Dispersal is a key aspect of population dynamics, but, for winged insects, we know little about the characteristics that make some better dispersers than others. Insects with relatively large and long, pointed wings are hypothetically better at powered flight and hence dispersal than those with relatively small, rounded wings. However, tests have been largely impossible given the difficulty of tracking dispersing individuals.
To solve this, we used a novel sampling design. In five pairs of hydrologically isolated streams that each share a catchment boundary, we sampled adult caddisflies at stream margins and up on the boundary, where there was no running water, i.e. caddisflies trapped on a boundary had to have dispersed. Of >130 spp., ~ half were trapped on a boundary (“boundary species”), while the other half were never found on a boundary (“lowland species”). We removed a single pair of wings from 10-30 males of multiple boundary and lowland species. Wings were mounted on microscope slides and photographed. Using images of coupled wings, we measured four wing metrics - wing span and area, aspect ratio and the second moment of wing area - to characterise wing sizes and shapes.
Boundary species typically had greater wing areas and wing spans and higher aspect ratios than lowland species. Species with the greatest wing areas (e.g. Triplectides similis, Leptoceridae) or highest aspect ratios (e.g. Hellyethira simplex, Hydroptilidae) were all boundary species. However, values of both those metrics spanned a wide range, and similar values were shared by some boundary and lowland species.
Our results show that wing area and span and aspect ratio can be used to identify good dispersers, and hence these metrics show promise for use as evidence-based, dispersal traits. However, overlap between some boundary and lowland species show that behaviour (i.e. choosing to disperse) must also be considered.