Beyond winglets: evolutionary scaling of flight-related morphology in stick insects (Phasmatodea)

Abstract

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.

Phasmid species that inhabit colder environments are less likely to have the ability to fly

“…However, if a phasmid species has long wings, they are generally assumed to be capable of flight but this is not always the case (e.g. Bank & Bradler, 2022;Zeng et al, 2020Zeng et al, , 2022. Here, I assume that all phasmids with long wings can fly.…”

Independent wing reductions and losses among stick and leaf insects (Phasmatodea), supported by new Cretaceous fossils in amber

“…As noted, the reduction and loss of wings is frequent in extant stick and leaf insects. There are also fully-winged species capable of good flight, but the wings of many relatively macropterous lineages are more often used to control free-fall descents from tree canopies [9][10][11][12]. Some brachypterous lineages have also evolved non-aerodynamic functions for wings, such as aposematic coloration or a stridulation capability [5].…”