Variation in transparency degree: response to light availability or part of mimicry syndrome?

Abstract

Transparency reduces prey detectability by predators. While transparent aquatic species hold higher transparency levels as the light availability of their habitat increases, less is known about such variation in terrestrial species. Lepidoptera species exhibiting transparent wings display various levels of transparency. Using two complementary approaches, we explore how the evolution of different transparency degrees relates to habitat openness, activity rhythm and mimicry syndrome (bee/wasp versus dead-leaf mimic). First, by exposing artificial moth-like prey to wild avian predators in a range of habitat openness, we show that survival is lower in more open habitats. We also found that less transparent morphs are more attacked than more transparent ones, regardless of habitat openness degree. Second, by analysing the evolution of wing features and ecological traits in 107 clearwing species, we found that diurnal species transmit more light than nocturnal species under certain conditions (when considering only forewings, at smaller clearwing surfaces and at larger wing lengths) and that species flying in open habitats and exhibiting large percentages of clearwing surface transmit slightly more light than those flying in closed habitats, although this is reversed at smaller percentages of clearwing surfaces. Additionally, bee/wasp mimics are more often diurnal and have higher and less variable light transmittances than dead-leaf mimics, which are more often nocturnal. Flying during the day, in open habitats and mimicking insects with transparent wings seem to promote high light transmittance under certain circumstances. Activity rhythm, habitat openness and species interactions play a crucial role in determining transparency design on land.