222 EVENINGS AT THE MICROSCOPE. 



can be either widely expanded, or brought together so as 

 to form a covering to the labium. They are pincer-form, 

 like the claws of a crab, the two fingers being strongly 

 toothed on their opposing surfaces. They thus form 

 effective prehensile instruments. These mandibles can be 

 advanced separately or together, and the whole head can 

 be elevated or depressed. 



In the water of ponds we may frequently see, playing 

 among the sub-aquatic vegetation, bright-coloured Mites ; 

 sometimes rich velvety green, sometimes purple, but more 

 commonly brilliant scarlet ; often curiously marked with 

 sinuous patterns or spots of black. They swim rapidly 

 and evenly by means of rapid rowings with their legs, 

 which are thickly fringed with long hairs. I have one 

 here, which seems to be the Hydrachna histrionica. It is 

 a little, flat, circular, cushion or cake-like creature, scarlet, 

 with four clouds of black on its back, about one-sixth of 

 an inch in diameter. You may notice the effective oars 

 which the legs form by means of their thick fringe of hair, 

 and in particular the power which the hind pair possess by 

 reason of the enormously dilated hip-joint, affording space 

 for broad and powerful muscles. 



The structure of the mouth differs greatly from the 

 same parts in the Cheese-mite. The palpi here are long 

 and perfectly free throughout ; the fourth joint is long 

 and slender, and is curiously hollowed at the end to re- 

 ceive the terminal joint, which forms a short claw, and 

 which falls down upon the former. The mandibles, too, 

 are not pincers, but consist each of a thick joint, cut off 

 obliquely behind, like the nib of a pen, while the other 

 extremity is blunt and broad, and bears a strong curved 

 claw ; the lip is oval, and cleft in the middle, and is wedged 

 in between the bases of the first pair of legs. 



