Phenotypic plasticity in a generalist insect herbivore with the combined use of direct and indirect cues

Ultimate causes of phenotypic plasticity in visual appearance are frequently related to increasing the degree of crypsis in a way specific to the environment. The cues used to elicit such plastic responses may be both direct (i.e. straightforward background matching) as well as indirect. In the latter case, cues other than the visual signals from of the environment are used to predict the phenotype best corresponding to the particular situation. On the basis of a series of laboratory experiments we show that the remarkable variability in the visual appearance of the larvae of the geometrid moth Ematurga atomaria, though genetically based in part, involves a substantial environmental component. Using multiple correspondence analysis, we transformed the multidimensional variation in colour and pattern into two dimensions interpretable as patterning and darkness. Plastic changes in the darkness of the larvae were elicited by direct cues: the larvae were darker when reared on dark host‐plants. Host‐specific degree of patterning was also induced in absolute darkness which indicates the use of an indirect cue. This was unexpected because the study species is broadly polyphagous, and thus not likely to have evolved adaptations specific to particular host‐plant species. Indeed, the larvae of E. atomaria originating from geographic populations using different host‐plants showed analogous plastic responses which indicates that the link between the indirect cue and visual appearance of the host needs not to be specific to plant species. In an additional experiment, we showed that surface roughness is a likely candidate to serve as the proximate cue for determination of some pattern elements, a case not reported for insect larvae earlier.