490 W. WILLIAMSON AND C. D. SOAR ON BRITISH HYDRACARINA : 



on the flexor surface, and two swimming hairs only. The small 

 spines of the sixth segment are generally six in number. 



The genital area lies about one-fifth of its length out of the 

 recess or bay formed by the inner ends of the epimera. The 

 genital area is about 0*24 mm. in length, and at its broadest 

 about 0*1 7 mm. The anterior and posterior sclerites for muscle 

 attachment are not very robust. The valves are about 0*22 mm. 

 in length, and have five pairs of coarse pores along their outer 

 margins, and numerous hair pores along their inner edges. The 

 hairs are long, reaching almost to the anus. The two anterior 

 pairs of acetabula are very large, the posterior pair almost 

 rectangular. The gland pores occupy the normal position. The 

 anus lies about midway between the genital area and the posterior 

 body margin. Sexual dimorphism appears to be limited, so far 

 as external appearance is concerned, to the posterior region 

 of the genital area being rather wider in the male than in 

 the female, and to the hair pores along the edge of the genital 

 valves being more numerous, viz. twenty-four to twenty-eight for 

 the male, and twelve to twenty for the female. 



Nymph. 



Along with the imaginal forms, Thor obtained two nymphs, 

 which bear so close a resemblance to the imago of this species, 

 that he was constrained to accept them as nymphs of tau- 

 insignita. The body in comparison is rather more extended, 

 and is 0*68 mm. in length and 0'43 mm. in breadth. The skin 

 is striate and minutely porose. The structure of the palpi is 

 very much as in the imago, but the equipment of hairs is very 

 much simpler. The hairs are confined to the extensor surface. 

 The first segment is devoid of any hairs. The second has one 

 small one midway, and two very long distal ones close together. 

 The third segment has one long distal bristle and one a little 

 way behind it. The fourth segment has two or three very fine 

 hairs, of which one is well back, about or a little behind the 

 middle. The flexor surface resembles that of the imago. The 

 epimera differ from the adult form in that the fourth pair has 

 the appearance of not being properly developed. The posterior 

 angle has a little papilla-like projection. The legs are about 

 0*38 mm. long in the first pair to 0*75 mm. long in the fourth 



