Moths, like all insects and arthropods, do not have an internal skeletal structure (endoskeleton). Instead, they rely entirely on a rigid external covering called an exoskeleton for structural support. This hard shell encases their internal organs and tissues. The exoskeleton provides protection and serves as an anchor for muscle attachment, but its external location presents unique challenges for growth and movement.
The Exoskeleton: Structure and Composition
The moth’s exoskeleton is a complex, multi-layered cuticle that functions as its armor and support system. The primary building block of this structure is chitin, a strong, flexible polysaccharide that is chemically similar to cellulose. Chitin microfibers are embedded in a matrix of proteins, which gives the cuticle its characteristic toughness and resilience.
The outermost layer is the epicuticle, a thin, waxy covering that lacks chitin and is water-resistant. This layer prevents water loss, a significant risk for small terrestrial organisms. Beneath this is the procuticle, which is stratified into the exocuticle and the endocuticle.
The exocuticle is the harder, rigid layer, having undergone sclerotization, or “tanning,” where protein molecules are cross-linked. This hardening provides the insect with stiff, protective plates. The inner endocuticle remains flexible, providing a tough but pliable inner lining with its fibrous chitin and proteins arranged in crisscrossing layers for strength.
How Moths Grow and Change
The rigid exoskeleton cannot stretch or expand as the moth grows. This limitation is overcome through molting, or ecdysis, which is hormonally triggered when the insect’s body reaches the physical limits of its current shell. Molting allows moths, primarily in their larval stage, to shed their outgrown cuticle and dramatically increase in size.
Before shedding, epidermal cells secrete a new, soft cuticle beneath the old one and release enzymes that digest the inner layer of the old exoskeleton. The insect then inflates its body, often by swallowing air or water, to split the old shell along lines of weakness. It struggles out of the old casing, which is left behind as the exuviae.
The newly exposed exoskeleton is soft and pale, making the moth or caterpillar highly vulnerable to predators during this time. The insect must quickly expand its body to a larger size before the new cuticle hardens through sclerotization, which can take several hours to days. This process enables the growth of the larva and the transformation into a winged adult moth, which typically ceases molting once it reaches its final form.
Internal Support and Locomotion
To facilitate movement, the exoskeleton is utilized as an anchor for muscles. Instead of muscles wrapping around bones, they attach directly to the inside surface of the cuticle. To provide better leverage and a larger surface area for attachment, the exoskeleton features inward folds and ridges called apodemes.
These apodemes function similarly to vertebrate tendons, serving as strong attachment points that allow muscles to contract and move the body segments, legs, and wings. The moth’s flight muscles are anchored to large, internal apodemes in the thorax, enabling the contractions necessary for flight.
Beyond the exoskeleton, the moth also relies on a hydrostatic skeleton, particularly in its larval stage. Internal body fluid, called hemolymph, circulates within the body cavity, providing pressure that helps maintain the insect’s shape and supports its internal organs. This fluid pressure is especially noticeable immediately after molting, as the moth uses the hemolymph to inflate and extend its crumpled wings before the new exoskeleton hardens.