222 EVENINGS AT THE MICROSCOPE. 



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 freely 

 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, and 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 enor- 

 mously dilated hip-joint, affording space for broad and 



powerful muscles. 



In the structure of the mouth it differs greatly 

 from 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 

 receive 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 oft 

 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. 



