376 British Hydrachnidce : The Genus Fiona. [Sess. 



common forms of the aperture being cherry or trefoil shaped. 

 The genital plates lie close on each side, and in some cases 

 extend backwards sufficiently far to enclose the anal opening. 

 The last segment of the third pair of legs is very much modi- 

 fied as compared with the corresponding segment of the first 

 and second pair of legs. The claws assume different forms in 

 different species. In spring-time, large numbers of the males 

 taken will be found to have their third pair of claws firmly 

 fixed in the genital pouch : indeed, so tightly are they fixed, 

 that it is often difficult to remove them without destroying 

 part of the structure. The fourth pair of legs also have a 

 peculiar formation, the patella being adapted for seizing and 

 retaining hold of the female. The distal end of this segment 

 has two or more long swimmiug-hairs, — a feature which is 

 rather important for identification. 



The colouring varies very much even in the same species, 

 as will be seen on reference to Fiona rufa. Some species are 

 almost colourless, while others are dark or brilliant, — Piona 

 longipalpis, Kren., being a good example of this. 



Briefly, the generic characters of Piona are : Body, soft- 

 skinned ; all legs supplied with swimming-hairs ; claws to all 

 feet ; third pair of feet of the male and the patella segments 

 of the fourth pair of legs much modified ; on each side of 

 genital aperture six or more acetabula, either on special plates 

 or free on body skin ; epimera in four distinct groups ; eyes 

 wide apart. 



In the Plates the bodies are all drawn to the same scale, 

 to allow of easy reference to their relative proportions. The 

 other figures are drawn to show structure and arrangements 

 of parts only, not for comparison in size. 



There are twenty-one species to consider, — twenty already 

 recorded, and one new one. Of the eleven species described in 

 * Science Gossip,' it is intended to leave P. ambigua out, as it 

 was only described from a nymph. 



There are only a few men who have worked at the Hydrach- 

 nids at all in Britain, and the ground they have covered is as 

 yet very small, so there is no reason why the present records 

 of Piona should not be very considerably augmented. In 

 Ireland we have records by Mr Halbert and Dr Freeman; 

 in Scotland by Mr Williamson, and a large collection made by 



