molt: one construct, many systems

In arthropods, moulting (ecdysis) is the periodic shedding of the rigid cuticle that a growing animal with an external skeleton cannot otherwise enlarge. A new, initially soft cuticle is laid down beneath the old one, the old exoskeleton splits along fixed lines and is cast off, and the animal swells and then re-hardens. Ewer, Gammie, and Truman's 1997 work on the endocrine control of insect ecdysis shows how a positive-feedback hormone cascade (eclosion hormone and ecdysis-triggering hormone) drives the shedding behaviour, and Chapman's The Insects: Structure and Function details the cuticular cycle. The referent is the discarding and renewal of the body's outer covering, the same construct seen when a bird replaces its plumage.[1][2]

Unit

The periodic shedding and renewal of the cuticular exoskeleton

Recovery means

The animal is briefly soft and vulnerable (teneral) until the new cuticle expands and sclerotises

Measured by

Counted as instars or moult cycles and timed by ecdysteroid peaks and ecdysis-triggering hormone release

In birds, moult is the scheduled replacement of feathers, which are dead structures that wear out and so must be shed and regrown, usually in an orderly sequence that preserves flight and insulation. Rohwer and colleagues' 2009 Allometry of the duration of flight feather molt in birds analyses how long this replacement takes and how its duration scales with body size across species. It is the same underlying construct as arthropod ecdysis (the periodic casting-off and renewal of the body's outer covering), differing mainly in that feathers are replaced in sequence rather than the whole integument at once.[3]

Unit

The scheduled shedding and regrowth of plumage

Recovery means

Flight and insulation are restored as replacement feathers grow in

Measured by

Scored by moult sequence, extent, and the duration of flight-feather replacement