380 SUBANTARCTIC ISLANDS OF NEW ZEALAND. [CoUetnbola. 



to this genus —T. rnichaelseni and T. rosea — are stated to be without this structure, 

 while in Willem's Triacanfhurus davatus it is distinctly present. Its vestigial con- 

 dition in the present species is therefore intermediate, and warns us against always 

 relying on its presence or absence as a safe generic character. 



Catch. — The catch, or retinaculum, on tlie 3rd abdominal segment (figs. 14, 15), 

 is of the usual form ; each of the appendages, fused at their bases, carries on its 

 distal outer edge 4 distinct teeth. 



SppaNG. — The spring in T. alha furniidies an interesting stage in the degeneration 

 of that organ. In the adult insect each united dens and mucro usually consists of a 

 finger-shaped process covered to the tip with the characteristic cuticular granulations, 

 except at the proximal anterior region, where 2 curved ridges are visible (fig. 16). 

 At an earlier stage (fig. 17) but little dift'erence can be seen, only a slight constriction, 

 which may indicate the boundary between dens and mucro. But in a voung speci- 

 men of 0-75 mm. in length the spring (fig. 18) has the mucronal region quite distinct, 

 and ending in a blunt tooth ; its condition resembles closely that figured in T. davntus 

 (Willem, 1902, pi. ii, fig. 1). In both T. rosea and T. rnichaelseni the mucro is large, 

 with several teeth, and the dens carries a broad scale -like structure at its tip ; pro- 

 bably this scale is represented by the small oval process alongside the mucro in the 

 young individual of T. alba now figured. In the present species the characters that 

 I have noticed and figured are not constant in individuals of the same age ; the 

 mucronal process (fig. 18) cannot be distinguished in all of the very young specimens, 

 while it is quite evident in some nearly full-grown insects. 



Musculature. — Several of the specimens afford a good display of the principal 

 muscles. There are dorsal, lateral, and ventral longitudinal strands (fig. 1, d, I, v) 

 in much the same position as described by Lubbock (1873, pp. 94-99) for Tnmocems, 

 and in each abdominal segment there are vertical (p) and oblique (o) muscles running 

 from the tergal to the sternal region. The muscles that work the legs arise from 

 the dorsal aspect of the thoracic segments. Two pairs of extensor and 2 of flexor 

 muscles can be determined in connection with the spring (fig. 1, e, /). 



DISTRIBUTIONAL NOTES. 



In a paper on Collewhola from the South Orkney Islands (190(i) I liave discussed 

 the bearing that the distribution of these frail insects has on problems of ancient 

 geography. There we have a faunistic group with distinctly northern, even Arctic, 

 affinities, and a characteristically Antarctic group, including some genera apparently 

 unrepresented elsewhere than in the far south. Here again in these islands to the 

 south of New Zealand the two species recorded in the present paper belong to the 

 same two geographical sections. Achorutes viaticus is distinctly a link with the 

 Northern Hemisphere, and even with the Arctic regions, while Triacnnthdla alba 

 recalls the subantarctic fauna of South America. Such species as the latter 

 suggest a former wide extension of the Antarctic Continent, perhaps in late 

 Mesozoic times. The existence of the northern Achorvf"s viaficus on these remote 

 southern islets ]>()ints to a still earlier period of connection with the great con- 

 tinental tracts. 



