Halali, D. | Search bioRxiv

bioRxiv2026

Toxin structure shapes palatability in a chemically defended butterfly

The toxicity of chemical defences is well studied, but the potential contribution of compound structure to predator deterrence remains largely unexplored. Whether predation acts more strongly on toxicity or unpalatability remains largely untested, partly because few systems allow toxin structure to vary independently of quantity. Heliconius sara larvae provide such a system: those reared on Passiflora auriculata sequester cyclopentenyl cyanogenic glucosides (CGs), while those reared on P. biflora biosynthesise comparable quantities of aliphatic CGs. Using two invertebrate predators, Camponotus floridanus ants and Hierodula membranacea mantids, we tested whether this structural difference affects palatability independent of toxicity. Mantids rejected larvae with cyclopentenyl CGs more often than larvae with aliphatic CGs, despite no detectable difference in total CG content. This pattern was mirrored in extract-based assays with ants, independently of cyanide release: extracts with cyclopentenyl CGs remained deterrent, while extracts with aliphatic CGs did not differ in deterrence from water. Live larvae, by contrast, elicited similar responses from ants regardless of CG structure. These results show that variation in toxin structure can strongly affect palatability, with some compounds conferring greater protection than others. This demonstrates the importance of chemical structural diversity in the evolution of chemical defences.

evolutionary biology↗

bioRxiv2023

Larval growth rate affects wing shape more than eyespot size in the seasonally polyphenic butterfly Melanitis leda

Butterflies often show adaptive phenotypic plasticity where environmental cues during early stages are used to produce a phenotype that maximizes fitness in the environment experienced by adults. Many tropical satyrine butterflies (Nymphalidae: Satyrinae) are seasonally polyphenic and produce distinct wet- and dry-season form adults providing tight environment-phenotype matching in seasonal environments. Dry-season forms, which are expressed in the dry season, can be induced in the laboratory by growing larvae at low temperatures or on poor food quality. Since both these factors also tend to reduce larval growth rate, larval growth rate may be an internal cue that translates the environmental cues into the expression of phenotypes. If this is the case, we predict that slower-growing larvae would be more likely to develop a dry-season phenotype. To test this hypothesis, we measured both larval growth rate and adult phenotype (eyespot size and wing shape) of individuals of the common evening brown butterfly (Melanitis leda), reared at various temperatures and on various host-plant species. We found that among treatments, larvae with lower growth rates (low temperature, particular host plants) were more likely to develop dry-season phenotypes (small eyespots, falcate wing tips), but within treatments, larval growth rate was mainly linked to wing shape, not eyespot size. These relationships tended to be stronger for males than females as males showed a wider range of eyespot sizes and wing shapes. Overall, only plasticity in wing shape appears to be (partly) mediated by larval growth, and in a sex-specific manner.

evolutionary biology↗