Aug 11, 2010

stone free to ride the breeze

This is the shed skin of a stonefly. Aquatic all its life, the insect crawled from Hazel Creek in July, cracked open its thorax, wriggled out, pumped its wings and took flight on misty night air. The white fibers on its head are like an umbilical cord, a remnant connection to the stonefly's former body.

I found the skin on a rock the next morning. It could not have been there long since the rock had been submerged by the prior day's afternoon rain. As the water receded, the stonefly crawled atop and began its brief aerial adulthood.

It will attempt to mate and contribute to the batch of eggs laid in the creek, timed to hatch as autumn leaves drop into the water, bringing nutrients on which the creek's ecosystem depends. Many types of insects and worms live on the rotting organic material that perpetually washes into a mountain stream, and these are prey for crayfish, darters and salamanders, in turn eaten by trout, raccoons and owls.

More than 100 species of stonefly occur in Southern Appalachain waters, with some likely undiscovered, some lost to science at the turn of the last century due to intensive logging, chestnut blight and slaughter of the passenger pigeon. There are more than 1,000 species of stonefly worldwide. Nine families occur in our region. The age of their oldest fossils (about 250 mya) combined with the geological history of the Appalachians makes it quite likely stoneflies evolved in these here hills.

The first winged insects were mayflies, which soon gave rise to damselflies and dragonflies. It was not long after that another mayfly lineage morphed into the first stonefly. All this took place in the Southern Appalachians some 300 million years ago as the shallow, inland sea west of the mountains receded, leaving behind the Cumberland Plateau and finally the Mississippi basin and the grand river herself.

The pictured skin is about an inch long, not counting the tails, and this is about as big as stoneflies get. They resemble roaches, and the similarity is not coincidence. It is likely the roach branch on the tree of life arose from the stonefly limb. In fact, the common ancestor of roaches, crickets and the whole Orthoptera assemblage may have been a stonefly.

Stoneflies spend most of their lives crawling slowly through the stones and muck of a streambed, grazing on crud, shedding occasionally to grow, then one day crawling from the water to sprout wings. Their flight is weak and slow, and they have little ability to manuever. If they run into something, they cling. Their search for a mate is clumsy and fraught with peril, but they have been succeeding for hundreds of millions of years, long enough to diversify into many genera and spawn a major assemblage of terrestrial insects.

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