The Mutillidae are a family of more than 3,000 species of wasps 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. Black and white specimens are sometimes known as panda ants due to their hair coloration resembling that of the giant panda. Their bright colors serve as aposematic signals. They are known for their extremely painful stings, (the sting of the species Dasymultila klugii rated a 3 on the Schmidt pain index and lasts up to 30 minutes), hence the common name cow killer or cow ant. However, mutillids are not aggressive and sting only in defense. In addition, the actual toxicity of their venom is much lower than that of honey bees or harvester ants. Unlike true ants, they are solitary, and lack complex social systems.
Mutillidae can be found worldwide with approximately 230 genera or subgenera and around 8,000 species worldwide. Over 400 species occur in the North American southwest.
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. Mutillids exhibit extreme sexual dimorphism. Like some related families in the Vespoidea, males have wings, but females are wingless. The males and females are distinct enough in their morphology, that it is often very hard to determine whether a given male and female belong to the same species, unless they are captured while mating. In some species, the male carries the smaller female aloft while mating, which is also seen in the related family Thynnidae.
As is the case for all Aculeates, only female mutillids are capable of inflicting a sting. The stinger is a modified female organ called an ovipositor; which is unusually long and maneuverable in mutillids. 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 two other vespoid families, (Bradynobaenidae and Chyphotidae) have felt lines, but the females of these families have a distinctive pronotum. Members of the family Myrmosidae, formerly classified as a subfamily of mutillids, also have a distinctive pronotum in females, but lack felt lines in both sexes.
Adult 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 that usually occur only after sunset.
Male mutillids fly in search of females; after mating, the female enters a host insect nest, typically a ground-nesting bee or wasp burrow, and deposits one egg near each larva or pupa. Only a few species are known to parasitize other types of hosts; exceptions include the European velvet ant, Mutilla europaea, one of the only species that attacks social bees (e.g., Bombus), and the genus Pappognatha, whose hosts are tree-dwelling orchid bees. The mutillid larvae then develop as idiobiont ectoparasitoids, eventually killing their immobile larval/pupal hosts within a week or two. Velvet ants exhibit haplodiploid sex determination, as do other members of the superfamily Vespoidea.
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