6.2. Alternative hypotheses on the origin of wings from either prono-tal lobes (a) or from modified gills (b).

6.2. Alternative hypotheses on the origin of wings from either prono-tal lobes (a) or from modified gills (b).

Exite or Gill Theory. The "gill theory" for wing origins hypothesizes that wings are serially homologous with the movable abdominal gills on aquatic naiads of mayflies (Kukalova-Peck, 1991). In this theory, the gill itself is a modified exite of a hypothetical, basal leg podite called the epicoxa. The epicoxa is considered to form the junction between the pleuron and the thoracic dorsum. The articulating exite of this leg segment then became modified into gills and into wings. Indeed, the gills of immature mayflies superficially resemble tiny wings, and their pattern of tracheation tantalizingly mimics that of wing venation. Non-homologous exites are present on the coxae and abdominal segments of some apterygote insects, but these cannot be considered evidence for an exite origin of wings because exites in these lineages are different appendicular podites. Recent developmental evidence (e.g., Averof and Cohen, 1997) has supported the exite theory by examining the expression of homologues in crustaceans for two wing-related genes in insects. The pattern of expression was associated with basal leg appendages and to some extent the lateral extremities of the thoracic dorsum.

An interesting idea not yet fully explored is a hybrid between the paranotal and exite theories. As noted, the wing is composed of two epidermal layers. It could be hypothesized that the dorsal epidermal layer is an evolutionary derivative of the tergum (i.e., a partial paranotal lobe), while the lower layer is derived from the pleuron (i.e., from tissues ancestrally associated with the basal segment of an appendage). There is presently no evidence, however, supporting such an idea.

Depending on the theory of wing origin, the exite or para-notal, insects evolved wings, respectively, either as sails at the water surface or as parachutes at the tips of branches. The exite theory was recently endorsed by observations that wings are used by some aquatic, basal groups of insects to sail along the surface ("skimming") (Marden and Kramer, 1994; Marden, 2003). In this scenario, gill-like exites or gill covers (Kukalova-Peck, 1978, 1991; Averhof and Cohen, 1997) expanded to catch surface breezes and carry the newly emerged adult insect along, which then evolved into structures capable of powered flight. This theory has been critiqued on several bases (e.g., Will, 1995; Rasnitsyn, 2003), some of which follow.

1. While skimming is widespread in adult Plecoptera (Marden, 2003), it is very sporadic or even rare in more basal pterygotes, the Ephemeroptera and Odonata.

2. The most basal insects (apterygotes), the Zygentoma and Archaeognatha, are fully terrestrial, suggesting that the ancestral pterygotes were also terrestrial.

3. A terrestrial origin of pterygotes is further supported by fossil evidence, which indicates that freshwater aquatic insects did not appear until the Triassic (Zherikhin in Rasnitsyn and Quicke, 2002), nearly 100 my after the earliest known winged insects. Indeed, the phylogeny of insect orders (Figure 4.24), and the different adaptations of nymphs in various aquatic insect orders (e.g., locations and structure of gills), indicate entirely independent colonizations of water by these orders.

4. Contrary to claims, mutational evidence does not provide evidence of homology, and, in fact, structures as complex as wings clearly have a highly polygenic basis. Also, if gills are homologous to wings, as this theory proposes, one would expect some structural similarity between the two, such as in the branching patterns of the gills and wing veins. Other than the gill covers and their dichotomous branching seen in some mayflies, there is little evidence for this similarity.

5. Many of the earliest winged insects from the mid-

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