One of my projects this year, which sadly has progressed at only a desultory pace, is to learn about our square-headed wasps, named for a conspicuous body feature of some species in this family. Biologists know the family as Crabronidae, and it’s well worth getting acquainted with them.
It’s a fairly diverse family, with about 9,000 species worldwide and about 1,200 in North America. Of these, only 30 have been solidly documented on the Vineyard, though the number actually present is surely higher than that. Many species are hard to identify from photographs, and so far, there has been no concerted, Island-wide study that has involved the collection of specimens for examination.
Like most wasps, the members of this family are predators, feeding their young on other insects (a few species prefer spiders). Most, maybe all, Crabronids are ground-nesters, with females digging burrows in sandy soil, excavating cells along the burrow for their young to occupy, and then laying eggs in each cell. The adult female finds a suitable prey item, paralyzes it with a sting, and drags it down into one of the cells, where it serves as food while the larval wasp develops.
While larval Crabronids enjoy tucking into a good arthropod for dinner, adults have a much more benign diet. They are flower visitors, taking pollen or nectar from any of the myriad types of flowers that offer these resources to insects, and because of this habit, Crabronids can be considered reasonably effective and probably fairly important pollinators.
checklist. —Matt Pelikan
It’s a ghoulish life history, to be sure, though it is shared by other groups of wasps. But it does mean that square-headed wasps play an important role in the ecosystem, helping to regulate the population sizes of their prey species. This type of nest provisioning is also a fascinating behavior to watch: If you spend time with this family, you grow familiar with the sight of a female wasp schlepping an immobile victim to a nest burrow and hauling it down to the basement.
I know little about it, but I’ve long been curious about how Crabronids, and indeed other predatory wasps, successfully locate their prey. Staying concealed has developed into an art form among arthropods, since not being noticed is a critical first step in not getting eaten. Yet predator wasps seem to have no trouble zeroing in on suitable prey.
I expect the method of detection varies. Some Crabronids, like the fly-eating species of the genus Oxybelus, look to me like they are visual hunters, spending much of their time cruising low over the ground in open habitat. Others, very likely, use chemical cues as at least part of their strategy, sniffing out their prey by detecting characteristic odors or pheromones. Very little seems to be known about the exact mechanisms used, but it’s a fun subject for speculation.
In the case of one of our most prominent Crabronids, I wonder if sound plays a role in prey detection. The Eastern cicada killer, Sphecius speciosus, preys, as its name suggests, on cicadas. What better way to find these loud insects than by zeroing in on their call? I have no evidence that this is the case, but it seems like too obvious a trick for evolution not to have invented it.
In any case, Sphecius speciosus is a massive wasp, pushing two inches in length in the case of large females, and they dig correspondingly impressive burrows: a half-inch-diameter hole surrounded by a mound of sandy spoil. While the males (which, like all male wasps, cannot sting) can appear aggressive, flying up to investigate intruders including humans, cicada killers are like all wasps, much more interested in living their lives than in harming humans.
Another interesting group of Crabronids are the bee wolves of the genus Philanthus. The Vineyard hosts at least five species of the genus. True to their name, members of this genus provision their nests with bees. One member of the genus, Philanthus gibbosus, is extremely common on the Vineyard ,and is one of our most commonly observed wasps.
Like many other Crabronids, P. gibbosus (like our other members of the genus) is a black insect boldly marked with yellow. There are typically bands of yellow ringing some or all of the segments of the abdomen, and the pattern of those bands is a powerful cue for species-level identification. Philanthus also all have boldly and distinctly patterned faces, with yellow patches of various size and shape. So if you’re trying to identify a member of this group, a clear head-on shot can be very useful.
I’ve just given a sample of this interesting family: We have members of five of the six North American subfamilies of Crabronidae, and members of at least 22 genera. Calling to mind the figure of 30 Vineyard species that I cited earlier, you’ll surmise correctly that so far, many genera are represented here by just a single firmly identified species. That’s a sign of a lot of work yet to be done. There are many members of this colorful and interesting family out there, waiting for someone to recognize their presence.
