A CARNIVOROUS WATER-BEETLE 33 



The mouth-parts of the adult are those of a predatory 

 beetle, but the mandibles are not slender and piercing like 

 those of a predatory land-beetle, but stout and double-toothed. 

 The feelers are long and slender, and the compound eyes large. 



The whole body is firmly knit together. The head is sunk 

 m the thorax, the prothorax bears on its under side a back- 

 ward-directed spine, which fits into a cavity in the mesothorax, 

 and the basal joints of the hind legs are soldered to the under- 

 side of the abdomen, which 

 therefore becomes particu- 

 larly rigid. 



The legs are of peculiar 

 form. The hind legs are 

 long, flattened, and fringed 

 with hairs, and it is chiefly 

 these which propel the body 

 through the water ; they can 

 be "feathered" like an oar, 

 so as to diminish the resist- 

 ance of the water during the 

 return stroke. Since the 

 basal joints are firmly at- 

 tached to the abdomen, 

 the movable part of the 

 leg acts from the middle 

 line of the body, and nearly 

 from the middle of its 

 length. I do not fully under- 

 stand the advantage gained 

 thereby, but am inclined to attribute to this disposition the 

 unusual power of changing its course, or the inclination of 

 its body, which this insect exhibits. The usual pair of stout 

 claws are insignificant in the hind legs, which are chiefly 

 serviceable as paddles. The intermediate legs appear to be 

 particularly useful in steering. For grasping the prey the 

 beetle relies on its fore legs, which are of very peculiar form, 

 especially in the male, where the first three joints of the 

 tarsus are enlarged and form a circular sucker. The joints 

 in question are studded with many cupules, or adhesive hairs, 

 and two of the cupules on the first joint of the tarsus are 

 enlarged to enormous dimensions. The adhesive power of 



Fig 23. — Male Dytiscus. 



