Field of Science

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.


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.

Giant Centipedes (That Aren't All Giants)

Scolopendra morsitans, copyright Jiri Lochman/Lochman Transparencies.

It was a dark night, but not stormy (nights tend to be dark, as a rule). We were out collecting for our regular survey when we encountered a large centipede (the same species as in the photo above) crossing the road, and decided to add it to our collection. Taking out the large 20-cm forceps that we had on hand for dealing with venomous animals, one of us used them to grab the centipede.

The response was electric. Rather than trying to escape its attacker, the centipede instantly whipped back and lashed itself around the forceps, doing its best to bite into them. Had the actual wielder of the forceps been within its reach, they would have been in for a world of pain. When dealing with scolopendrid centipedes, you should always remember three things: they are big, they are fast, and they are mean.

Scolopendridae are unmistakeable. They include the giants of the centipede world, with the largest species (the South American Scolopendra gigantea) reaching up to a foot in length. Even the smaller species are relatively robust compared to other centipedes. Like all centipedes, the first pair of legs is modified into a robust pair of 'fangs' used for delivering venom (when I referred above to a centipede 'biting', this is what I was properly referring to). Large scolopendrids have the most dangerous centipede stings, potentially causing intense pain, though fatalities are very rare (Bush et al., 2001, noted that no centipede fatalities were known from the US, though they did refer to a single known child fatality in the Philippines). Their hunting prowess is amply demonstrated in this video of a large scolopendrid hunting bats by quite literally snatching them out of the air:
ARKive video - Amazonian giant centipede hunting bats inside a cave

But lest you think that scolopendrids are all venom and viciousness, let me point out that they also have their endearing qualities. Female scolopendrids make devoted mothers, coiling around their egg clutches and regularly cleaning them to prevent fungal attack. Even after the eggs hatch, the female continues to coddle and groom her young. Brunhuber (1970) recorded that females of Cormocephalus anceps spent at least three months (from late September to late December) caring for young before they struck out on their own. Even after becoming independent, the young do not reach sexual maturity until they are at least two years old. Other scolopendrids may mature more quickly, at about one year (Lewis 1972). Individual centipedes may live for several years.

Female Scolopendra morsitans cleaning her eggs, copyright H. J. B..

The Scolopendridae belong to a larger centipede group called the Scolopendromorpha. Most scolopendromorphs have bodies with 21 or 23 leg-bearing segments, except for one remarkable scolopendrid species from central Brazil that has 39 or 43 leg-bearing segments (Chagas-Junior et al. 2008). Non-scolopendrid species are often much smaller than the Scolopendridae, with some being only about 10 mm in length. These smaller scolopendromorphs also differ in eye morphology: Scolopendridae have a patch of four ocelli on either side of the head, but other scolopendromorphs are mostly blind and lack ocelli. In the past the blind scolopendromorphs have been treated as a single family Cryptopidae, but recent authors have mostly recognised three separate families Cryptopidae, Scolopocryptopidae and Plutoniumidae in light of uncertainty about the monophyly of a broader Cryptopidae. Nevertheless, a recent phylogenetic analysis by Vahtera et al. (2012) combining both morphological and molecular data did support a single blind clade. Unfortunately, no phylogenetic analysis to date has been able to include Mimops orientalis, an odd scolopendromorph known from a single specimen collected in China in 1903 and placed by Lewis (2006) in its own distinct family. Mimops possesses but a single ocellus on either side of the head, potentially making it very intriguing for the question of whether blindness has evolved in scolopendromorphs more than once.

The blind scolopendromorph Scolopocryptops sexspinosus, copyright Troy Bartlett.


Brunhuber, B. S. 1970. Egg laying, maternal care and development of young in the scolopendromorph centipede, Cormocephalus anceps anceps Porat. Zoological Journal of the Linnean Society 49 (3): 225-234.

