Insects and other “bugs” do possess skeletons, though not in the same manner as humans or other vertebrates. Instead of an internal bone structure, these creatures feature a unique external skeletal system known as an exoskeleton. This rigid outer covering provides support and form.
The External Framework
The exoskeleton is a multi-layered outer covering that defines arthropods, including insects, arachnids, crustaceans, and myriapods. Its primary component is chitin, a robust polysaccharide, second only to cellulose in abundance. Chitin forms fibrous chains embedded within a matrix of proteins, which can be modified for varying degrees of hardness and flexibility.
This external framework consists of several distinct layers. The outermost layer is the epicuticle, a thin protein layer often containing waxes that provide a water-tight barrier, particularly in terrestrial arthropods. Beneath this lies the procuticle, a thicker layer composed of chitin and proteins. The procuticle itself is divided into an outer exocuticle and an inner endocuticle.
Hardening occurs in the exocuticle through sclerotization, involving protein cross-linking that provides rigidity. In crustaceans, additional strength comes from the deposition of calcium carbonate minerals within the chitin-protein matrix, a process known as biomineralization. The endocuticle, conversely, remains more flexible. This layered structure allows for both strength and elasticity, important for arthropod survival.
More Than Just Support
The exoskeleton offers many functions beyond structural support. It acts as a protective barrier, shielding internal organs from physical damage and providing defense against predators.
The exoskeleton also plays an important role in preventing desiccation, or water loss, which is important for terrestrial species. The waxy epicuticle reduces evaporation from the body surface. Without this barrier, arthropods would quickly lose moisture.
For movement, muscles attach to the inside surface of the exoskeleton. These internal attachment points, often infoldings of the exoskeleton called apodemes, allow the exoskeleton and muscles to function as a lever system. This arrangement enables efficient locomotion and a wide range of movements, despite the rigid external covering.
Growing Pains: The Molting Process
Because the exoskeleton is a rigid external covering, it does not grow with the arthropod. To increase in size, arthropods must periodically shed their old exoskeleton in a process called molting, or ecdysis. This hormonally controlled event allows for growth and development.
Molting begins with a preparatory phase where the arthropod stops feeding and seeks a safe location. The epidermis, the living cell layer beneath the old exoskeleton, secretes enzymes that digest and separate the inner layers of the old cuticle from the body. Simultaneously, a new, soft, and flexible exoskeleton forms underneath the old one.
Once the old exoskeleton is detached, it splits, often along predetermined lines, and the arthropod wriggles out. During this emergence, the animal is vulnerable as its new exoskeleton is still soft. The arthropod then inflates its body, often by taking in air or water, to stretch the new cuticle.
The new exoskeleton then hardens and darkens through sclerotization, a process that can take hours or even days depending on the species. This hardening provides the necessary rigidity and protection for the arthropod to resume its normal activities. The entire molting process is regulated by hormones, such as ecdysone, which trigger the sequence of events.