Cow Killer) Fact Sheet
- Cow Killer at the University of Nebraska-Lincoln Extension in Lancaster County, Nebraska
- Mutillidae at BugGuide.Net
- Mutillidae at the University of Florida Institute of Food and Agricultural Sciences
- Mutillidae at Texas A&M University