The first winged insects evolved from a wingless ancestor, but details of the transition from wingless to winged morphology remains unclear. Studying extant pterygotes undergoing secondary flight loss may help to understand such a transition in reverse temporal sequence. Stick insects (Order Phasmatodea) exhibit differently sized partial wings and frequent transitions between fully-winged and wingless forms, along with robust near-isometric scaling of wing area and body mass, with a correspondingly wide range of wing loading variation. To address how other flight-related morphological traits (including the flight apparatus and body-leg system) might correlate with wing size evolution, we studied wing and body shape in fifty different phasmid taxa over a wing loading range from 1.4 — 2300 Nm-2. Wing shape evolution showed sex-specific trends, with a linear reduction of aspect ratio over the wing loading range of 2.2 — 23 Nm-2 in female insects, but a positive correlation with wing size in the males. Also, with reduced wing size and increased wing loading, wing venation exhibited structural reconfiguration across the wing loading range 6 — 25 Nm-2, and the wingbase shifted from the anterior half of the body to nearer the center of body mass. Nonetheless, masses of the wings and flight muscle, and the shape of the body-leg system, were predominately predicted by specific allometric scaling relationships and not by wing loading. Such morphological reorganization relative to wing size evolution likely reflects interplay between selection on flight performance and on wing and body size within different ecological contexts. These results reveal complex reconfiguration of flight-related traits on the wing loading landscape during wing and body size evolution in phasmids.

The first winged insects evolved from a wingless ancestor, but details of the transition to a fully-winged morphology remain unclear. Studying extant pterygotes with partial wings, such as the stick insects (order Phasmatodea), may help us to understand such a transition. Here, we address how a series of flight-related morphological parameters may correlate with flight evolution by studying different phasmids representing a volancy continuum ranging from miniaturized to full-sized wings. Variation in phasmid wing shape, venation, wing mass and the mass of flight muscle can be described by specific scaling laws referenced to wing length and wing loading. Also, the mass distribution of the body-leg system is conserved in spite of a wide range of variation in body shape. With reduced wing size and increased wing loading, the longitudinal position of the wing-bearing thoracic segments is shifted closer to the insects’ centre of body mass. These results demonstrate complex reconfiguration of the flight system during wing morphological transitions in phasmids, with various anatomical features potentially correlated with reduced flight performance attained with partial wings. Although these data represent phasmid-specific features of the flight apparatus and body plan, the associated scaling relationships can provide insight into the functionality of intermediate conditions between wingless and fully-winged insects more generally.

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