In terms of numbers of the species, insects are the earth's most successful animals, and we lack a true picture of their actual numbers. Of the insects, the beetles are the most numerous, and among them, the weevils form the world's largest family of animals. Although much maligned and rarely kept as aquarium animals, aquatic insects can provide a lifetime of fascination to those willing to look closely. One advantage in keeping such creatures is that they can be collected almost anywhere. And, indeed, this is half the fun of keeping them. Each new trip to even the tiniest of temporary ponds will yield new discoveries. Another reason to keep aquatic insects in an aquarium is the fact that you can usually observe all their behaviors, including reproduction, within the confines of a relatively small aquarium.

A true terror in its world, the giant water bug is a voracious predator. The first pair of legs have developed into large, piercing hooks, which can grasp and hold sizable prey. Species from temperate regions grow to be over 2 inches long, and tropical types can be twice that large. The rear legs have become adapted as effective swimming paddles, and the body terminates in a breathing tube, which can be thrust above the surface of the water.

Giant water bugs of all species are quite hardy in the aquarium and extremely interesting to observe. In some species the female glues her eggs to water plants at the water line, but in many she deposits them on the male's back, where they are quite safe from all but the largest predators. Giant water bugs can be maintained in an unfiltered bowl or a filtered aquarium. Be sure to provide upright sticks upon which they can rest and adequate space for them to move about, because they are quite active. In the wild, they feed upon all manner of insects, tadpoles, and even small fish and frogs. In captivity they are content with crickets or other insects, and they take quite readily to feeding upon bits of raw meat.

It is quite impossible to even touch upon the great variety of aquatic insects that are suitable for maintenance in the aquarium. Go out and explore, and experiment with the animals that you find. Maintenance for each must be tailored to the individual species, but some generalizations are possible.

Backswimmers and the closely related water boatman are quite comical to watch as they row madly (and, often, upside down) through the water with their greatly enlarged, hair-lined rear legs. They require aquatic plants or sticks on which to anchor themselves when they are not swimming and will feed on smaller insects and pieces of meat. Water scorpions are amazingly well camouflaged as waterlogged sticks. They lie in wait attached to aquatic plants, usually at the water's surface with their breathing tubes extending into the air, to snare small invertebrates.

One of the most entertaining insects that you can keep in the aquarium is the whirligig beetle. Possessing eyes that see simultaneously above and below the water's surface, these active beetles occur in large groups, remaining fairly motionless until they are disturbed or detect food. At that time they begin to spin madly about in circles (the source of their common name), and it is quite impossible to keep your eye on any one individual. They are extremely sensitive to the presence of food in the water; a bit of tropical fish flake food tapped into their aquarium will set them stirring. They also enjoy feeding upon other insects and will attack a cricket that is put onto the water's surface, tearing out bits of flesh with their welldeveloped mouth parts. They are quite easy to maintain on a diet consisting of other insects and tropical fish food.

Water scavenger beetles are, as their name suggests, useful scavengers in the invertebrate aquarium. They feed on all manner of decaying plants and animals and readily accept pelleted and flake fish food. One of the more interesting aquatic insects to observe is the larvae, or nymphs, of dragonflies. They stalk about the bottom of the fish tank in the manner of a hunting cat, pausing when they observe a possible meal. They slowly move toward their victim and, when about a body length away, rapidly extend the lower lip. This unique organ is hinged and tipped by a small cup with several grasping hooks. This implement snares the meal and pulls it to the mouth. Dragonfly nymphs seem to require live food, or at least the stimulation of movement to induce them to feed. They are quite easily raised on black worms and brine shrimp, and the largest species will also eat other insects, tadpoles, and even small fish. Be sure to provide some sticks that extend up above the surface of the water in the dragonfly larvae's tank. When it is time to transform into an adult, they will climb up out of the water, split their exoskeleton and unfurl their transparent wings. This generally occurs that night, and the adults will usually be found still on their sticks, hardening the wings, in the morning. If this occurs while you are not at home, the adults will usually fly toward the nearest widow, whereupon they can be released to perform their important task of feeding upon noxious insects such as mosquitoes.