Wednesday, September 12, 2012

Wasp Wednesday: Dasymutilla bioculata

Last Sunday I wrote about a brightly-colored jumping spider, Phidippus apacheanus, and suggested that it was probably a mimic of wingless female wasps called velvet ants. Today I would like to introduce you to one of those wasps. Velvet ants comprise the family Mutillidae, and can be found across most of the North American continent. They are most common in arid habitats like deserts and prairies. Here in Colorado Springs, Dasymutilla bioculata is one of the most frequently encountered species.

Many velvet ants are bright orange, red, or yellow, and black. This pattern advertises the fact that females can sting. So powerful is that sting that one of the larger species is called the “cow killer.” Folklore has it that the pain is enough to kill livestock. It is no mystery why less well-defended insects, or spiders, would want to look like velvet ants. No sensible predator wants to mess with them.

I have seen female Dasymutilla bioculata scurrying over sandy paths in open fields, usually on overcast days or around dusk when the air temperature begins to cool slightly. Both genders like to clamber around on the stems and leaves of Common Sunflower, Helianthus annuus. They find the sweet, sticky secretions of the plant to their liking.

Male velvet ants have the customary two pairs of wings and fly well, but they can also be colored very differently from their female counterparts. This has resulted in much confusion, even among experts who seek to associate the males with the appropriate females and vice versa.

Dasymutilla bioculata is an excellent example of “lumping,” whereby scientists decide that several different species are actually variations of just one species. A paper published in 2010 resulted in lumping no fewer than twenty-one former species and subspecies under the name Dasymutilla bioculata. The authors of the publication used molecular analysis to find common nuclear ribosomal RNA markers among all the former species and subspecies. Morphological differences (what you see when you look at the external structure of the insects) were deemed too inconsistent to be the sole determining factors in differentiating species.

No wonder I didn’t recognize these Colorado specimens. George Waldren, a velvet ant expert and volunteer editor at Bugguide.net was kind enough to set me straight, identifying my images of the females. The images of the male shown here may or may not be the same species. You just can’t tell from images alone.

Dasymutilla bioculata ranges across the entire continent, and from southern Canada to Mexico. Adults vary in size from about 8 millimeters to 16 or 17 millimeters in body length. The bigger the specimen, the better it fed as a larva.

The life histories of velvet ants are often as mysterious as their classification, or simply unknown. Those species we do know well are parasitic on other insects, especially other solitary wasps and bees. Female velvet ants scour the ground for signs of their hosts, which often dig burrows in the soil. Velvet ants can even detect closed burrows, digging them open to gain access. Once inside, the female lays an egg in the cell or cells at the end of the underground tunnel. The larval wasp that hatches will attack the pupa of the host, or a larva in diapause (inactive state).

The known hosts for Dasymutilla bioculata are sand wasps in the genera Bembix and Microbembex. Should an adult sand wasp discover a velvet ant invading her nest, she will of course attack it. Velvet ants have an extra-thick exoskeleton that effectively deflects the bites and stings of their enemies.

I would advise anyone searching for velvet ants to avoid picking them up. Do scoop one into a vial some time and put it up to your ear. Do you hear that? Both genders of these wasps can “squeak” by rapidly rubbing their abdominal segments together. Music to my own ears anyway.

Sources: Kits, Joel, et al. 2011. “Species Dasymutilla bioculata,” Bugguide.net.
Krombein, Karl V. and Paul D. Hurd. 1979. Catalog of Hymenoptera in America North of Mexico. Washington, DC: Smithsonian Institution Press. 2188 pp.
Williams, K.A., D.G. Manley, E.M. Pilgrim, C.D. von Dohlen & J.P. Pitts. 2010. “Multifaceted assessment of species validity in the Dasymutilla bioculata species group (Hymenoptera: Mutillidae). Syst Entomol, 36(1): 180-191.

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