Psenulus: Silk-Weaving Wasps

Female Psenulus pallipes carrying an aphid back to her nest. Copyright Jeremy Early.

Because I am an obsessive-compulsive weirdo, I spend a good chunk of my spare time at home sorting through biology publications and pulling out names (you can seen some of the results of this at my other site, The Variety of Life). Back on July 1, I tweeted: "And from tonight, I delve into sphecoids. To species level. This could take even longer than the oribatids." Never, as it turns out, were truer words spoken, as we have now very nearly reached the end of July, and I am still only a relatively small part of the way through this diverse group of wasps (to be more specific, I've been taking stuff out of Bohart & Menke's [1976] Sphecid Wasps of the World, and I've only gotten as far as p. 179 of what is a 695-page book: there are over 7500 species listed in that book, a depressing high proportion of which appear to have originally been placed in the genus Sphex). And seeing as so much of my time recently has been spent on sphecoids, it is only appropriate that my semi-random selection for this week's post has been one: the pemphredonine Psenulus trisulcus.

The sphecoids are a group of solitary wasps including such beasts as the digger wasps and sand wasps. Bohart & Menke (1976) placed them all in a single family Sphecidae, but this does not represent a monophyletic group, as some 'sphecoids' are more closely related to bees than to other sphecoids. As a result, most recent authors have divided the sphecoids between three families: the Ampulicidae (cockroach wasps), Sphecidae (digger wasps, etc.) and Crabronidae (sand wasps, etc.) The Pemphredoninae are a group of mostly quite small wasps in the last of these families. Psenulus is a genus of about 120 species of pemphredonines found on most continents except South America; P. trisulcus is one of only a small number of Psenulus species found in North America (the genus is most diverse in the Oriental region). Like other sphecoids, females of Psenulus species provision their nests with paralysed prey insects for their larvae to feed on after hatching. While more familiar sphecoids such as digger or sand wasps may dig tunnels in which to construct their nest cells, Psenulus species use hollows such as beetle borings in plant stems. Krombein (1979) listed P. trisulcus as nesting in elder stems; another Psenulus species has been recorded constructing cells in hollow grass stems floating on water (Bohart & Menke 1976). I have not been able to find a record of the preferred prey of P. trisulcus itself, but closely related species such as P. pallipes, a Holarctic species shown in the photo at the top of this post, attack aphids. In the case of P. pallipes, a single nest cell may be packed with as many as 27 aphids, providing plenty of food for an emerging larva. Other Psenulus species may collect other Hemiptera, such as psyllids (plant-lice) or leafhoppers. Psenulus trisulcus resembles P. pallipes in its overall black coloration, and the characters distinguishing the two would not be visible without a close microscopic examination: in P. trisulcus, the ridge running between the antennae is marked by longitudinal grooves that are not present in P. pallipes, and the petiole of P. trisulcus has a ridge along its underside (Malloch 1933*).

*As corrected by Pate (1944), who noted that Malloch's "trisulcus" was actually a different species that he named "parenosas" (subsequently regarded as a subspecies of pallipes), and that the true trisulcus was actually Malloch's "sulcatus".

Pinned specimen of Psenulus trisulcus, copyright York University.

The nests of Psenulus trisulcus and P. pallipes are also unusual in being lined with silk, with silk also being used to construct the partitions between cells. While many insects produce silk as larvae, it is more uncommon for them to continue doing so as adults (and only the females do so in the case of Psenulus). The source of Psenulus' silk was long uncertain (with one researcher suggesting that it was extruded from the labial palps), until Melo (1997) established that it was secreted from bristle-like spinnerets that form fringes on the hind margins of the fourth and fifth sternites of the gaster. However, not all Psenulus species have such fringes: Melo (1997) examined three spinneret-less species and found that their silk glands opened directly on the underside of the gaster (with long erect setae possibly assisting in the spreading of silk in these species). This makes for an interesting comparison with spiders, in which the fossil Attercopus suggests the evolution of spinnerets from previously disassociated silk glands. Unfortunately, we don't yet really know what the relationships are within Psenulus, and whether the spinneret-less model is truly ancestral.

REFERENCES

Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World: a generic revision. University of California Press.

Krombein, K. V. 1979. Catalog of Hymenoptera in America North of Mexico vol. 2. Apocrita (Aculeata). Smithsonian Institution Press.

Malloch, J. R. 1933 Review of the wasps of the subfamily Pseninae of North America (Hymenoptera: Aculeata). Proceedings of The United States National Museum 82 (26): 1-60.

Melo, G. A. R. 1997. Silk glands in adult sphecid wasps (Hymenoptera, Sphecidae, Pemphredoninae). Journal of Hymenoptera Research 6: 1-9.

Pate, V. S. L. 1944. Synonymical notes on the psenine wasps (Hymenoptera, Sphecidae). Canadian Entomologist 76 (7): 133.