FORUM Intraspecific Variability in Number of Larval Instars in Insects

ABSTRACT Thenumberof larval instars varieswidely across insect species.Although instar number is frequently considered to be invariable within species, intraspeciÞc variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies,we show that intraspeciÞc variability in thenumber of larval instars iswidespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, foodquality andquantity, humidity, rearing density, physical condition, inheritance, and sex are themost common factors inßuencing the number of instars.We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according towhich instar number increases in adverse conditions when larvae fail to reach a species-speciÞc threshold size formetamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more speciÞc cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional gen