J 34 
INSECTS. 
COMMON WHIRLIGIG 
beetle, Gyrinus 
natator (enlarged). 
is swimming about under the water; but the air meanwhile is being used up in 
breathing by means of the thoracic and abdominal spiracles. 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 drying 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 upper 
side of the head and the other underneath. These beetles 
appear, in consequence, to have four eyes; one pair, as it is said, 
though there is 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 antennas 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 species, 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 attention by the ease and rapidity of their movements as they skim 
about on the surface 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 collecting 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 
abbreviated wing-cases, are known as the Brachyelytra. This tribe to which, 
however, 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 
narrow and elongated form of their bodies, the rove-beetles have an easily recognised 
and characteristic appearance. The head is generally large and flat with a narrow 
neck behind where it fits into the prothorax. The antennae—composed of eleven, 
or occasionally twelve joints—are usually filiform, but are often slightly thickened 
towards the extremity, and in some cases end in a distinct club. Though prominent 
and conspicuous in a few genera, the eyes are, as a rule, raised but very little above 
the general surface of the head. It is interesting to note that ocelli, which are of 
such rare occurrence in adult beetles, are to be found in certain groups of this 
