The Problem with Sacesphorus

Probably not the subject of today's post: an unidentified assamiid from Thailand, from here.

One of the most frustrating things about many older taxonomic resources can be the shortage of illustrations. Up until the early part of the twentieth century, at least, it was something of a rarity for a publication to include extensive figures of their subject(s). So when I was presented for my semi-random subject of the week with the Burmese assamiid Sacesphorus maculatus, described by T. Thorell in 1889, I was not entirely surprised to discover that this Asian harvestman has never actually been illustrated.

At present, Sacesphorus maculatus is the only recognised species in its genus, known only from the Bago region in southern Burma. Unfortunately, that in itself doesn't necessarily indicate much. The Assamiidae are perhaps the most diverse group of Laniatores (short-legged harvestmen) in the tropics of the Old World, but they are also some of the least studied. The last extensive revision of the family was by our old friend Carl-Friedrich Roewer in 1935, and like many of Roewer's classifications its accuracy is suspect. Roewer divided the assamiids between seventeen subfamilies, but the characters separating most of these subfamilies are fairly superficial and probably do not reflect actual relationships (Staręga implicitly synonymised some of the African subfamilies in 1992 when he synonymised genera from different 'subfamilies' together). Roewer placed Sacesphorus in the Erecinae, which he characterised by features such as the absence of a pseudonychium (a claw-like process between the two true claws at the end of each leg), smooth leg claws, a two-segmented telotarsus on the front legs, and the absence of a median spine along the front edge of the carapace. All of these are fairly generalised characters, and some (such as tarsal segment number) are probably more variable than Roewer realised. Many short-legged harvestmen possess a pseudonychium as nymphs but lose it as they grow into adulthood, and the distribution of this feature in the assamiids may require more investigation.

Similar issues attend the identification of assamiid genera. Thorell (1889) originally distinguished Sacesphorus from the genus Pygoplus, also found in Burma and eastern India, by the presence in the former of a small spine in the middle of the eyemound. Roewer recognised a number of 'erecine' genera in Burma and eastern India, largely on the basis of tarsal segment numbers and armature of the dorsum. The relationship between all these genera deserves a second look. Many other groups of harvestmen have been successfully raised from the Roewerian quagmire in recent years; the assamiids are still waiting.

REFERENCES

Roewer, C.-F. 1935. Alte und neue Assamiidae. Weitere Weberknechte VIII. (8. Ergänzung der “Weberknechte der Erde” 1923). Veröffentlichungen aus dem Deutschen Kolonial- und Uebersee-Museum in Bremen 1: 1–168.

Staręga, W. 1992. An annotated check-list of Afrotropical harvestmen, excluding the Phalangiidae (Opiliones). Annals of the Natal Museum 33 (2): 271–336.

Thorell, T. 1889. Viaggio di Leonardo Fea in Birmania e regioni vicine. XXI.—Aracnidi Artrogastri Birmani raccolti da L. Fea nel 1885–1887. Annali del Museo Civico di Storia Naturale di Genova, Serie 2a 7: 521–729.