Bush, S. P., B. O. King, R. L. Norris & S. A. Stockwell. 2001. Centipede envenomation. Wilderness and Environmental Medicine 12 (2): 93-99.

Chagas-Junior, A., G. D. Edgecombe & A. Minelli. 2008. Variability in trunk segmentation in the centipede order Scolopendromorpha: a remarkable new species of Scolopendropsis Brandt (Chilopoda: Scolopendridae) from Brazil. Zootaxa 1888: 36-46.

Lewis, J. G. E. 1972. The life histories and distribution of the centipedes Rhysida nuda togoensis and Ethmostigmus trigonopodus (Scolopendromorpha: Scolopendridae) in Nigeria. Journal of Zoology 167 (4): 399-414.

Lewis, J. G. E. 2006. On the scolopendromorph centipede genus Mimops Kraepelin, 1903, with a description of a new family (Chilopoda: Scolopendromorpha). Journal of Natural History 40 (19-20): 1231-1239.

Vahtera, V., G. D. Edgecombe & G. Giribet. 2012. Evolution of blindness in scolopendromorph centipedes (Chilopoda: Scolopendromorpha): insight from an expanded sampling of molecular data. Cladistics 28: 4-20.

The World's Scorpion

Pair of lesser brown scorpions Isometrus maculatus in captivity, from here. The more elongate individual at lower left is a male; his stouter companion is female.

With their elongate, sting-tipped tails, scorpions are instantly distinguishable from any other arachnid. Depending on how you look at it, they are either charismatic or infamous (not many invertebrates have constellations named after them). Yet though the world diversity of scorpions is not unrespectable (about 1750 species have been described so far), distinguishing one scorpion species from another can be a challenging prospect. They tend, as a whole, to be a morphologically conservative group.

As a result, new species of scorpion continue to be described, at a rate limited only by the relatively small number of people taking up the challenge. Isometrus is a genus of about thirty species of scorpion found mostly from in southern Asia and Australasia, from Pakistan to northern Australia and New Caledonia. A good third of those species have only been described since 2000, and probably more remain to be described. They are small, relatively slender scorpions, often with the 'fingers' of the chelae noticeably darker than the 'palms' (as in the photo above). They have a sting that has been referred to as painful, but doesn't seem to have lead to recorded fatalities in humans.

Isometrus has been divided between two subgenera, Isometrus sensu stricto and Reddyanus (Kovařík 2003). The two subgenera are distinguished primarily by the locations of trichobothria, long sensory hairs, on the chelae. The higher diversity of species belong to Reddyanus, but Isometrus is by far the more widespread subgenus. This is due to its inclusion of one particular species: I. maculatus, commonly given the rather underwhelming name of 'lesser brown scorpion'. For some reason, Isometrus maculatus has proven itself very amenable to transport by humans. It may have been originally native to Sri Lanka (as cited here) but from there it has spread to tropical regions around the world. It is found in North America, South America, Africa and Australia, and on various oceanic islands such as Hawaii, Saint Helena and the Seychelles. In Europe, it is only known from southern Spain, though it may have been the species originally indicated by Linnaeus' 'Scorpio europaeus' (Fet et al. 2002; some authors have consequently used the name 'Isometrus europaeus' for this species, but Linnaeus' name was declared invalid by the ICZN due to the uncertainty of its identity). Currently, I. maculatus is regarded as the world's most widespread scorpion species. No other Isometrus species has been subject to the same degree of spread, though I am personally inclined to wonder about the distribution of I. heimi, recorded by Kovařík (2003) from both New Guinea and New Caledonia.


Fet, V., M. E. Braunwalder & H. D. Cameron. 2002. Scorpions (Arachnida, Scorpiones) described by Linnaeus. Bull. Br. Arachnol. Soc. 12 (4): 176-182.

Kovařík, F. 2003. A review of the genus Isometrus Ehrenberg, 1828 (Scorpiones: Buthidae) with descriptions of four new species from Asia and Australia. Euscorpius 10: 1-19.


