A Tale of Winglets: Evolution of Flight Morphology in Stick Insects

Yu Zeng,Xin Chen,Sonal Singhal,Robert Dudley,Connor O'Malley,Sehoon Park,Faszly Rahim

doi:10.3389/fevo.2020.00121

Abstract

Author(s): Zeng, Yu; O'Malley, Connor; Singhal, Sonal; Rahim, Faszly; Park, Sehoon; Chen, Xin; Dudley, Robert | Abstract: The evolutionary transition between winglessness and a full-winged morphology requires selective advantage for intermediate forms. Conversely, repeated secondary wing reductions among the pterygotes indicates relaxation of such selection. However, evolutionary trajectories of such transitions are not well-characterized. The stick insects (Phasmatodea) exhibit diverse wing sizes at both interspecific and intersexual levels, and thus provide a system for examining how selection on flight capability, along with other selective forces, drives the evolution of flight-related morphology. Here, we examine variation in relevant morphology for stick insects using data from 1,100+ individuals representing 765 species. Although wing size varies along a continuous spectrum, taxa with either long or miniaturized wings are the most common, whereas those with intermediate-sized wings are relatively rare. In a morphological space defined by wing and body size, the aerodynamically relevant parameter termed wing loading (the average pressure exerted on the air by the wings) varies according to sex-specific scaling laws; volant but also flightless forms are the most common outcomes in both sexes. Using phylogenetically-informed analyses, we show that relative wing size and body size are inversely correlated in long-winged insects regardless of sexual differences in morphology and ecology. These results demonstrate the diversity of flight-related morphology in stick insects, and also provide a general framework for addressing evolutionary coupling between wing and body dimensions. We also find indirect evidence for a “fitness valley” associated with intermediate-sized wings, suggesting relatively rapid evolutionary transitions between wingless and volant forms.

Highlights

Flight is fundamental to the ecology and evolutionary diversification of pterygote insects by allowing for three-dimensional mobility and greater access to nutritional resources (Dudley, 2000)
We examine the correlation between Sexual wing dimorphism (SWD) and sexual size dimorphism (SSD) to address the role of sex-specific ecology in driving the diversity of flight morphology
Few species have intermediate-sized wings, suggesting the presence of a fitness valley defined by two ‘adaptive peaks’: one peak consists of wingless taxa and those with miniaturized wings (i.e., Q < ∼0.3), and another represents volant taxa (i.e., Q > ∼0.6)

Summary

Introduction

Flight is fundamental to the ecology and evolutionary diversification of pterygote insects by allowing for three-dimensional mobility and greater access to nutritional resources (Dudley, 2000). Flight Morphology Evolution in Phasmids various morphological and ecological factors, such as developmental tradeoffs, enhanced female fecundity, and reduced demand for aerial mobility in certain habitats (Roff, 1990, 1994) In these cases, smaller wings exhibit reduced aerodynamic capability, but may serve secondarily derived nonaerodynamic functions such as use in protection, stridulation, and startle displays (see Dudley, 2000). Reduced body mass enables greater maneuverability in flight (e.g., more rapid translational and rotational accelerations), numerous factors influence insect size evolution (see Blanckenhorn, 2000; Chown and Gaston, 2010). Both flight capacity and body size can be subject to sex-specific selection. It is of interest to consider patterns of sexual dimorphism in both wing and body size within a phylogenetic context

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