Wings

John Grehan jrg13 at psu.edu
Mon Sep 14 11:00:12 EDT 1998


I was interested to read the several comments of evolution of insect wings and
development of metamorphisis that arose out of my questions regarding the
application
of purpose to "explain" insect wings (or anything else for that matter) in
terms of
empirical observation.

I appreciated the outline Rikki Hall provided on Brodsky's Evolution of
Flight in
Insects as it is a field out of my detailed expertise. There were several
statements which
clearly avoided purpose by specifying functional conseqeuences to the
existence of a
particular morphology -

These gills had a musculature....ALLOWED the organism to move them
through the water, enhancing oxygen absorption.  This ALLOWED the
organism to invade oxygen-poor habitats such as marshes and swamps,
where the first insect appears to have evolved.

This is better (from my perspective at least) than saying the musculature
evolved
TO allow the organisms to move or invade habitats etc.

However, I would question the statement that
"Predation pressure probably drove these mating swarms out of the water,
where thoracic gill flapping became weak flight became strong flight."

To be "driven out" implies that they could already get out and fly in the
first place. As
far as I am aware there is no empirical evidence of such predation
pressure. Is it
possible that they moved out without force being applied?


Aside from its value as a predator-avoidance mechanism,
>flight may have also allowed these early insects to disperse upstream
>more easily than they could have in the water.

But they managed ok before they had wings.

  Similarly, during the
>invasion of stagnant waters, the protoinsect would become more dependent
>on crawling or swimming to get around, whereas its predecessors might
>have relied on tides and currents to get around.

Were the insects necessarily "invaders"?

>
>That's the basic story as I understand it.  There is solid evidence to
>back this story up:

There may be solid evidence of homology between gills and wings,
that gliding has evolved from flapping on numerous occassions, and this
is consistent with the fossil record etc., but none of these evidences have any
contribution to supporting the invokation of predation pressures which is,
as far as I am aware, purely hypothetical, and not much better than the
grasshopper
hypothesis. Its possible, as an alternative, that aquatic ancestors (if
this is indeed
the case) were exposed to a terrestrial environment, and as they survived,
what once functioned as a gill now functioned as a wing (albeit as a long
process).

Sincerely, John Grehan




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