39 20 Proceedings of the Royal Irish Academy. 



has been described from Switzerland by Dr. Walter (32, p. 529). The pro- 

 visional genital area of one of these immature forms is figured in the present 

 paper (PL II., fig. 17). 



Distribution. — The known range is Norway ; Germany; Switzerland; Italy; 

 and Britain (7). 



Megapus sprnipes (C. L. Koch), nov. var. montanus. (Plate II., fig. 15). 



Stream on Croaghpatriek ; Bunowen stream at Louisburgh. 



The specimens from these localities are to be ref erred to the large steno- 

 thermal form of M. spinipes, for which it would be convenient to have a 

 distinctive varietal name. Compared with the smaller eurythermal form this 

 variety, for which I propose the name montanus, is distinguished by the 

 following characters: — 



montanus nov. var. 



Decidedly larger, the Irish specimens ranging from 1 to l - 40 mm. in length. 

 More richly coloured than the typical form, some specimens are suffused with 

 a pinkish tinge ; the legs are hyaline or pale yellow. 



The most distinctive character is the absence of swimming-hairs (in the 

 typical form, as it occui's in stagnant and slowly flowing water, there are a 

 few of these hairs on the last three pairs of legs). The claw-arniature, 

 especially of the first pair, is more strongly developed, with a large blade-like 

 basal part. The terminal segment of the first leg (PI. II., fig. 15) is rather more 

 strongly bent. 1 Length of first pair of legs l - 280 mm., fourth pair 1740 mm. 



The palps (480 mm.) are relatively longer, and of more robust structure 

 than in the lowland form, with a less truncated apex to the fourth segment, 

 and the sword-like spine on its inner side is much stronger. Certain of these 

 characters are such as we should expect to find in a stenothermal form. The 

 absence of swimming-hairs, combined with the greater development of the 

 claw-armature, is an adaptation for life amongst aquatic mosses, or for lurking 

 under the stones of the rapid highland streams in which this form reaches its 

 greatest development. 



The variety montanus brings to mind the interesting stenothermal form, 

 Diplodontu-s torrenticolu-s recently described by Dr. Walter from Italy (" grossen 

 Quelle bei Sarno"). In this species there is a great reduction in the number 



1 The terminal segment of the first leg of the typical eurythermal form is figured (PI. II., fig. 16) 

 for comparison; the specimen from which it was drawn occurred in quarry holes near Ciurnlin, 

 Co. Dublin. 



