August 10, 2015
I’m standing on the banks of the Casselman River, in the heart of the Catoctin Mountains of Western Maryland. It’s late summer, the day after Labor Day. The sun is shining brightly, but a cool mountain breeze is blowing in my face. Below me, I can hear the water gurgling over the rocks. From an unseen location amidst the riparian vegetation, crickets are trilling. Amidst the riot of green, splashes of color: daisies, goldenrod, orange jewelweed, blood-red columbine, and bayberries the color of indigo.
A doe exits from the thicket alongside the riverbank and glances at me, regarding me coolly for a few moments before turning away and trotting off unhurriedly. Two more deer emerge, both bucks, still wearing the summer coating of velvet on their antlers. They, too, regard me coolly for a few moments before trotting away. To my left, a dragonfly is skimming over the water surface like an Exocet missile.
I’ve always been fascinated by dragonflies. With their speed, maneuverability, and brilliant metallic colors, they truly are the Ferraris of the insect world.
The dragonflies, along with their cousins the damselflies, constitute the Order Odonata. Together with the Ephemeroptera, or the mayflies, they constitute a major group of insects called the Paleoptera. All paleopterous insects are found close to bodies of fresh water, as this is where they grow up. The females of this group all lay their eggs in the water, and the newly hatched individual is called a naiad, which is the Greek word for “water nymph.”
The central fact of insect life is that all insects are possessed of a rigid exoskeleton which cannot grow. Therefore the insect must periodically make a new exoskeleton and shed the old one, a process known as molting. The newly molted individual is soft and defenseless and usually goes into hiding. It takes in air or water to expend the new exoskeleton to give itself room to grow, and remains in hiding for several hours until the new exoskeleton hardens.
The mayfly naiads are mostly herbivorous, content to graze on diatoms and other algae encrusting the rocks. By contrast the dragonfly and damselfly naiads are voracious predators, dining on mosquito larvae and other aquatic invertebrates and even small fish and tadpoles. Members of both groups take one to several years to grow up, going through as many as fifty molts before attaining adult size. When that happens, the individual crawls out of the water, sheds its last larval exoskeleton, and unfurls its wings. Soon it is ready to take flight.
True to their name, the ephemeropterans spend only a few hours or at most days in the adult stage, usually not even bothering to eat. They have only one thing on their minds. After mating, the gravid female hits the water surface. Her swollen abdomen bursts, releasing her fertilized eggs hither and yon. By contrast, the odonates spend several weeks in the adult stage. Like their youthful waterborne counterparts, they, too, are voracious predators, devouring mosquitoes and other flying insects with gusto. Males guard territories along the water’s edge containing emergent vegetation, which is used by the female to enter the water to lay her eggs. The male mates with any female who enters his territory for this purpose, and males fight with each other over prime oviposition sites.
The word Paleoptera means “ancient wing.” Ancient they may be, but backward they are not. This group includes the fastest flying insects, the dragonflies, with top speeds in excess of 35 miles per hour. A dragonfly can fly forwards, backwards, side to side, up and down vertically, and can hover for up to a minute.
The Paleoptera is the oldest group of winged insects which has living descendants, but they were not the first of their kind to take to the air. The oldest flying insects appear in the fossil record over 300 million years ago. These were bizarre, clumsy-looking creatures with multiple wings, in stark contrast to the clean lines and simplicity of design of their modern-day successors. They call to mind our own species’ first attempts in the nineteenth century to build powered flying machines—bizarre, clumsy-looking contraptions with multiple wings, in stark contrast to the clean lines and simplicity of design of the Wright Brothers’ first successful airplane.
After watching the dragonfly zoom off, I turn my attention to the matter at hand. I’ve come here to gather specimens for my teaching collection, the kind of excursion naturalists have been going on for generations. I’m armed with a fine-mesh net mounted on a triangular frame, attached to a stout wooden pole. I step into the stream and place the base of the triangle firmly on the bottom. I grasp the pole firmly in one hand, using my weight to hold the net in place, as I stoop over and upend rocks with my free hand, allowing the water current to sweep any dislodged invertebrates into the net. Then I haul the net up and scrutinize the results of my work.
