3098 THE JOINTED ANIMALS 



furnished underneath with sucker-like hairs; while in this sex also the back is gen- 

 erally smooth and glossy, the elytra of the females frequently have a ribbed or cor- 

 rugated surface. The Dytiscidce seem especially fond of stagnant waters, and some 

 of the species are common objects in our ponds and ditches. They come to the sur- 

 face when it is necessary to take in a fresh supply of air beneath the elytra. These 

 organs fit very closely against the sides of the body, and so prevent the air from 

 escaping while the beetle is swimming about under the water; but the air mean- 

 while is being used up in breathing by means of the thoracic and abdominal spira- 

 cles. The beetles fly strongly, and on fine summer evenings may sometimes be seen 

 winging their way to new quarters, a change which is often necessitated by the dry- 

 ing up of the pools in which they had previously been living. Dytiscus marginalis, 

 one of the largest British species, is also one of the commonest and best known. 

 Another common species, Acilius sulcatus, is also represented in our figure. 



The Gyrinidce or whirligig beetles are a small but very well-defined group, and 

 in many points of structure are sharply distinguished from the other families of the 

 tribe Adephaga. In their oval shapes they resemble the Dytiscidce, though they are 

 usually somewhat flatter below and a little more convex on the upper side. But in 

 the relative proportions of the three pairs of legs they are 

 entirely different. The fore-legs are long and slender, and 

 when stretched out look like arms, whereas the two hinder 

 pairs are short and broad, being modified for use as paddles in 

 swimming. Another very distinctive feature is presented by 

 the eyes, each of which is divided by a ridge on the side of the 

 head into two widely-separated portions, one lying on the 

 COMMON WHIRUGIG upper side of the head and the other underneath. These beetles 



BEETLE, Gynnus a pp ear> j n consequence, to have four eyes; one pair, as it is said, 

 natator (enlarged.) . , , t . . . ., r - , . 



though there i!s no proof of the fact, for espying objects above 



them, the other for looking at things in the water below. From the Dytiscidce and 

 Carabidce they differ further in having their antennae shorter than the head, and the 

 outer lobe of the maxillae either completely atrophied or else in the form of a slender 

 spine. The Gyrinidce, though widely distributed and represented in almost all parts 

 of the world, include altogether rather less than three hundred known species. The 

 genera are few in number and two only occur in Europe. Some of the British spe- 

 cies, such as Gyrinus natator, are commonly to be seen in ponds and canals or 

 "holes" in reedy sluggish streams, where the shiny little beetles attract atten- 

 tion by the ease and rapidity of their movements as they skim about on the sur- 

 face of the water, performing a variety of intricate evolutions, some sweeping 

 along in graceful curves, others going round in circles or spiral tracks, now all col- 

 lecting together in groups, and then, if startled, suddenly darting off with amazing 

 speed in every direction. 



The next beetles we have to consider are those which, on account of their ab- 

 breviated wing cases, are known as the Brachy elytra. This tribe to which, how- 

 ever, not all beetles with short elytra belong, contains a single very large family 

 the Staphylinidce . Owing to the shortness of their elytra, and the usually nar- 

 row and elongated form of their bodies, the rove beetles have an easily recognized 



