HYDRACHNID/li IN ( ACARIDES OF BERWICKSHIRE ' 29 



on the fact that in each case there are two small genital plates, each 

 with two discs. It is well known now that what was then considered 

 to be a distinct species, is only a stage in the life-history the 

 nymphal period. 



In 1892, after pointing out that Nesaa reticulata Kramer was 

 the nymphal form of the species which Haller later described as 

 Hygrobates gracilis, Koenike established Hygrobates reticulatus Kram. 

 as the name of the species. 



Piersig, in his large treatise on the German water-mites 

 (" Zoologica," H. xxii.), and again in his work on the family 

 Hydrachnidse ("Das Tierreich," Lief, xiii.), makes use of Kramer's 

 Hygrobates reticulatus. 



In recording specimens found within the Britannic area, C. F. 

 George, Halbert, Soar, and I have also continued the use of the 

 same name. 



As indicated by Haller, the main points in identification are the 

 structure of the palpus and the reticulated skin. Fortunately, Dr. 

 Johnston, when describing his specimen, described these two points 

 in detail. 



The only literature on mites available to Dr. Johnston was the 

 works of Miiller and Duges, together with Koch's " Ubersicht." Not 

 being able to identify his specimen from these, with some diffidence he 

 described it as a new species. His diagnosis is : " H. orbicularis 

 albescens maculis fuscis fuse signata, oculis 2, pedibus pellucidis 

 albis sparse spinosis, palpis articulis inferioribus crassis minutissime 

 serrulatis." 



After describing the shape and colour of the body, he gives the 

 following description of the palpus : " pediform, proportionately 

 large, thicker than the legs, 4-jointed, the ist and 2nd joints 

 thick and short, minutely serrulated on the outer edge, a rough or 

 granulous mucro at the articulation of the first joint, 3rd joint 

 elongate and slender, 4th small and unquiform, and terminated 

 with two minute claws." 



The normal number of segments in the hydrachnid palpus is 

 five, but a reference to John- 

 ston's figure shows that he has 

 not taken the basal segment 

 into consideration. Conse- 

 quently, the first, second, third, 

 and fourth segments of John- 

 ston's figure are respectively 

 the second, third, fourth, and PALPUS OF HYDRACHNA NAICA 



fifth segments of normal speci- (AFTER JOHNSTON). 



mens. 



Johnston's figure does not show the two hairs which stand one 

 behind the other on the flexor edge of the penultimate segment, 



