Dimorphic cocoons of the robin moth, Hyalophora cecropia, reflect the existence of two distinct architectural syndromes

The architectural design of animal structures forms part of an individual's extended phenotype that can be subjected to strong selection pressures. We examined cocoon architectural dimorphism in robin moths (Hyalophora cecropia), which construct multilayered silk-woven cocoons that possess either ‘baggy’ or ‘compact’ morphology. These dimorphic cocoons reflect extended phenotypes that can enable survival during a critical developmental period (pupal stage to adult emergence), with cocoons occurring either sympatrically or as monomorphic groups across different climatic regions in North America. We hypothesized that cocoon dimorphism is related to the cocoon's role as a mediating barrier for moisture. We predicted that the macro- and micro-architectural differences between the cocoon-morphs would be consistent with this function. We compared the cocoon-morphs in their orientation when spun under natural field conditions, examined how these orientations affected cocoon water absorption under simulated rain trials, and performed material surface tests to compare the hydrophobicity of cocoons. We found that compact cocoons had traits that increased water resistance, as they had significantly greater hydrophobicity than baggy cocoons, because they absorbed less water and released water vapor faster. In contrast, the increased water absorptiveness of baggy cocoons can allow for greater moisture retention, a function related to the prevention of desiccation. Our study suggests that cocoon dimorphism in robin moths reflects distinct architectural syndromes, in which cocoons are spun to optimize either water resistance or retention. These different functions are consistent with strategies that act to respond to uncertain external environmental conditions that an individual might encounter during development.

Expansion du papillon cécropia dans la zone tempérée mixte du Québec

Depuis le tournant du présent siècle, le majestueux papillon cécropia (Hyalophora cecropia) a étendu progressivement son aire de répartition au Québec dans la zone tempérée mixte et dans le domaine bioclimatique de la sapinière à bouleau blanc en zone boréale. Vers 2006-2007, cette saturnie aurait simultanément atteint les basses terres de la Haute-Côte-Nord, du Lac-Saint-Jean et d’Amos en Abitibi. Les signalements par le public du papillon et de la chenille au Saguenay–Lac-Saint-Jean ont été nettement plus fréquents en 2011 ; une centaine de cocons ont été trouvés tout autour du lac Saint-Jean dans des lisières arbustives en milieu rudéral, agricole, riverain et résidentiel, principalement sur le cerisier (Prunus spp.), le saule (Salix spp.), l’aulne (Alnus rugosa) et le cornouiller (Cornus stolonifera). La diminution du couvert de conifères au profit de jeunes peuplements mixtes et feuillus et la clémence de certains hivers au cours des années 2000 auraient occasionné cette expansion de l’espèce au nord du 47e parallèle.