Wings

[Joseph G. Kunkel]

Richard Hall wrote:
> 
> Mark Walker wrote:
> >
> > Rikki Hall has posted a fascinating account of Brodsky's hypothesis on
> > insect wing evolution, and I must admit that it was interesting reading.  I
> > don't think it provides any insight as to why or how the ancestors would
> > have evolved down the metamorphosis path to get there.
> 
> As far as metamorphosis goes, it is just a special case of the basic
> skin-shedding process that all arthopods go through in order to grow
> larger.  Mayflies experience "incomplete metamorphosis", wherein the
> larval stages resemble the adult, while butterflies experience "complete
> metamorphosis", wherein pupation results in a dramatically different
> form.
> (snip>
> They may not be as colorful as Leps, but aquatic insects are way cool.
> 
> Rikki Hall

I think that the most enigmatic evolutionary step is the evolution of
the maggot/caterpillar stage.  The pupa is like a little adult
shrink-wrapped in its cuticle and having all the parts of the adult but
not yet expanded or freed up.  It is the larva with its primitive or
absent legs that amazes me.
I support a theory born of my interest in insect serum proteins that
agrees with an old thory of Hinton who suggested that the larval stages
of the caterpillar/maggot are actually derived from the embryonic
molting stages.  The last embryonic molting cycle or rather the last
stadium in the egg would correspond to the pupa (Thus, a cockroach egg
has the preformed appearance of a pupa just before it hatches.). The
earlier embryonic stadia do not have legs yet but do have a cuticle that
is much thiner and larval-like. Also, Vince Dethier told me that most
maggot/larva are very depauperate in sensory organs.  The moth
caterpillar has only 5 taste sensilla for instance while an adult fly
has orders of magnitude more.  Looking at the egg stages the sensillae
of cockroaches develop in the next to last embryonic molt (the last
being hatching).  Thus the cockroach has a pseudo maggot stage that
changes to a pseudo-pupa just at that last embryonic molting cycle.
The evolution of complete metamorphosis could have been the freeing up
of an embryonic stage to wiggle about in the environment to find more
food.  This transition could potentially have occured in eggs that were
retained in the females bursa/uterus, as there are some insects that
have developed ways of providing nutrients to eggs that hatch in utero.
In any event I subscribed to Hinton's theory based on the fact that I
found the cycling of serum storage proteins needed for cuticle formation
during the embryonic molting cycles.  These are proteins that cycle
during the larval stages of most insects and often dissappear in the
adults.  Thus the mechanisms for cyclical production of cuticle exist in
the embryonic stage.  The evolution of a egg/maggot form is not such a
stretch and the maggot would go on to perform its normal metamorphosis
into a big legged adult by passing though the now real-pupa stage.  So
complete metamorphosis is not as big a mystery as we thought, if you
accept Hinton's theory.
What do you think of that!
Joe
-- 
--------------------
Joseph G. Kunkel, Professor
Biology Department             joe at bio.umass.edu
University of Massachusetts    http://www.bio.umass.edu/biology/kunkel
Amherst MA 01003