Richard Hall hall at
Fri Sep 11 04:48:33 EDT 1998

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

The interesting thing is that it is the larval form that has become
specialized.  In other words, caterpillars are the miracle and
butterflies just another winged adult.  Mayflies begat Caddisflies, and
Caddisflies begat Lepidoptera.  It was during the Caddisfly stage that
the larval form became more worm- or caterpillar-like, with reduced legs
and reduced sclerotization.  As Caddisflies evolved into Leps, this
transition continued to the point that the legs became unsclerotized
prolegs, and only the head retained a chitinous covering.  In both the
Caddisfly and the caterpillar, this reduction of the hard shell skin
allowed the animal to stretch as it grew, reducing the need for numerous
molts and also allowing it to build up nutrient reserves to fuel its
adult form.  Some Caddisflies and a few Leps compensated for the
vulnerability of a soft body by constructing cases of sand, twigs,
leaves, or other debris, woven together with the silk that they can
produce with their mouthparts.

It is also worth noting that in some Mayfly families, the larvae are
heavily sclerotized and closely resemble the adults, while in the more
shrimp-like families, larvae have reduced sclerotization on the ventral
surface of the abdomen to give it flexibility, as that is how they
propel themselves.  So the transition toward reduced sclerotization
starts even within the Ephemeroptera.  Also, completely unique among
insects, some Mayflies actually have a molt after the metamorphosis into
a winged form.  It too is winged.  This hints that metamorphosis to the
winged adult form may have taken some time to become reduced to a single

The transition from incomplete metamorphosis to pupation, when you
consider the intermediate stages between Mayfly and Lepidoptera, does
not seem particularly abrupt or unlikely to me.  There are lots of
intermediate steps and lots of intermediate orders, families, genera,
and species too.  Not that it isn't a miracle or a wonder of nature, I
just think that God left behind lots of hints as to how she pulled it

They may not be as colorful as Leps, but aquatic insects are way cool.

Rikki Hall

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