Natural History, December 2017–January 2018
Skimming over large bodies of water is a favorite pastime all over North America. Whirligig beetles (family Gyrinidae) have been doing it for at least 200 million years, from at least the beginning of the Jurassic period. If threatened, they may dive and stay underwater, breathing oxygen from an air bubble held on their back, under their wing covers. As it is used up, the oxygen in the bubble is replenished by partially extruding it to the water (where the bubble acts like a physical gill; oxygen from the water diffuses in, and carbon dioxide from the respiration of the beetle diffuses out). The beetles may also fly to another pond, lake, or stream if the one they are on or in becomes unsuitable. Usually, though, these shiny black beetles, up to a centimeter in length, idle conspicuously on the water’s surface. I found thousands of them in the late 1970s on Lake Itasca in northern Minnesota, where I helped teach a field course at the University of Minnesota’s Field Station located at the edge of the lake.
Along the shore of Lake Itasca, I saw these whirligigs aggregated into tight groups, or pods. Hundreds and sometimes tens of thousands of individuals occupied a square meter. They barely moved except when I paddled within several meters of them, at which point they made the surface boil with their frantic swimming; then, within ten to fifteen seconds, they reaggregated and settled down in the same place. What could these beetles possibly be doing, loitering all day in large flotillas? I wondered. Yet after dark, the individual beetles’ V-shaped wave tracks were conspicuous as they skimmed along near shore, far from any pod. I wondered where the isolated beetles had come from, and where were they speeding to. I thought the question might be answered by chasing the beetles by canoe, and because canoe skills would be essential for following these fast skimmers, I recruited Dr. F. Daniel Vogt, then a biology student and an athletic outdoorsman, to help.
Over the next three weeks the beetles put our canoeing skills to the test, as Dan and I almost daily and sometimes nightly canoed a survey course along the twenty-two-kilometer shoreline. We found twenty-seven beetle pods, totaling approximately 400,000 beetles. Each pod contained from 50 to an estimated 200,000 beetles. Day after day each pod remained in its same place, but the number of beetles in some increased, while those in others decreased. Yet we seldom saw a beetle singly during our daily excursions in sunshine along the idyllic shores, where the dragonflies gamboled among nodding wild rice and waving bulrushes. On the other hand, we routinely saw individuals in the dead of night and during cold dawns. Contrary to what had been supposed, we found that the beetles were nocturnal.
The beetles started to move about twenty minutes after sunset, when the water surface around a pod became roiled. Periods when the beetles were milling about alternated with times when they stayed quiescent. As it got darker the pods expanded, and beetles, alone or in small trains, started to leave the pod vicinity and travel in straight lines along the shoreline. Those that approached our canoe grabbed mosquitoes that we swatted and dropped down to them; thus, they were apparently out on their scavenging hunts. They were not likely finding prey only by sight (although their eyes are double, with one part underwater and the other above it), but by a different mechanism, using their antennae.
The antennae of most insects are long and sometimes also plumose, and are used primarily as scent detectors. The whirligigs’ antennae are short—barely visible to our naked eye. They are instead adapted as a sonar device, for detecting obstacles and prey.
The base section of each antenna rides on the water surface (while a short club-shaped portion is raised into the air just above the water surface). The antennae have mechanoreceptors that stimulate a nerve at the respective antennal base, when the respective right, left, or both antennae are lifted due to a disturbance of the water floating the antennae. The beetle’s brain decodes the incoming mechanical information from the antennae, in terms of the direction and nature of the water disturbance (Friedrich Eggers, 1927). The way the beetles hunt for food—by bursts of swimming, which we saw among beetles at night—creates pulses of waves that then inform the beetle about the nature and behavior of an object immediately ahead of it by the waves that are reflected from it. (The same principle of animal sonar was discovered in 1940 by Donald Griffin, who proved that bats locate flying insects in the dark by pulsing sound waves and detecting the echoes reflected from them.)
Those beetles in the vicinity of their pods zigzagged or moved in small circles, making rapid movements but achieving little if any net forward motion. Those leaving their pod traveled in fairly straight lines at about thirty meters per minute (about fifty body lengths per second), fast enough to potentially move 0.8 kilometers (the average distance between rafts) in thirteen minutes.
