DB. JOHNSTON ON THB ACARIDE8 OF BKBWICKSHIRK. 315 



insertions, and 6'jointed ; the first pair scarcely stouter bat 

 longer than the second and third, which are nearly equal, 

 but the hinder pair are the longest and extend far beyond 

 the body ; Ist joint very small, 2d longer, 3d still longer, 4th 

 twice as long as the third, 5th as long as the fourth, 6th (Fig. 

 b, c) about the length of the fifth, slenderer, obtuse, obliquely 

 truncate and excavated at the extremity, which is armed 

 with a few short spines and weak bristles, and with three ? 

 claws moving in the same direction: Bristles smooth and 

 sharp, patent, not longer than the diameter of the joints ex- 

 cepting those at the articulations, the bristles on the middle 

 joints in whorls : JSkin when highly magnified minutely ar- 

 eolar. 



Found amongst submerged plants in the Whitadder. 



This is a small species, about one-half the size of the Atax 

 histrionicus. It is very conspicuous, however, in the water 

 from the whiteness of its markings, for the colour is other- 

 wise liable to great variation. When I first found the insect 

 the body was beautifully marbled with irregular black patches, 

 divided by greenish-yellow irregularly branched lines ; and 

 instead of a large white patch beneath, pale lines only ra- 

 diated from the vent. The insect was then, I suppose, 

 gorged with food, and it was not until being kept some time 

 in a phial of water that the markings were defined as we 

 have described them. In investigating the species, this va- 

 riation in the colouring must be kept in view. It moves 

 with quickness, keeping the palpi bent. It creeps on the 

 bottom and over plants, but I did not observe it ever to swim 

 in the water, and the structure of the legs would lead us to 

 believe that it cannot do so, for the legs are not furnished 

 with the long cilia which garnish those of most other aquatic 

 species. It readily throws itself on its back. The structure 

 of the end of the tarsal joint is very curious : it is dilated a 

 little and excavated like a spoon on one side, the edge armed 

 with one or two small spines and weak bristles, but the claws 

 are attached to a moveable centre-bit or tendon, and by its 

 means can be withdrawn into the spoon-shaped sinus or 

 pushed beyond its rim (Fig. 6, c). The advantage of this ad- 

 mirable structure to an insect that creeps amidst hair-like and 

 matted confervae is obvious, for, with the claws withdrawn, 

 it can glide through the entangled masses without check and 



