Sphaeriodesmidae, Humbert & De Saussure, 1869

The Sphaeriodesmidae View in CoL

is the only family in which either tergum 3 or both terga 3 and 4 can be enlarged ( Fig. 2 View Figs 1–11 ). Conglobation implies that the lateralmost, narrowed parts of the paraterga of several segments both subsequent to and in front of the broadest segment (3 or 4) are hidden under the latter. Starting from body segment 4 or 5, the lateral, subacuminate end of each subsequent paratergum is hidden under and behind the previous one, thus displaying an overlap pattern which can be termed typical. The telson usually is flattened, with a fully exposed, broad, plate-like epiproct; during complete volvation of the animal it is tightly oppressed to the dorsal side of body segment 5 or 6.

This family is characterised, among other things, by long and slender antennae and legs ( Figs 3, 4 View Figs 1–11 ); antennomere 5 is longer and often larger than antennomere 6; the paraterga are very prominent and relatively simple ( Fig. 3 View Figs 1–11 ), the dorsum is smooth to roughly tuberculate, the ozopores are very small to missing; the tergal limbus is evident but nearly smooth; the gonopod aperture is transversely oval and modest in size; the gonopods are simple to relatively complex ( Figs 5, 6 View Figs 1–11 ), highly variable but subcylindrical coxae nearly always have a sternal rudiment, the cannula is normal to hypertrophied basally. The body is usually medium-sized and 15–35 mm long. Further structural details can be found in Hoffman (1982 b 1990) and Simonsen (1990). Nearly completely volvated members of the subfamily Desmoninae are depicted in Shelley (2000).

Sphaeriodesmids currently comprise 14–15 genera and about 90 described species, but the actual diversity exceeds 200 species (Hoffman 1990; Hoffman et al. 2002). The family is endemic to Central America and the Greater Antilles, ranging from between north of Panama to Missouri and Kentucky in the USA.

What seems particularly remarkable is that this family still comprises at least one genus and species, the cave-dwelling Mexican Proeilodesmus mecistonyx Hoffman, 1990 , that is apparently not capable of volvation (cp. Figs 3 & 4 View Figs 1–11 ). Most of the characters of this creature seem plesiomorphic and suggest a link between a presumed flat-bodied chelodesmidean ancestor and the remaining, convex, truly volvatory members of the family (Hoffman 1990).

In Sphaeriodesminae, the prozona are reduced to virtually absent ventrally, paralleling the situation in Oniscomorpha , but in Desmoninae , a group of Sphaeriodesmidae , the prozona are about the same length all the way around. Desmonines thus resemble a flattened oblate spheroid (= disc) rather than a true sphere as in species of Sphaeriodesmus Peters, 1864 . Being about 15 mm wide, S. mexicanus (De Saussure, 1859) , from Mexico, is probably the largest polydesmidan currently known to roll up.

The phylogenetic relations of the Sphaeriodesmidae seem particularly close to the Holistophallidae , a small group endemic to Central America and incapable of volvation (Hoffman 1980 1982 b; Hoffman et al. 2002).

In all other ‘oniscoid’ polydesmidan families, it is only the second tergum that is more or less enlarged and adapted for volvation. In addition, the antennae, and usually the legs, are relatively short; the latter are sometimes separated to a varying degree, even on the same body segment ( Fig. 42 View Figs 37–44 ).