Back in 2011, I presented you with a post on the southern African flowering bulb genus Ledebouria. In that post, I mentioned that Ledebouria was just one of a wide diversity of ornamental plants found in that part of the world.

Lachenalia elegans var. flava, from the Pacific Bulb Society.

Lachenalia, sometimes known as Cape cowslips, is a genus of over 100 species found in Namibia and South Africa. Most Lachenalia species sprout and flower in the winter. Lachenalia is not too distant a relative of Ledebouria—both are classified in the squill tribe Massonieae—and bears a distinct resemblance to the latter with its fleshy leaves that are often blotched with purple. Some species of Lachenalia share the geophyllous habit I described in the earlier post for some Ledebouria, with the leaves growing pressed closely to the ground. However, Lachenalia differs from Ledebouria in having flowers with well-developed bracts, and anthers arranged in two series. Also, while the scales of Ledebouria bulbs are often loose, though of Lachenalia bulbs are always tightly packed (Manning et al. 2004).

Lachenalia zebrina f. zebrina, photographed by Alan Horstmann.

Lachenalia species include some popular garden plants, to the extent that some are known as invasive weeds here in the Perth region. Nevertheless, a simple image search immediately shows why they are so popular. Varieties of this genus are available in reds, pinks, yellows, purples... One species, L. viridiflora, has flowers of a quite remarkable turqouise colour. Though revered in cultivation, L. viridiflora is critically endangered in the wild, with a range of only 19 km2 in which it is threatened by grazing, housing development and (almost ironically) the collection of specimens for horticulture.

Lachenalia viridiflora, photographed by A. Harrower.


Manning, J. C., P. Goldblatt & M. F. Fay. 2004. A revised generic synopsis of Hyacinthaceae in sub-Saharan Africa, based on molecular evidence, including new combinations and the new tribe Pseudoprospereae. Edinburgh Journal of Botany 60 (3): 533-568.

There's No Such Thing as Caddids

Caddo agilis, from here.

Long-time readers of this site may recall my previous rants on the subject of the prolific, but not entirely reliable, arachnologist Carl-Friedrich Roewer. Hopefully, this post will serve to rehabilitate Roewer's image a little, because occasionally something comes along about which he was right in the first place.

Among Roewer's innovations in Die Weberknechte der Erde, his 1923 revision of the world Opiliones fauna, was the introduction of a new family for Acropsopilio, an odd little harvestman from South America. He placed this new family in the Dyspnoi, a subgroup of the Palpatores (long-legged harvestmen) that is otherwise found in Eurasia and North America. Acropsopilio was a distinctive beast, a tiny harvestman with relatively massive eyes (just take a look at the picture below!) Over time, other authors added to the Acropsopilionidae: species are now known from Australia, New Zealand and South Africa. They are nowhere comon, though.

Specimen of Acropsopilio neozelandiae, photographed by Stephen Thorpe.

In 1975, the acropsopilionids were revised by Shear (1975), who proposed that they were related to Caddo, a genus of harvestmen found in north-eastern Asian and north-eastern North America. That's not a typo, by the way: the range of this genus includes Japan and New England, but not the spaces in between. To make things just that extra bit wierder, the genus includes two species, C. agilis and C. pepperella, both of which are found in both the sections of its overall range. Genetic analysis has demonstrated that this wierdness is real, and not just convergence or one variable species (Shultz & Regier 2009). Caddo had previously been classed as a member of the Eupnoi, the other main subgroup within the Palpatores, but resembled acropsopilionids in features such as the small size and large eyemound. Shear proposed classing them all as a single family, Caddidae, with two subfamilies: one for Caddo and one for the Acropsopilioninae. Subsequent authors have followed his lead, and the Caddidae has come to be placed within the Eupnoi as the sister taxon to the Phalangioidea (the group including the familiar long-legged harvestmen such as the field harvestman Phalangium opilio).

