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!

REFERENCES

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.