Zipping about on the surface of the water the whirligig beetle is quite a sight to see. These are the only beetles that swim within the surface film of water creating a whirling motion with their shiny bodies that makes them easy to see even from quite a distance away. Whirligig beetles are from the Gyrinidae family of insects in the Coleoptera order which also means beetles. These mesmerizing little insects are difficult to get close to since they are very fast and wary of any movement around them.
But even from a distance, this intriguing beetle has many interesting features to their movement. Undisturbed the move across the water in inches without making even a ripple in the water but when they are disturbed they begin whirling around in continuous small arcs making a wake of ripples in front of each themselves while remaining in a tight group.
This is the movement for which they are named and what is fascinating about this motion is that they make this motion without ever bumping into one another. The third motion which is the least common is jerky, creating concentric ripples around their bodies as the beetle remains in the center.
Mating of the whirligig beetle takes place in early spring with both the male and female staying together after mating for anywhere from a few minutes to many hours. The females laying masses or rows of eggs on the stems of submerged vegetation. Once mating is complete and the eggs have been laid both adults will die within a few weeks. The larvae hatch in about two weeks and begin feeding underwater on insects or other small animals until they reach the adult stage which can take up to three months. As adults, they will leave the water to make pupal cells near their homes.
This is often done on the stems of plants or in muddy areas with the beetles remaining for around a week prior to emerging as new adults to form large aggregations on the surface of the water. In large lakes, these massive aggregations can contain up to 200,000 beetles. In the fall whirligig, beetles will move underwater where they can overwinter among the mud and debris at the bottom.
This fascinating insect is usually between a one-half inch to one-quarter inch long depending on the genera. There are two genera, the Dieutas and Gyrinus, both of which have a thin pair of front legs that are used for grasping their prey. The flattened second and third pairs of legs are much shorter but well suited for swimming.
This beetle swims with half its body out of the water and half-submerged as it watches for prey and predators in both areas. This is done with the remarkable adaptation of the eyes which are split into two different parts. One half of the eye is adapted for seeing beneath the water while the other half is adapted for seeing above the water. The antennae are used to detect any ripples on the surface of the water since an organ at their base called the Johnston’s organ can detect even the slightest change.
These antennae are held on the surface of the water where changes are detected in relation to the angle of the antennae. This alerts the whirligig beetle when other insects are struggling on the surface of the water or if an object is in their path that will need to be avoided. It is believed that this is done in the manner of an echo system in the water that is similar to the sound wave echo system used by bats.