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Re: bauplan convergence (long)



On Thu, 15 Jun 2000, Timothy Williams wrote:
> Ronald Orenstein wrote:
> >They also remind us that the earliest fossil winged insects had wings on
> >every thoracic and abdominal segment - just what you would expect, I
> >submit, if they evolved from gills of aquatic arthropods into rowing
> >structures (think of the banks of oars on a galley).
> 
> I like this idea, and it is probably correct that the first insect may have 
> had a pair of "wings" (or proto-wings, since they were not originally used 
> for flight but maybe for aquatic respiration) on each segment.  Every 
> segment also bore a pair of limbs - the mouthparts of insects (maxillae, 
> mandibles, labium) are known to be highly modified limbs, as are the 
> postantennal appendages (but not the antennae themselves, which have a 
> different origin).
> 
> In the proto-insect, the gills perhaps came to be used for locomotion (used 
> as, as Ronald states, as rowing structures, akin to the banks of oars of a 
> galley), then subsequently for skipping on the _surface_ of the water.  
> Fully-fledged wings came next - by which time the number of wings was 
> consolidated to two pairs, one each on the second and third segments of the 
> thorax.  (Some winged Carboniferous insects apparently retain a pair of 
> flaps on the first thoracic segment, which never went on to become true 
> wings, and were soon lost in the course of evolution.)
> 
> That's one hypothesis for the origin of insect wings.  For a contrary 
> hypothesis (which favors the idea that insect wings evolved from flaps used 
> for gliding) check out:
> 
> http://entmuseum9.ucr.edu/ent205/snodgras/snod10.html

Something of interest here appeared recently in the May 6th Science
News:

 Leggy beetles show how insects lost limbs

 Inactivating two genes in red flour beetles causes grubs to
 grow lots of legs and provides clues to the puzzle of the
 evolution of the six-legged body plan.

This article is not online (a shame, as there's a very interesting
picture of a larvae with legs on each segment), but what material
there is can be found at

 http://www.sciencenews.org/20000506/fob3ref.asp



> [...]

rich