The critical period for wing pattern induction in the polyphenic tropical butterfly Bicyclus anynana (Satyrinae)

Adults of the butterfly Bicyclus anynana express striking phenotypic plasti city. A wet season form has conspicuous marginal eyespots and a medial pale band which are much reduced in the dry season form. These alternative form s are produced after rearing at high or low temperatures, respectively. We used 'window' experiments involving switching of larvae and pupae between h igh and low temperatures at different stages during development to examine the timing of sensitivity to environmental temperature. The final, fifth la rval instar is shown to be especially sensitive. The fourth larval instar a nd the very early pupal period are also sensitive. It is argued that an inc reasing sensitivity during growth is ecologically adaptive since the late l arval environment will be the most accurate predictor for the adult environ ment in which the wing phenotype is subject to selection. The period of sen sitivity is not as short as a few days. This may minimize the chance of any 'mistakes in matching the adult phenotype to the season because of short-t erm environmental fluctuations during the larval period. The observed sensi tivity occurs as late as possible during growth since the wing pattern is d evelopmentally determined at the end of the early part of the pupal stage. (C) 1999 Elsevier Science Ltd. All rights reserved.