Molting, scientifically known as ecdysis, is a fundamental biological process for insects and all other arthropods. This complex, hormonally regulated cycle involves the periodic shedding of the insect’s rigid outer covering. This process is necessary because the hard external skeleton provides protection but limits continuous growth, requiring the organism to shed the covering to increase in size and advance through developmental stages.
Why Insects Must Shed Their Skin
Insects are covered by a stiff, non-living external skeleton called the exoskeleton, which is primarily composed of the tough carbohydrate chitin and various proteins. This rigid outer layer provides structural support and protection for the insect’s soft internal tissues, serving as a point of attachment for muscles. The major drawback of this “suit of armor” is that it cannot expand with the insect’s body as it grows, placing a physical constraint on its size.
To accommodate an increase in body mass and volume, the insect must periodically replace its old, outgrown exoskeleton with a new, larger one. This process is triggered and controlled by hormones, most notably ecdysteroids such as 20-hydroxyecdysone, which initiate the physiological changes beneath the old cuticle. The period between two successive molts is called an instar, and the frequency of these molts is influenced by factors like temperature and food availability.
The Step-by-Step Process of Ecdysis
The process begins with apolysis, which is the separation of the old cuticle from the underlying epidermal cells, creating a small space between the two layers. The epidermal cells then secrete an inactive molting fluid into this space and begin constructing a new, soft cuticle beneath the old one.
Once the new cuticle is partially formed, enzymes in the molting fluid activate, starting the chemical digestion of the old exoskeleton’s inner layers. The insect reabsorbs most of the digested material, such as proteins and chitin, to recycle these components for the new cuticle. Ecdysis is the physical act of shedding the remnants of the old exoskeleton (exuviae), which typically splits along predetermined lines of weakness. After emerging, the newly molted insect’s cuticle is soft and pale (teneral) and must rapidly expand its body by taking in air or water. The new exoskeleton then hardens and darkens through sclerotization, restoring the insect’s protective armor.
When Molting Stops: Immature Stages vs. Adults
For the vast majority of insects, molting is strictly limited to the immature phases of their life cycle, such as the larval or nymphal stages. Once an insect completes its final molt and reaches the adult stage (imago), it generally stops growing and does not molt again. In insects with incomplete metamorphosis, like grasshoppers or cockroaches, the nymph molts repeatedly until the final molt produces a sexually mature adult with fully developed wings.
Insects with complete metamorphosis, such as butterflies and beetles, undergo several larval molts before transforming into a pupa, a non-feeding, transformative stage. The final molt from the pupa produces the adult form, and the glands that produce the molting hormones often disintegrate at this point. The cessation of molting in adults allows the insect to channel its energy entirely into reproduction. However, a notable exception exists among primitive, wingless insect orders, like silverfish and firebrats (Zygentoma), which continue to molt throughout their entire adult lives.