1905 - 6 .] Collembola from the South Orkney Islands. 
475 
In the shape of the ridge surrounding its post* antennal organ, 
I. Brucei agrees closely with I. Beselsii ; hut while the latter has 
the sixteen ocelli which are usually present in species of Isotoma 
the former has only twelve, the two posterior ocelli of the inner 
row of four, on each side, being absent. The antennal organ 
consists, as in I. Beselsii , of two prominent hemispherical papillae 
at the extreme apex of the fourth antennal segment (fig. 2, a. <>.). 
The feet in the present species have, as in I. Beselsii , untoothed 
claws and no tenent hairs; the apex of the smaller claw is, 
however, drawn out in a slender process, which is not so fully 
developed in the northern species. 
The mouth-parts of Isotoma Brucei show several interesting 
peculiarities. The mandibles are remarkably narrow and parallel- 
sided at the apex (fig. 3), and exhibit two very prominent 
acuminate processes at the hind dorsal corner of the grinding 
area (fig. 3, mo.). The maxilluke (fig. 4, mxl.) have prominent 
apexes, armed with several stout curved bristles ; the spines, 
arranged in series along the inner edge of the basal region of the 
maxillula, are elongate and sharp-pointed. 
In the structure of the maxilla (fig. 4, mx.) I. Brucei differs 
from other species of Isotoma, and indeed from members of its 
family generally, by the slender and elongate form of the “ head ” 
(compare the typical semi-globose “head” of Cryptopygus, fig. 19). 
The head in the present species is composed of a strongly 
chitinised dorsal lobe or “galea” (fig. 4, ga.), terminating in three 
prominent teeth. Ventral to this, and protruding beyond it, is a 
delicate falcate lamella (fig. 4, la.'), fringed with long delicate hairs, 
while a smaller lamella, also fringed with fine hairs (fig. 4, la ."), 
lies internal to the galea. The palp (fig. 4, pa.} carries six 
prominent bristles, the most distal being inserted on a long 
acuminate process. The stipes of the maxilla (fig. 4, sti.) 
articulates with the cardo (fig. 4, car.), which is itself in connection, 
as usual, with the supporting “foot” and ligament (fig. 4, pd. lig .) 
of the tongue (fig. 4, tin.). 
Several at least of these characteristic features of the jaws in 
1. Brucei may also be detected in its northern ally I. Beselsii. 
The form of the retinaculum in 1. Brucei, as seen from the 
side, is shown under high magnification in fig. 8. 
