Does wing reduction influence the relationship between altitude and insect body size? A case study using New Zealand's diverse stonefly fauna.

Researchers have long been intrigued by evolutionary processes that explain biological diversity. Numerous studies have reported strong associations between animal body size and altitude, but insect analyses have often yielded equivocal results. Here, we analyze a collection database of New Zealand's diverse endemic stonefly fauna (106 species across 21 genera) to test for relationships between altitude and plecopteran body size. This insect assemblage includes a variety of wing-reduced (26 spp) and fully winged (80 spp) taxa and covers a broad range of altitudes (0-2,000 m). We detected significant relationships between altitude and body size for wing-reduced, but not fully winged, stonefly taxa. These results suggest that, while the maintenance of flight apparatus might place a constraint on body size in some fully winged species, the loss of flight may free insects from this evolutionary constraint. We suggest that rapid switches in insect dispersal ability may facilitate rapid evolutionary shifts across a number of biological attributes and may explain the inconsistent results from previous macroecological analyses of insect assemblages.