Walking on water

Scanning a sunlit pond floor for crayfish, I was distracted by seven dark spots gliding in a tight formation. Six crisp oval shadows surrounded a faint, less distinct silhouette. The shapes slid slowly and then, with a rapid motion, accelerated before slowing to another glide. I can remember seeing this pattern as a child, in my first explorations of pond life.

Water strider shadows are far larger than the insects casting them. To visualize the surprising proportion of legs to body, it may help to think in human scale. For mathematical simplicity, picture a six-foot-tall man lying flat on the water surface. Imagine that attached near his hips he has a pair of seven-foot-long, stick-skinny legs pointing back at a 45 degree angle. Just forward of these spindles he has another pair pointing forward at a 45 degree angle; these are nine feet long. A pair of three-foot-long arms point forward and each has a single claw protruding from the palm.

The legs are long for good reason; they distribute body weight over a wide area, and aided by water repellent hairs, allow the insect to coast across the water’s surface tension. The minute leg hairs are densely packed and each has many air-trapping surface grooves. According to the Chinese scientists who discovered the grooves, water striders displace enough water to float up to fifteen times their own body weight. This extreme buoyancy is enough to keep the water strider’s body high and dry above the water, even during rainfall and choppy conditions.

Because the insects literally walk on water, some call them “Jesus bugs.” When fish or backswimmers approach, the water striders are well positioned to make an aerial getaway. Their super buoyancy means that they can use their long legs to jump straight up from the water surface, and once airborne, they can spread their wings (yes, they have wings) and fly to safer haunts.

Slow motion video reveals how water striders move. The longer middle legs sweep back rapidly like oars, pushing against the surface tension to drive the insect forward. Human rowers lift their oars out of the water on the recovery stroke to reduce drag, and rapidly moving water striders do the same thing. However, when moving more slowly, they drag their middle legs forward along the water surface. The rear legs trail and change angles like twin rudders steering the insect towards food, or mates, or away from hazards.

All the while, the front legs rest on the water surface just forward of the insect’s head. Theirs is a murderous function, allowing the water strider to find and seize its next meal. Subtle ripples made by surfacing aquatic insects including mosquito larvae, or struggling terrestrial insects on the water surface function like tugs on a spider web, leading the water strider to its prey. The single-clawed forelegs grapple the prey while the insect’s piercing mouthparts stab through the cuticle, consuming bodily fluids as if through a drinking straw.

To see this first hand, my Saint Michael’s College students and I dropped a few large carpenter ants onto the water surface of some ponds in Winooski. It took only seconds for a water strider to grab the first ant. Others were rapidly scooped up and carried off. A braver student dunked a yellow jacket, trapping her in the surface tension. The water striders investigated but took a pass on that risky meal. The yellow jacket climbed out on some vegetation a little the worse for wear.

My students and I were also curious to see if the insects were faithful to particular pools or if they moved around. We used paper correction fluid (“Wite-Out”) to mark a dozen water striders and released them where we caught them. The following day, we found marked water striders in their home pool, but also in pools upstream and downstream. We frequently observed water striders fighting each other. Perhaps territoriality and competition drives them to seek other living space?

As summer arrives, I have noticed that the water striders are back in force from their winter hideouts among the pond-side leaf litter. I’d welcome a little sun any day now so that their spectacular shadows may also return.

Declan McCabe teaches biology at Saint Michael’s College. His work with student researchers on insect communities in the Champlain Basin is funded by Vermont EPSCoR’s Grant NSF EPS Award #1556770 from the National Science Foundation.

This article was written for Northern Woodlands Magazine's Outside Story and first published on June 12th 2017

Download the Article © by the author; this article may not be copied or reproduced without the author's consent.

Beetles and ballet

The tree dance with flowers awaiting pollination.

 

Seated with my children on folding chairs at Philo Ridge Farm in Charlotte; strains of Vivaldi pierced the evening air.  There’s nothing quite like live music.  But we were not there just for the music.  A pair of dancers pirouetted onto the ‘stage’, or should I say ‘lawn’.  An expectant hush moved through the audience as the dancers met, and exchanged glances of feigned indifference for the beginning of their performance. Strangely, their gazes seemed focused behind their partner’s backs. 

The performers turned and parted, the side-long view revealing unusual costume choices for classical ballet.  Each had a flickering, glowing, insect-like abdomen extending from beneath beetle-esk wings. This was Farm to Ballet, and we were being treated to ‘The Firefly Dance’.

Perhaps only in Vermont, would a ballet company take to the fields, contributing to the charm that keeps us all here. The dancing was amazing as ballerina and ballerino circled and posed, all the while, each examining the other’s flashing light pattern.  Initially lack of synchrony between their abdominal lights seem to cause repulsion between the dancers.

Real fireflies are innocuous beetles, easily overlooked during daylight hours. Their larvae eat insects and snails and may well be important pest control agents.  They are found in vegetation, leaf litter, and along the soil surface where they are frequently pit trapped by Saint Michael’s College student researchers. 

It is only at night that fireflies come into their own and truly capture the imagination.  The adult beetles use flashing light pulses and larger eyes than those of many beetles to communicate with and attract mates.  Males flash as they fly a few feet off the ground, and choosey females sit in darkness returning the male’s signal only when a particular attractive flash pattern tickles their fancy.  Female fireflies can afford to be picky, at least early in the season when they may well be outnumbered two to one by males.  Mark Branham and Michael Greenfield used computer controlled lights to demonstrate that females of one firefly species really preferred fast flashing males; in fact they responded best to flash patterns that were even faster than most male beetles could ever achieve.

Because fireflies use light to find mates, biologists have frequently expressed concern that light pollution might disrupt their mating habits.  Anyone who has made the mistake of leaving the lights on with a window open can confirm that artificial lights attract insects; entomologists rely on this quirk of insect biology to trap study specimens.  Firefly numbers seem to have declined in recent years as light pollution has increased, but the precise link between fireflies and lights was more clearly established this year when Kevin Costin and April Boulton demonstrated that even a single powerful light placed at a field site could reduce firefly flashing by half. 

If the Farm to Ballet dancers were at all concerned about light pollution, they did not mention it to me.  But creative director Chatch Pregger did confirm my perception of the developing firefly dance.  As he danced with his partner Avi Waring, their lights gradually changed from random, asynchronous flashes, to tightly choreographed patterns that flashed in unison.  The ballerina’s initial distain for her partner was replaced by harmoniously flowing parallel moves and lifts as they simultaneously achieved synchrony of light flashes and choreography that captivated their audience.

According to Pregger, the light flashes controlled remotely by an off-stage cast member were designed to gradually synchronize by the end of the dance.  The synchronous flashing lights mimic the patterns used by the same firefly species studied by Branham and Greenfield.  Females of Photinus consimilis, respond to the males using flash patterns that are dimmer than, but remarkably similar in frequency and duration to those of the males.  If the female sends an appropriate signal to the male, they achieve their own form of highly evolved choreography.

As the evening drew to a close and the crowd filtered out, the subtle sparks of real fireflies continuing their evolutionary journey went largely unnoticed by dancers and audience members.  With summer over, dancers have abandoned the fields in favor their South Burlington Vermont studio.  As the Farm to Ballet troupe develops the following season’s choreography, next year’s firefly larvae have burrowed into the soil following choreography that has sustained them over eons.