Well, the blog is staying put, actually, but the author is moving from Tucson, Arizona to Colorado Springs, Colorado next weekend. This blog will be on hiatus until he gets settled and hooked up with whatever internet provider serves his neighborhood there. Thank you in advance for your loyalty and patience. I am optimistic that I'll be back by mid-October.
Have you ever been involved in a hit-and-run accident? If so, then you know what it feels like to be a scarab beetle grub being attacked by a scoliid wasp. The only difference is that the insect encounter is no accident. Female scoliid wasps actively seek out the beetle grubs as hosts for their larval offspring. Here in southern Arizona, one of the largest and most conspicuous of the scoliids is Triscolia ardens.
T. ardens is the only member of its genus in North America north of Mexico. It occurs from Texas to southern California and is relatively common. Females are robust, with short antennae. Males have long antennae and sport a three-pronged "pseudosting" at the tip of the abdomen. The remainder of this post will treat scoliids in general because so little is known about the biology of individual species.
Scoliids of both genders can be found visiting flowers, especially milkweed, seep willow (Baccharis), saltcedar (Tamarix), desert willow (Chilopsis), mesquite (Prosopis), scalebroom (Lepidospartum), and buckwheat (Eriogonum). At the end of the day, the wasps bed down singly on vegetation, remaining alert but not not terribly motivated to move as the desert temperatures begin to fall.
Their life cycle can be generalized as follows. The female wasps fly low over the ground, somehow divining the presence of subterranean scarab beetle grubs. Once one is detected, the wasp digs it up, using her densely spiny legs to send the soil flying. Once she unearths the grub, she stings it into paralysis. This allows her to lay a single egg on the grub. After she accomplishes her mission, she re-buries the grub and flees the scene of the crime (some species have been observed moving the grub deeper into the soil and fashioning an earthen cell around it before depositing an egg and sealing the tunnel). The beetle grub apparently never recovers from its coma. The egg of the wasp hatches, and the larva that emerges will feed as an external parasite on its host for about a week or two before spinning a silken cocoon and pupating. Most North American scoliids overwinter in the pupal stage.
The size of the mature wasp is dependent on the size of the host beetle grub. Even the largest Triscolia ardens seldom exceed about 30 millimeters in body length, but one frequently finds “dwarfs” that obviously suffered a lack of nutrition in their youth. Tropical scoliids, however, are among the largest (certainly heaviest) wasps in the world, exceeding 50 millimeters in body length.
Other online resources include this PDF on scoliid wasps of Florida. It is an excellent introduction to the family as a whole, treating Florida scoliids in particular. Another PDF on Scoliidae of California addresses some southwestern species but it appears incomplete.
Clearly, much remains to be learned about these highly attractive and obvious wasps. Any documentation with video is likely to be a first, or nearly so; and host associations are almost entirely lacking for our native scoliids (some species have been introduced to the U.S. to control exotic scarab pests).
Arrrr, matey! In honor of International Talk Like a Pirate Day (tomorrow, September 19), I bring ye the story of pirate spiders in the family Mimetidae. I’m not pulling your beard, there really are such creatures.
The mimetids are represented by a total of three genera and eighteen species in North America north of Mexico, though another ten remain undescribed (awaiting the assignment of names by arachnologists).
Pirate spiders are easily identified by their four eye patches and two wooden legs. But seriously, folks, the long spines on their legs help differentiate them from the cobweb spiders and sheet-web weavers that they might otherwise be mistaken for. Their resemblance to other spiders is further complicated by the fact that you sometimes find pirate spiders in the snares of cobweb weavers. Their appearance there is as sinister as their name suggests.
Pirate spiders eat cobweb spiders, as well as orb weavers and other spiders. They stealthily stalk the rightful owner of the web, finally dispatching it by biting the other spider on its legs. The pirate spider feeds on one leg after the other until its victim is totally drained. One reason for attacking the legs of its prey might be that the jaws of pirate spiders are fused at their base, not permitting the spider to open its mouthparts wide enough to bite other parts of its victims.
Pirate spiders can also feed on insects when presented with the opportunity, but their venom is designed to kill other spiders quickly. They will also eat the egg sacs of their victims. This lifestyle has given rise to another popular name for them: “cannibal spiders.”
