Abstract
The larvae of the giant silk moth (Hyalophora cecropia) spin strikingly dimorphic, multilayered cocoons that are either large and fluffy (baggy) or significantly smaller and tightly woven (compact). Although these cocoon-morphs share the same function (i.e., housing for pupal to adult development during overwintering), previous work has been unable to determine why cocoon dimorphism exists. We addressed this issue in cecropia moth cocoons collected along power line right-of-way habitats in Massachusetts. We first characterized the architectural differences between cocoon-morphs for all three cocoon sections (outer and inner envelopes, and the intermediate layer separating the two). We show that outer envelope structural and ultrastructural differences are what underlie dimorphism. Using a common spinning arena, we next show that the behavioral suites used to construct the outer envelopes of the two morphs are significantly different in behavioral time investment and patterning, as well as in the location of silk placement in the common spinning arena. Finally, we compared the cocoon-morphs in response to various environmental stressors to ask whether dimorphism is an adaptive response to such pressures. In contrast to compact cocoons, we find that baggy cocoons act as heat sinks and allow greater moisture permeability; differences in outer envelope architecture underlie these characteristics. These two biophysical properties could be advantageous for pupae in baggy cocoons, during unseasonably cold or dry conditions encountered during development prior to adult emergence. Our results suggest that cocoon dimorphism in the cecropia moth may provide a bet-hedging strategy for dealing with varying environmental conditions in Massachusetts and perhaps over its entire habitat range, during pupal to adult development.
Highlights
IntroductionDimorphic cocoon construction in the cecropia moth pupal stage, and from which individuals will later emerge (eclose) as adults
Baggy cocoons are significantly larger than compact cocoons, it is unlikely that their larger size biased our field sampling
We found that architectural dimorphism in cocoons made by caterpillars of the cecropia moth is manifested at the level of the outer envelope, in which baggy and compact cocoons differ from one another in both structural [4] and ultrastructural characteristics
Summary
Dimorphic cocoon construction in the cecropia moth pupal stage, and from which individuals will later emerge (eclose) as adults. Caterpillars will spin a cocoon that is stereotyped and species-specific, such that it can be considered an extended phenotype of the individual [3]. These differences in cocoon morphology across species may reflect specific adaptations and different strategies to maximize survivorship or proper development during the pupal stage. This includes species-specific responses to different topographies or environmental conditions during cocoon construction
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