Relationship between starvation and supernumerary ecdysis and recognition of the penultimate-larval instar in the common cutworm, Spodoptera litura

Supernumerary ecdysis of the common cutworm, Spodoptera litura was induced by crowded rearing and starvation. Long periods of starvation in the 3rd- and 4th-larval instars induced higher proportions of extra ecdyses, up to 100%. The pupal weights were essentially similar in spite of different instar numbers. However, starvation in the 5th (penultimate) and 6th instars hardly induced supernumerary ecdysis. Starvation caused a fluctuation in the head-capsule sizes of the larvae, but those larvae with head capsules wider than 1.65 mm were destined to become last instar at the next ecdysis. The significance of a threshold for the penultimate instar is discussed in relation to phase polymorphism.Crowded rearing also caused an increase in the proportion of extra ecdyses in the following generation, whereas isolation decreased it. It is concluded that supernumerary ecdysis is not a phase character, but is for recovery from reduced size due to food shortage.