Agenioideus: Average Spider Hawks

I have commented in earlier posts on the challenges of identifying spider hawks of the family Pompilidae, resulting from this wasp family's combination of high species diversity with a mostly conservative body plan. As a result of this conservatism, pompilid classification has tended to drift towards a situation where the majority of species are included in a relatively small number of somewhat vaguely defined genera. Each of the species included in one of these genera can be associated with other species in the genus, and groups of species approach each other closely enough that clear lines cannot be settled upon, but identifying features shared by all members of the genus can prove difficult. A good example of one such genus is Agenioideus.

Female Agenioideus birkmanni, from the University of Texas at Austin.

Species assigned to Agenioideus can be found pretty much worldwide though the greatest diversity occurs in warmer parts of the Holarctic. Though there does not seem to be a great deal of disagreement over which species should be placed in this genus, it seems a little difficult to say exactly what makes an Agenioideus. If anything, Agenioideus species seem to be associated by how relentlessly average they are, possessing a unique combination of characters that are none of them individually unique. They have wings with three submarginal cells, a broad metapostnotum in front of the propodeum, and legs ending in a small arolium with a weak comb of setae between a pair of long claws, mostly with a single small ventral tooth (Krogmann & Austin 2012). If you don't know exactly what those terms mean, just know that they are all quite unspecialised features for pompilids. Males often have asymmetrical claws on the forelegs, with the inner claw strongly bent and bifid while the outer claw is like those on the other legs, and the pterostigma (the dark node at the front of the fore wings) is relatively large compared to other genera. Females often have a comb of longer spines on the inner margin of the fore tarsi. But these last, more derived, features may not be universally present across all species of the genus.

Female Agenioideus nigricornis with redback spider Latrodectus hasselti as prey, copyright Mark Newton.

As befits their unspecialised appearance, most Agenioideus species (as far as we know) are relatively unspecialised in their nesting behaviour (Shimizu 1997). Like other pompilids, they lay their eggs on paralysed spiders that will provide food for the larva when it hatches. Most Agenioideus species construct simple nests with a single brood cell containing a single spider for each nest. One European species, A. nubecula, is known to produce slightly more extensive nests with up to four cells. The nest may be made by digging in loose soil or by using a pre-existing cavity; whether the wasp is more likely to do one or the other is correlated with whether she possesses a well-developed tarsal comb. A Japanese species, A. ishikawai, is known to at least partially dig a nest before capturing a spider, completing construction after bringing it back. The most specialised provisioning behaviour known for the genus, however, is found in another European species, A. coronatus. This species hunts jumping spiders which she paralyses with her sting as is standard. The paralysis, however, is only temporary, lasting just a few minutes, just long enough for the female to deposit an egg near the base of the spider's abdomen where it cannot easily remove it. The spider is then freed to go about its business without being placed in a nest, until the wasp larva hatches and feeds on its host in the manner of a parasitoid.

REFERENCES

Krogmann, L., & A. D. Austin. 2012. Systematics of Australian Agenioideus Ashmead (Hymenoptera: Pompilidae) with the first record of a spider wasp parasitizing Latrodectus hasselti Thorell (redback spider). Australian Journal of Entomology 51: 166–174.

Shimizu, A. 1997. Taxonomic studies on the Pompilidae occurring in Japan north of the Ryukyus: the genus Agenioideus Ashmead (Hymenoptera). Japanese Journal of Entomology 65 (1): 143–167.