Size compensation in moth larvae: attention to larval instars

Environmental perturbations such as starvation and poor diet often prevent animals from attaining their optimal sizes. When the perturbation has a transient character, compensatory responses are expected in terms of faster growth or a prolonged developmental period. In the case of insect larvae, details of such responses are insufficiently known at the proximate level. Attention to responses at the level of particular larval instars should promote an understanding of insect developmental plasticity also in a more general context. To provide an instar‐specific analysis of compensatory growth, larvae of the moth Orgyia antiqua (L.) are reared on inferior diet during one larval instar. Responses in growth parameters are recorded in the course of the manipulated instars, as well as at the level of the entire larval period. The negative relationship between development time and size in response to the inferior food quality, typical of the entire larval periods, is also observed within the manipulated instars taken separately. The manipulated larvae remain smaller than the larvae of the control group (significant in males only), even by the end of the subsequent instar during which all individuals are provided with superior host. In males, close to full size compensation by the time of pupation is achieved only by means of adding an extra larval instar. The inability of larvae to fully compensate during one and even two instars is considered as an indication of the presence of constraints on the within‐instar growth pattern. An alternative, adaptational explanation for the incomplete compensation could be based on the cost of prolonged development period. Given the ecological context of the species' life history, such an explanation appears less likely.