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

The article focuses on the morphological and biomechanical transitions underlying flight evolution in stick insects (order Phasmatodea), which exhibit a continuum from winglessness to fully developed wings. By analyzing wing shape, venation, wing and flight muscle mass, and body-leg mass distribution across multiple phasmid taxa, the study identifies scaling relationships correlated with wing length, relative wing size (Q), and wing loading (pw, the ratio of body weight to wing area). Key findings include sex-specific changes in wing aspect ratio, a critical threshold of wing loading (~10 N m⁻²) associated with sharp reductions in wing venation complexity, and a posterior shift of wing-bearing thoracic segments closer to the insect’s center of mass as wings reduce. These morphological reconfigurations suggest complex adaptations during flight loss or gain, providing a model for understanding intermediate stages in the origin of insect flight, especially given the lack of fossil intermediates in early pterygotes.