Nevertheless, there was still a bit of humming and hawing going on behind the scenes. Despite the overall similarities in habitus between Caddo and acropsopilionines, several of the finer details (such as the structure of the pedipalps and genitalia) were quite different. Phylogenetic studies commenting on the position of caddoids within the Opiliones had generally included Caddo only, and not included any representatives of the acropsopilionines. And so it is quite welcome to see a new publication by Groh & Giribet (in press) in which they produced a molecular phylogenetic analysis of the caddids as a whole. The result, as hinted in the first paragraph, is that the caddids are not supported as a monophyletic group. Caddo remains in its accustomed position within the Eupnoi, but the acropsopilionids are placed as the sister clade to the Dyspnoi. Roewer, it turns out, had them in the right place to begin with.

This has some interesting implications: for instance, the otherwise entirely Holarctic Dyspnoi have just acquired a Gondwanan basal group. Also, the large eyemound is either a convergent feature between Caddo and acropsopilionines, or a retained primitive feature from the palpatorean common ancestor. Groh & Giribet suggest the latter, but I suspect the former to be just as likely (it may be related to small size: some phalangioids, such as the Mediterranean Platybuninae and the Western Australian Megalopsalis tanisphyros, also have large-ish eyemounds). But the greatest surprise for yours truly was something else: one particular 'acropsopilionine' genus, Hesperopilio, was not placed either with Caddo or the other acropsopilionines. Instead, it was placed closer to the the phalangioid family Neopilionidae: the subject of my own research.

When I produced my revision of the Australasian phalangioid family Monoscutidae (which I ended up synonymising with Neopilionidae), I included Caddo as an outgroup taxon in my morphological phylogenetic analysis. At the time, my supervisor asked me why I didn't include an acropsopilionine as well, but I demurred on two points. One was that, as rare as acropsopilionines were in collections, males were even rarer (there is evidence that they are commonly parthenogenetic, as for that matter is Caddo). The other was that acropsopilionine genitalia were truly bizarre, and I couldn't determine which parts of the acropsopilionine penis corresponded to where on the monoscutid organ.

I was basing that judgment on Acropsopilio and the South African genus Caddella (offhand, there is a longstanding tradition in harvestman taxonomy that whenever the name Caddella appears in a paper, it must be mis-spelled at least once). I still stand by that judgment. But upon seeing the results of Groh and Giribet's molecular analysis, I looked up the description of Hesperopilio (Shear 1996), which includes a drawing of the male genitalia. And suddenly, I was struck by the possibility that they could indeed be neopilionid-like. So I tried entering Hesperopilio into my original data set using the published descriptions. The result? Though missing a fair amount of data (my coding would need to be checked against actual specimens), a rough run suggests that morphology supports Hesperopilio as a neopilionid too!

The simplified version of what I end up with. Remember, this is by no means a thoroughly vetted result; this is just me going "what if I do this?"

So let that be a lesson, I suppose. Because of the belief that Hesperopilio was an acropsopilionine, I had never even considered taking a closer look at it. As it turns out, I really should have!


Groh, S., & G. Giribet (in press) Polyphyly of Caddoidea, reinstatement of the family Acropsopilionidae in Dyspnoi, and a revised classification system of Palpatores (Arachnida, Opiliones). Cladistics.

Shear, W. A. 1975. The opilionid family Caddidae in North America, with notes on species from other regions (Opiliones, Palpatores, Caddoidea). Journal of Arachnology 2: 65–88.

Shear, W.A. 1996. Hesperopilio mainae, a new genus and species of harvestman from Western Australia (Opiliones: Caddidae: Acropsopilioninae). Records of the Western Australian Museum 17: 455–460.

Shultz, J. W., & J. C. Regier. 2009. Caddo agilis and C. pepperella (Opiliones, Caddidae) diverged phylogenetically before acquiring their disjunct, sympatric distributions in Japan and North America. Journal of Arachnology 37: 238–240.