At any point over a hundred meters from a pod, net beetle traffic was away from it in the early part of the night, in both directions at midnight, and primarily back toward it before dawn. Perhaps some beetles “homed” to the rafts of their origin each dawn, as this data suggested. To find out, we had to identify some beetles with respect to their pod of origin, so we captured and marked 680 beetles from one large pod.
Our capture strategy was simple: We positioned our canoe about a hundred meters from our targeted pod, with one of us standing poised in the bow of the canoe, holding an insect net. The other, in the stern, paddled as vigorously as possible directly toward, into, and through the raft. The catcher with the net made several quick swipes through the pod before the beetles had time to disperse or dive. We then daubed our beetles with red paint on their elytra and immediately released them to rejoin their pod, as it came back to rest.
That night, as usual, numerous beetles left their pods to travel up and down the shoreline of the lake. Within two days, we had resurveyed all our previously located pods. Marked beetles had reappeared not only in their original pod, but also in nearly all of the pods in the two large arms of the lake. This showed that the beetles were capable of joining pods without homing. Subsequent observations of beetles in the vicinity of pods also provided clues as to how the beetles may aggregate to produce the relatively permanent pods, even without homing. The mechanism involves following one another, as we found by following them.
Many of the beetles remained at or near their pods all night, foraging in the vicinity of the pod by zigzagging about individually. Near dawn, however, the beetles started following one another. They produced trains of several arranged one behind another, which joined up with still others. The more beetles in a group, the slower its net rate of movement, as one beetle swirled around another. The more they followed in the crowd, the less net progress was made in moving elsewhere. Thus, swarms of beetles contracted into the original pod cluster, the nucleus of which was defined by those that had stayed.
Whirligig beetles feed only at the water surface, where they scavenge and capture prey caught in the surface tension. Their ability to find food depends on close encounters because their sonar system works only within several centimeters. Speed of travel translates to distance traveled, and to the number of prey encounters per unit of time. Therefore, the beetles’ water-skimming speed has likely been under intense selective pressure, accounting for their highly streamlined and smoothly tapered form. Their oily covering likely also minimizes drag.
The beetles’ first pair of legs serves as a tong specialized for grasping, while the last two pairs are adapted as paddles. The middle pair rows at twenty-five to thirty times per second and serves also in maneuvering. But the last pair is the beetles’ main analogy to a boat propeller, churning the water at fifty to sixty beats per second (twice as fast as a dragonfly beats its wings).
Waves are a large part of a whirligig beetle’s world, and wave mechanics are important to it, as they are to a boat’s performance. Objects on the water surface create two kinds of waves. One, called capillary waves, are those pushed up at the bow. The other, gravity waves, are those left in the stern of the moving object (or vice versa). The bow wave provides a barrier, and hence resistance to speed, and ships take advantage of that phenomenon for energy economy by adjusting their speed accordingly. Similarly, for maximum speed and lowest energy expenditure, whirligig beetles swim not over the waves, but in the trough between a capillary wave in the front and a gravity wave behind.
Although the beetles’ morphology and physiology explain many aspects of their capabilities, including how they can hunt at night, they did not reveal to us why they were nocturnal on Lake Itasca, nor why they aggregated into pods. However, other features, and comparative biology, did give hints of likely explanations.
A beetle skimming along on the water surface is easy to locate because the wave it leaves behind on each side of it on the water is visible from afar. A moving beetle could therefore be an easy target; a fish hunting by sight would find it almost impossible to miss. But if the fish recognizes a beetle by its wake, as a bird recognizes the red color of a ladybird beetle, it would learn to avoid the obnoxious taste of its foul-smelling defensive secretion (gyrinidol, a sesquiterpenoid aldehyde). Some ladybird beetles also gather into huge aggregations. They may be highly visible due to their aposematic coloration (color that serves as a warning), but they are also protected by foul-smelling secretions.
An aggregation of gyrinid beetles should be a safe place for each individual. Not only does it dilute the individual’s risk of attack, but also it pools their noxiousness, enhancing the defense: the bass or sunfish living in the pod area would soon become familiar with their sight and taste, and would avoid them. However, fish in the middle of a large lake, far removed from a pod there, might perceive the beetles’ waves as a strong stimulus for attack. To test whether such fish are potentially attracted, we ferried beetles out to the center of the lake and released them. As predicted, fish rose to catch them there, but we did not see any beetle get taken that we released at or near one of their pods. The beetles’ aggregations and nocturnal foraging therefore likely relate to a defensive strategy against fish predation.