Forest Preserve District of Cook County, Illinois
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Water Beetles
Nature Bulletin No. 639-A   April 29, 1961
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis Supt. of Conservation

WATER BEETLES
The world is full of beetles. They live everywhere except in the oceans and in the polar regions. There are more of them than any other kind of insect. A quarter of a million species are known and new ones are being discovered every year. Whether it is a microscopic mushroom beetle a hundredth of an inch long, or a giant six-inch Hercules beetle from South America, it can be recognized by its wings. The upper pair forms a hard shell curving like a shield over the thin folded lower wings and the abdomen. In flight, the upper pair is extended like the wings of an airplane and the lower two become buzzing propellers.

One or more destructive kinds of beetles attack almost everything that we grow, or eat, or wear. Some, like the lady beetle, are beneficial because they destroy plant lice and other crop pests. However, the great majority of the world's beetles do not affect us directly, either pro or con. Among these are a few especially interesting types which spend their active larval and adult lives in fresh water.

Whirligig beetles are fun to watch as they glide like tiny motor boats over the surface of our ponds and streams. Hundreds of them often crowd together in a sheltered spot out of the wind and current. If a pebble is tossed among them, they race about at high speed in a mad frenzy of circles, loops and zigzags -- hence, they are sometimes called "Write-My-Names. " When pursued, they dive to the bottom and hide. The whirligigs are oval, flattened, and black with a metallic sheen. The larger species are about a half inch long and the smaller ones about a fourth inch. The second and third pairs of legs are short broad paddles. The front legs are used for holding the mosquitoes, midges, and other surface insects on which they prey. The eyes are one of their most interesting features. Each is divided into two parts by the edge of the head; thus, one pair looks for enemies from above and the other pair from below. Some kinds give off a protective milky fluid with an odor like apple seeds.

The Diving Beetles are adapted for a life in water but are awkward and almost helpless on land. The flattened hind legs are used together like oars. A number of different local species range from one-eighth inch to over an inch in length. They are oval in shape and colored shiny black or brownish. At rest they hang head down with the breathing pores on the tip of the abdomen above the water surface. Like most water beetles they carry an air supply trapped under the wing covers when they dive. Both the adult and its larval young, called Water Tiger, are blood- thirsty predators on the other animal life in our ponds. They attack, kill, and eat dragonfly nymphs, worms, snails, tadpoles and small fish -- often several times their own size. Sometimes they cause severe losses in fish hatcheries. The water tiger grabs its prey with the large hollow sickle-shaped jaws and injects digestive juice into it. After the flesh has become liquefied, it is sucked back into the stomach.

The Water Scavenger Beetles resemble the diving beetles in sizes, shape and color. In swimming they paddle alternately with the right and left legs, which gives them an erratic course. Their diet is both plant and animal matter, living or dead. In China they are dried, ground into powder and used to flavor sauces. The Crawling Water Beetles are a group of very small spotted species that creep about feeding on algae. The strangest one of all is the Water Penny. It looks like a round brown flat scale attached to a rock in rapid streams or on wave-washed shores. It is really the young of a very ordinary appearing beetle. Who says you can't get change for a penny.


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