1

30 fascinating suggestion that surface skimming, as seen in some living stoneflies (Plecoptera)

and the subadult stage of some mayflies (Ephemeroptera), may represent this intermediate phase. Essentially, surface skimming is running on the water surface, using the weak flapping movements of the wings to generate propulsion. Because the water supports the weight of the insect's body, the muscular demands of skimming are far less than those required in a fully airborne insect. Thus, stoneflies with quite small wings and weak flight muscles can surface skim. Thomas et al. (2000) combined a molecular phylogenetic analysis of the Plecoptera with an examination of locomotor behavior and wing structure in representatives of families across the order. Their study showed that surface skimming, along with weak flight, is a retained ancestral trait in stoneflies, supporting the hypothesis that the first winged insects were surface skimmers. Marden and Thomas (2003) have provided further support for Kukalova-Peck's proposals by studying the Chilean stonefly Diamphipnopsis samali. The weakly flying adults of D. samali use their forewings as oars to row across the water surface. Further, they retain abdominal gills. The larval stage is amphibious, living by day in fast-moving streams, but foraging at the water's edge by night. Thus, D. samali may represent a very early stage on the road to true flight: an amphibious lifestyle, the co-occurrence of wings and gills, and the ability to row on the water surface.

In addition to her views on wing origin, Kukalova-Peck has also speculated on the evolution of fused wing pads in juveniles and wing folding. Noting that the earliest flying insects had wings that stuck out at right angles to the body, Kukalova-Peck pointed out that, as they developed (in an ontogenetic sense), the insects would be subjected to two selection pressures. One, exerted in the adult stage, would be toward improvement of flying ability; the other, which acted on juvenile instars, would promote changes that enabled them to escape or hide more easily under vegetation, etc. In other words, it would lead to a streamlining of body shape in juveniles. In most Paleoptera streamlining was achieved through the evolution of wings that in early instars were curved so that the tips were directed backward. At each molt, the curvature of the wings became less until the "straight-out" position of the fully developed wings was achieved. Two other groups of paleopteran insects became more streamlined as juveniles through the evolution of a wing-folding mechanism, a feature that was also advantageous to, and was therefore retained in, the adult stage. The first of these groups, the fossil order Diaphanopterodea, remained primitive in other respects and is included therefore in the infraclass Paleoptera (Table 2.1 and Figure 2.6). The second group, whose wing-folding mechanism was different from that of Diaphanopterodea, contained the ancestors of the Neoptera. The greatest selection pressure would be exerted on the older juvenile instars, which could neither fly nor hide easily. In Kukalova-Peck's scheme, the older juvenile instars were eventually replaced by a single metamorphic instar in which the increasing change of form between juvenile and adult could be accomplished. To further aid streamlining and, in the final juvenile instar, to protect the increasingly more delicate wings developing within, the wings of juveniles became firmly fused with the terga and more sclerotized, that is, wing pads. This state is comparable to that in modern exopterygote (hemimetabolous) insects. Further reduction of adult structures to the point at which they exist until metamorphosis as undifferentiated embryonic tissues (imaginal discs) beneath the juvenile integument led to the endopterygote (holometabolous) condition, that is, the evolution of the pupal stage (Section 3.3).

Regardless of their origin, the wings of the earliest flying insects were presumably well-sclerotized, heavy structures with numerous ill-defined veins. Slight traces of fluting (the formation of alternating concave and convex longitudinal veins for added strength) may have been apparent (Hamilton, 1971). The wings (and flight efficiency) were improved

TABLE 2.1. The Major Groups of Pterygota

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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