premolt

Premolt is a preparatory phase in the molt cycle of many arthropods, occurring between a previous molt and the actual shedding of the exoskeleton (ecdysis). During premolt, animals prepare a new cuticle beneath the old one and reabsorb minerals from the old exoskeleton; the old cuticle typically separates from the underlying epidermis in a process called apolysis, creating space for the new cuticle.

In crustaceans, the transition is hormonally regulated by a balance between molt-inhibiting hormone (MIH) produced by the X-organ-sinus gland complex and ecdysteroids released by the Y-organ. A decline in MIH activity permits Y-organ activation and ecdysteroid pulses that drive epidermal cell growth, cuticle secretion, and prep for ecdysis. In insects and other arthropods, ecdysteroids promote similar processes of apolysis and the production of a new cuticle layers.

The premolt period is variable in duration; feeding often decreases, locomotion is reduced, and pigments may migrate, affecting coloration. The new cuticle remains soft until hardening (sclerotization or calcification) after the old cuticle is shed during ecdysis.

The premolt phase is critical for growth, and disruption can lead to molting defects or mortality.