The Whirligig-beetles are easy to recognize on account of their characteristic circular gyrations when disturbed, and by their habit of associating in large numbers in quiet places. When one of these, groups is disturbed they exhibit such activity that they well deserve their name, "Crazy-bugs." The eyes of these beetles are very peculiar in that each eye is divided into an upper and lower part. Thus the insect has practically an upper and lower pair, one adapted for sight at the surface, and the other for vision under water.
The Whirligigs do not seem to be very particular about their food, as they will accept both live and dead insects which fall into the water, and even under some circumstances will feed upon plants. When a beetle plunges beneath the surface, as he often does when frightened, he carries down a small bubble of air between the ends of his horny wings and the tip of his body. On account of his body being lighter than water it takes some effort to dive, but none to rise to the surface. The two hind pairs of legs which are used so much in swimming are very much flattened and plate like, making excellent paddles, as is shown by their exceedingly rapid movements.
The Water-skaters, or Striders, prefer quiet waters, as do the Whirligigs, but do not have the decided social disposition, shown in the latter to such a marked degree. These Skaters, on account of their long legs and short bodies, are the "Daddy long-legs" of the water. These characteristics and their habitat make them easy to recognize. They are nervous, active insects in their movements, jumping and skimming about on the surface with but little show of grace and ease as compared with the ordinary graceful curves of the whirligigs. Their food habits are very similar to those of other surface insects, that is, dead and dying insects found floating on the water. But their method of taking food is quite different from that of the Whirligigs, because of the great difference in the structure of their mouth parts. The Whirligig, being a beetle, has the typical biting mouth parts, while the Strider has a slender beak or proboscis, by means of which it sucks the juices from its prey, as do other bugs. The fore legs are used to seize the prey and bring it within reach of the beak. The middle and hind pair are used for rowing over the surface, the latter pair, primarily, for steerage, the fine hairs on the legs making it possible, as mentioned before, to make use of the surface tension. Large dimples are formed on the surface of the water where the feet touch it. One would hardly expect it possible for an insect standing on the surface of water to get its feet dirty, yet the great care which they give to cleaning their feet clearly shows that dirt is of common occurrence, even there. The white or gray color on the lower side of the body is due to the reflection of light from minute hairs which cover the surface, and keep the insect dry even when submerged.
The marine relatives of our Striders have some curious habits. Some of them live out at sea, hundreds of miles from land, where they are thought to feed upon the dead bodies of small animals. When the surface of the sea is calm they glide in colonies quickly over the surface, showing great skill in diving, but if the sea begins to become agitated they immediately disappear from the surface.
Perhaps the most remarkable habit which a surface insect has is that possessed by some of the allies of the Skaters, which not only swim in the water, but actually run on the under side of the surface film. It would be very interesting to know how such a habit was acquired.
Another interesting group of insects are those which breathe air, as the surface film insects, yet seek their food below the surface. These insects are compelled, on account of their air-breathing habits, to repeatedly visit the surface or communicate in some way with a fresh supply of air.
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| FROM COL. CHI. ACAD. SCIENCES. | INSECTS. Life-size. | COPYRIGHT 1900, BY A. W. MUMFORD, CHICAGO. |
| Archimerus calcarator | Nezara hilaris | Metapodeus femoratus |
| Oncopeltus fasciatus | Leptoglossus phyllopus | Conorhinus sanguisugus |
| Ranatra fusca | Benacus griseus | Cicada septemdecim |
| Galgulus oculatus | Zelus bilobus | Platycotis sagittata. |
We have two families of the large-sized water beetles, common in our ponds and streams. The Predaceous water-beetles and the Water-scavenger beetles. These are easily recognized, because in the former, the antennæ are thread-like and not enlarged at the tip. While the members of the Water-scavenger family have the antennæ enlarged or club-shaped at the tip. The Predaceous water-beetles are often quite common under electric lights, where they have been attracted by the intense light. Their large size and clumsy movements, when out of water, attract attention. But when seen in water their skill as swimmers is in striking contrast to their awkward movements made on land. The hind legs are flattened and very powerful, the surface being increased by a fringe of strong hairs on the inner side. In swimming the stroke is made by both legs at once. Perhaps the most interesting facts about these beetles are those associated with their method of breathing.
The horny wing cases covering the abdomen are very thick and fit close against the abdomen, except at the extreme posterior end of the body. The space between the wing cases and the upper surface of the abdomen forms a large air space. The spiracles, or openings into the respiratory system, are situated at the margins of the upper side of the abdomen. When the beetle comes to the surface for a fresh supply of air it exposes the tip of the body and then by a depression of the tip of the abdomen allows a fresh supply of air to enter into the cavity below the wing covers; this cavity is then closed and the beetle is ready for another trip under the water. When resting in the water they float with their head downward and the end of the abdomen slightly projecting from the water; thus a fresh supply of air is easy to secure. In their food habits these beetles are predaceous, and in addition to other insects, will even kill small fish.
The Water-scavenger beetles are not such perfect swimmers as the Predaceous ones. When the latter makes a stroke in swimming it strikes with both hind legs, while the Scavengers strike alternatingly with the hind legs. Their method of securing and carrying air, as with other water-beetles, is remarkable. In addition to the air reservoir under the wings, they have on the under side of the body large hairy areas which communicate with the one under the wings. All the air spaces are thus in direct communication. The respiratory openings in the Predaceous water-beetles open on the upper side of the abdomen, but in these beetles they are on the lower side and surrounded by short hairs which preserve the air film on the lower surface. When the fresh air supply has been exhausted the beetle comes to the surface, tips the body slightly, so as to bring the region on one side of the body just behind the head, to the surface. The long antenna which is folded backward and reaches to the rear part of the head, occupies an air space in its apical half, and in addition is covered by fine hairs, thus being doubly protected from being wetted. At the moment the beetle reaches the surface, by a stroke of the antenna (on the side which is nearest the surface, the body being tilted), the film from the air space in which the antenna rests is carried upward and outward to the surface of the water, thus forming an opening to the exterior. By movements of the wings, aided by bellows-like contractions and expansions of the body, a fresh supply of air is pumped into the air reservoir.