BEETLES 347 



Family 3: Water Beetles(Dyticidae). 



The Common Water Beetle (Dytieus marginalis) . 



(Length about 1\ inches.) 



The water beetle and its allies are, in contrast to all other beetles, 

 aquatic animals, and hence in many parts of their anatomy exhibit an 

 entirely different structure from that of other Coleoptera. To understand 

 its structure and habits, the insect should be put into an aquarium. If 

 in thus transferring it the beetle is somewhat roughly handled, it emits 

 from the anterior and posterior margins of the prothoracic shield an 

 evil-smelling milk-white fluid by which it tries to alarm us (and any 

 other enemies). (Compare with gold beetle, etc.) Like many other 

 aquatic animals (give examples), the beetle is much darker on its upper 

 than on its under surface, and when at rest on the dark bottom of a 

 pond or among the confused tangle of water-plants cannot easily be 

 distinguished (under side yellowish-brown, upper side dark olive green ; 

 prothoracic shield and elytra with yellow margins — hence its specific 

 name). As, like all other insects, the beetle breathes by trachea, it has 

 periodically to ascend to the surface of the water. In doing this, it 

 protrudes the end of the abdomen for a moment above the water and 

 slightly raises the wing-covers, under which are situated the spiracles 

 (show these). In this way the used-up air is expelled and pure air 

 taken up. Having thus supplied itself with a fresh store of air, in the 

 shape of a silvery bubble of air adhering to the hairs of the back under 

 the elytra, the beetle once more dives below the surface (compare with 

 the water spider). Notice how nimbly and rapidly it swims, its flattened, 

 sharply-margined body cleaving the water with ease, and the hind-legs 

 acting as effective oars. Notice also the peculiar structure of these 

 limbs (natatorial legs). They are very long (the third thoracic segment 

 is for this reason the largest; see cockchafer), and are capable of being 

 moved only from behind forwards, and in the reverse direction (by what 

 arrangement is the motion thus limited ?) ; all their parts are strongly 

 compressed laterally, and the tibia and joints of the tarsus are covered 

 with two rows of strong hairs, by which the rowing surface is consider- 

 ably increased. The first and second pairs of legs, though locally 

 furnished with hairs, are in other respects constructed like those of land 

 beetles, being, in fact, used much less for swimming than for climbing 

 among and holding on to the tangle of water-plants and for progress 

 on land (see also below). The three first tarsal joints of the front-legs 

 of the male assume the form of large disc-like suckers, which are used 



