1168 THE STRUCTURE OF EVOLUTIONARY THEORY
Averof and Cohen (1997) found apterous expression in the sheet-like dorsal branch
of respiratory epipodites in a branchiopod crustacean, thus supporting the exite theory
for the origin of wings. Shubin et al. (1997, p. 645) draw a reasonable phyletic
conclusion consistent with this section's theme of evolution from homonomy to
specialization: "This suggests that Recent wings evolved from the respiratory lobe of
an ancestral polyramous limb, probably first appearing in the immature aquatic stages
as gill-like structures, such as those found on all trunk segments of extinct
Paleodictyoptera or extant mayfly larvae."
On the related issue of evolutionary suppression of wings on most segments and
their restriction to two pairs in most insects, and to one in dipterans (see Fig. 10-28),
Carroll et al. (1995, p. 58) demonstrate that "wing formation is not promoted by any
homeotic gene, but is repressed in different segments by different homeotic genes."
Against the older view (consistent with Lewis's original additive model of phenotypic
complexification) that Antp positively regulates the formation of wings and halteres
on T2 and T3 of Drosophila, Carroll et al. (1995) present evidence that other Hox
genes repress wing primordia on the remaining body segments. For example, Scr is
expressed in both labial and Tl segments; in mutant embryos lacking Scr expression,
"flight appendage primordia arise in the Tl segment" (p. 58). As for suppression of
more posterior wings, the oldest information about homeotic mutations in Drosophila
documented the development of complete wings on T3, where the vestigial halteres
usually form. We now know that this bithorax phenotype (which gave its name to the
previous designation of the posterior Drosophila Hox series as the "bithorax
complex" or BX-C) results from a mutation that represses the Ubx gene in T3.
Another mutation of Ubx leads to the growth of wing primordia on Al as well.
Carroll et al. (1995) propose that when wings existed on all post-oral body
segments of a homonomous ancestor, "there was no homeotic gene input into their
number or design" (p. 59). Carroll et al. then hypothesize that elimination of wings
from most segments occurred as the Hox genes became
10 - 28. Differentiation of flight in the evolution of insects as a consequence of repression of
wings on posterior segments by various Hox genes. The fossil nymph in B possessed wings on
all segments. These are reduced but still present on all segments in the fossil mayfly nymph at
C. From Carroll et al., 1995.