Amidst the leaves and other debris is a squirming grayish-black insect, wet, glistening, about an inch in length. Three slender filaments emerging from the terminal abdominal segment allow me to identify it as a mayfly naiad. Each of the last two thoracic segments bears a pair of wing buds, while each abdominal segment bears a pair of flattened, paddle-shaped appendages called tracheal gills, which the animal uses to breathe underwater. I know that if I were to look at these appendages under a microscope, I would find each one was invested with numerous vein-like tubules called tracheae which convey oxygen to the tissues. A lay person might say these tracheal gills resemble tiny wings, but it would be more accurate to say that wings are over-sized tracheal gills.
When Charles Darwin proposed his theory of evolution by means of gradual accumulation of many small successive changes, one of his contemporaries, George Mivart, raised the perfectly reasonable objection: What good is 50% of a wing? It won’t get you off the ground. The answer to this question may be found in the concept of exaptation—the idea that a feature that arises in response to one kind of selection pressure, just by chance, fits its owner for an entirely new role.
We may imagine the ancestor of winged insects as a simple, inconspicuous-looking creature, similar to a modern-day silverfish. We may imagine this creature spent its days crawling amidst the rocks alongside a stream, into which it would venture from time to time, perhaps to search for food, perhaps to avoid predators. Now, any slight extension of the lateral body wall would be favored by natural selection, because it would increase the surface area for oxygen uptake, allowing this animal to stay underwater longer. We may imagine these extensions increased in size until they developed into the tracheal gills we see in modern-day mayfly naiads, enabling these creatures to spend their entire lives underwater.
Once these appendages reached a certain size, they could have used for an entirely new purpose, enabling these insects to be blown around by the wind. This would be of obvious advantage for an animal that lived in fresh water, because fresh-water habitats are ephemeral—they dry up—and any feature that allowed these insects to find a new home would provide an obvious advantage.
Having taken on this new role, these appendages would be subject to selection pressure for further increase in their size. We may imagine that the next step was the addition of muscles onto the inner walls of the thorax, for attitude adjustment, and from here it would be but a short step to powered flight.
The oldest winged insects had one pair of wings for each thoracic and abdominal segment. But natural selection is relentless in her quest for efficiency of design, and soon the superfluous wings were jettisoned, and flying insects adopted the same streamlined design we see today.
Of course, no one was there to watch all this unfold. But this theory, known as the tracheal gill theory of insect wing origins, plausibly explains how a complex structure such as an insect wing could have arisen through the gradual accumulation of many small successive changes, and it also neatly accounts for the fact that all the members of the oldest living group of flying insects begin their lives in the water. The mayflies, which are acknowledged to be the most primitive living order of flying insects, still retain the paired abdominal tracheal gills of their ancestors in their aquatic larval stages, although these gills are absent in the terrestrial adult stage. The dragonfly and damselfly naiads have dispensed with these appendages altogether, having developed other means of breathing underwater.
Two days later, I find myself in Ruxton, Baltimore, just north of the city-county-line. It’s a brilliantly hot afternoon. I’m standing by the side of the light rail track, listening to the hum of the approaching train. As soon as the coast is clear, ignoring the sign that says “Trespassers will be prosecuted,” I sprint across the tracks—giving the big buck on the other side the surprise of his life. With an explosive burst of speed, he crashes through the briars on the opposite embankment and makes his getaway. I follow as best as I can, ignoring the thorns and stinging nettles lacerating the exposed skin of my arms and legs.
I follow a well-worn trail alongside a field of goldenrod and reach the banks of Roland Run. The plan is to follow the course of the river until it empties into the north end of Lake Roland, hoping that somewhere along the way I will encounter suitable habitat for dragonfly naiads.