The specimen imaged above is Mimetus puritanus, the most common species in the eastern United States. I found this one in South Deerfield, Massachusetts on October 21, 2009. The male specimen below (note the modified pedipalps) was discovered on the ceiling of my residence on July 23, 2009. At only 3-7 millimeters in body length, pirate spiders in general are not terribly imposing for creatures that make their living attacking other spiders.
My girlfriend found the egg sac of a Mimetus in Colorado Springs, Colorado on July 28 of last year. The “fluffy” appearance of the sac’s loosely-woven silk is fairly diagnostic for the genus.
I encountered what appears to be a different species of Mimetus at Bentsen Rio Grande Valley State Park in Mission, Texas on June 7, 2010. It was occupying the web of a theridiid cobweb weaver, and until I cropped my images, that is exactly what I thought it was.
Well, my swashbuckling friends, keep yer eyes peeled fer them thar buccaneer arachnids we call pirate spiders. Arrr!
Here in southern Arizona there are several species of paper wasps in the genus Polistes. What intrigues me is that you seldom find more than one species in any particular location. At the least, one species is usually dominant and other species scarce. Here in the city of Tucson, one of the more abundant urban species is P. arizonensis.
At first glance, this species could be mistaken for Polistes exclamans, a species common in the eastern and southern U.S. Indeed, P. arizonensis was once considered a subspecies of P. exclamans. The Arizona wasp only ranges from Arizona to southern California and Mexico.
One of the favorite places for P. arizonensis to nest is under collapsed palm fronds. Several large colonies can coexist in close proximity under these weathered but sturdy leaves. Here is an example I found at the Sweetwater Wetlands in northwest Tucson.
I have found several female (worker) wasps overwintering on such insulated nests, so the large combs may represent the expansions of the previous year’s occupants. Well, that was my theory until I took some of these old nests as souvenirs and found they harbored huge numbers of dermestid beetle larvae that eventually became adult beetles flying freely around my apartment. The invasion of scavengers like dermestids suggests that the combs held deceased wasps (probably in the pupal stage), and those cells are not likely to be re-used.
Interestingly, Polistes arizonensis seems just as comfortable nesting amid tangled shrubbery, which leaves them perhaps more exposed to the elements, and their prime enemies: ants. The wasps do have an answer for the ants, though. The female wasps secrete a substance that repels ants, and apply this blackish goo liberally to the paper pedicel that attaches the nest to the substrate (stem, frond, ceiling, whatever). The repellant is produced by sternal glands located internally on the underside of the abdomen. The secretion consists of long-chain carboxylic acids. One study showed that the unsaturated acids (palmitoleic, linoleic, and oleic) in the secretion were effective in repelling at least three species of ants, for up to four days (Dani, et al., 1996).
Like most paper wasps, P. arizonensis seems to prefer hunting caterpillars as food for the larvae back at the nest. Notice the worker chewing up a caterpillar in the image below, prior to feeding the morsel to a larva in one of the cells in the nest.
This nest was located under the sheltering roof of a “ramada” overlooking one of the ponds at Sweetwater Wetlands.
You can tell the gender of many kinds of paper wasps by looking them in the face. Females usually have dark, triangular faces, whereas males have square, yellow faces. Such is the case with P. arizonensis. Also note the hooked tips on the antennae, and blunt tip of the abdomen in the male specimen imaged below.
I find that this species is remarkably tolerant of close approaches to its nests. I was able to get these images without the occupants even taking an alarm stance. When paper wasps stand on tip-toe (tip-tarsi?), with wings raised and splayed, it means “You have come too close! Back off now or suffer the consequences.” By retreating even a short distance after such a warning you prevent an attack and the wasps go back to business as usual. Understand that I am not recommending you trespass active colonies, but do use your own judgment and enjoy observing their interactions.
Sources: Dani, F. R., S. Cannoni, S. Turillazzi, and E. D. Morgan. 1996. “Ant repellent effect of the sterna gland secretion of Polistes dominulus (Christ) and P. sulcifer (Zimmermann). (Hymenoptera: Vespidae),” Jour. Chem. Ecol. 22: 37-48.
