  Evolution of Insect Flight   Pterosaurs, birds and bats took
to the air from evolutionary runways that scientists believe
they understand fairly well, but insects began flying so much
longer ago that details of their stepwise conquest of flight
remain obscure.  Scientists at Pennsylvania State University
hypothesize, however, that a present-day flightless insect
called the stone-fly may be closely related to ancestral insects
that first learned to fly more than 330 million years ago.  Last
February, Dr. James H. Marden, a biologist at Pennsylvania State
University, and Melissa G. Kramer, his student, began studying
the behavior and biology of stoneflies - the immature nymphs of
which are familiar to many fishermen as delicacies for trout.
The nymphs begin life in river or pond water and then develop
primitive wings enabling them to skim across water at high speed
without actually taking to the air. Marden and Ms. Kramer have
concluded that the humble ancestor of such expert fliers as
mosquitoes and wasps m