Hand Lens - May 1, 2020

Issue №5: Larval Forms

by W.L.


In one of the later snowfall events in the historical record, this past week saw well over an inch of snow deposited overnight in this part of southeastern Vermont. In the valleys the subsequent rain washed almost all of this away by the early morning hours, but at elevations only a few hundred feet higher, snow remained on the ground throughout the day.

While it hasn’t been cold enough to grow any ice on the streams or lakes, the still, shallow vernal ponds have not been so lucky. Ice covered many of the more shaded pools, with a few unlucky frog eggs near the surface being trapped in the ice.

And yet, despite of water temperatures only a few degrees above freezing, the tadpoles have begun to emerge from their eggs. At these temperatures, they are most easily distinguished from the insect larvae they share the pools with by their sluggish movement, compared with the spasmodic on-off motion of the arthropods.

Just a few feet away from where the remains of their eggs floated as a loose mat of torn, algae-tinted slime, dozens of tadpoles congregated on the leaf matter at the bed of the pool. The cold-induced sluggishness really showed itself now as I waded into the pool for a closer look. Apparently undisturbed by the massive footfall drawing near to them, they only began to move in appreciable numbers when I disturbed the leaf itself. A few did not move at all—casualties, I suspect, of the cold.

Judging by the truly heroic layers of scat that have collected on every flat surface of this fractured chunk of the local bedrock, this boulder has sheltered porcupines for many years. The cracks in this particular boulder provide ideal porcupine habitat—cool, northwest-facing openings with relatively narrow, defensible entrances leading into larger cavities. When I encountered it, the porcupine had been walking along the base of the rock; once alerted to my presence, it made its deliberate way up the rock and into the largest crevice—and then left its back half exposed at the entrance until I’d come much closer, perhaps fifteen feet away.

The bugs that carved this elaborate pattern left long ago. When this tree still had its bark—though probably after it had entered a decline, and was no longer able to defend itself—an individual of some species of bark beetle bored through the bark and then carved out the broad central “gallery” in the cambium of the tree. This was to be their “parental gallery”, along the walls of which their eggs would be evenly spaced out. As each egg hatched, the beetle larvae could then begin to burrow their own galleries through the cambium, growing as they fed—notice how the galleries slowly widen as they depart from the parental gallery. Each gallery that reaches out to a wide tail represents a beetle larvae that, having spent its entire childhood in total darkness while eating nothing but wood, reached adulthood and bored its way out, seeking a mate and eventually another dying-yet-not-dead tree. 
Any entomologist who can identify the species of beetle by the pattern left in the wood should feel free to email their candidates to handlens@tinyletter.com. I’d tell you the species of tree, but this particular hunk of dead wood was rooted in right at the edge of the river—all the traces I’d usually use to identify it had all been water-worn away.

Despite its age, this water-heater heeds the call to return to the waters it returns to every year—the fresh-water stream where it hatched from its egg many years ago. Notice how it swims against the swift current, sleek steel flesh cutting through the water as it readies to leap its substantial bulk up the minor falls that stand in its way—like the many miles of such rapids it has already crossed to reach here, and the many further miles before it reaches the spawning ground. Without the great deal of pressure and heat it has stored in its robust frame over months as an open-seas predator, it could not hope to finish this journey—hence the clear morphological distinctions between these species of water-heaters and the instant water-heater species, which spawn in brackish waters near the coast.