This is easier said than done. There is no path, as such, to follow. The water has cut a deep channel through the mud, leaving sheer embankments of dubious stability overgrown with brambles right up the edge. The only way to go here is to wade right through the water, which in some places is ankle-deep, in others waist-deep, and surprisingly cold. Several times the stream divides and I have to decide which is the main channel. More than once I decide wrong, reach a blind end, and have to backtrack. Eventually it occurs to me that I am lost. I have no idea where I am.
I keep walking. A great blue heron soars overhead, silently, majestically.
Through an opening in the trees I espy open water and begin heading toward it. The sycamores give way to willows, which give way to cattails, which give way to watercress, which gives way to duckweed and filamentous green algae. I have found what I am looking for. This is Dragonfly Central. Flitting to and fro above the water surface are hundreds of dragonflies, a variety of makes and models and colors: emerald, ruby, cerulean, and jet-black.
I step into the pond, which feels hot as bath water, and immediately sink halfway up to my knees in the soft, stinking black mud. Using the net handle as a kind of third leg, I maneuver myself into position and jab the net at the base of some cattails, then take a look at what I have found.
I see several dragonfly naiads, each about an inch long, sleek, torpedo-shaped, an indeterminate grayish-green in color. Each one sports a pair of prominent bug eyes and two short spiky antennae. I know that if were to look at them under the microscope, I would see each one also bears a protrusible mouth appendage called a labrum, armed with a pair of curved dagger-like spines it uses to impale unwary prey. I select half a dozen of the best specimens, and, with only a momentary twinge of regret, consign them to their deaths in a vial of alcohol.
I notice an adult male hovering over a willow twig emerging from the water, on which it comes to rest. I pause for a moment to admire his form – electric-blue body, outstretched wings refracting the sun’s rays like diamonds. His huge eyes, which take up almost the whole of his head, turn alertly this way and that. Another male enters his territory, and he takes to the air and veers straight at the intruder, who zooms off.
It’s time to go back. I walk back to the stream and begin heading back the way I believe I came. But the terrain looks unfamiliar, the water is getting deeper, and my footing on the soft muddy bottom feels increasingly treacherous. I decide making my way through the brambles is the lesser of two evils. Eventually I happen upon an abandoned railroad bed and decide to follow it, confident that it must lead to somewhere. I emerge into a clearing and view the waters at the south end of Lake Roland cascading over the walls of a concrete dam into the Jones Falls River. I am miles away from where I thought I was, but at least now I know where I am.
I follow the smooth blacktop driveway out to Lake Avenue, turn left, walk for a mile and a half, turn left again onto Charles Street, walk for another mile, and turn left onto Bellona Avenue, which despite the citified-sounding name is actually a steeply descending narrow winding country road. In the treetops overhead I hear the cicadas chorusing, singing their song of another summer about to end. I arrive back at my car tired, aching, thirsty, scratched, bleeding, and spattered with dried black mud. It’s been a great day.
I reach into my pocket and extract a small, hard, rectangular plastic object. It’s my cellphone, soaking wet, ruined. I toss the phone into a nearby trash can, toss the net into the back of my car, and turn for one last look behind me before heading home. The sun is already beginning to set, splattering the western sky with palette of colors—rose, salmon, peach, gold, and even violet.
The sun is beginning to set on my life was well. I definitely have more years behind me than in front of me. I think back to my boyhood, growing up in rural Pennsylvania. It seems like yesterday, but it was over forty years ago.
I remember I had an aquarium stocked entirely with creatures I had captured from the local streams and ponds—tadpoles, minnows, water striders, whirligig beetles, snails, flatworms, and my pride and joy—two crayfish, each of which I had daringly plucked from the water with my own fingers before sprinting home with my prize.
People don’t really change with the passing of years—they just become more like themselves.
Patrick D. Hahn, Ph.D., has been an affiliate professor of biology since 2009 at Loyola University Maryland, where he teaches Invertebrate Zoology. His writing has also appeared in Biology-Online, the Canada Free Press, and the Baltimore Sun. He can be reached at email@example.com.