The term “bugs” commonly refers to small creatures like insects, arachnids, myriapods, and crustaceans. These diverse groups, found globally, often raise questions about their internal structure, particularly whether they possess bones like familiar vertebrates. Understanding their unique anatomy clarifies how these creatures are built and thrive.
The Answer: No Bones
Insects, arachnids, myriapods, and crustaceans—commonly called “bugs”—do not possess an internal bony skeleton. Unlike vertebrates with their endoskeletons of bones and cartilage, these animals use an external framework: the exoskeleton. This rigid outer covering functions much like an internal skeleton, providing support and shape. Composed primarily of chitin, a tough yet flexible polysaccharide, and various proteins, the exoskeleton can be further strengthened by calcium carbonate in groups like crustaceans.
The Exoskeleton System
The exoskeleton is a hard, protective casing that completely encloses the animal’s body. This external armor provides structural support and protection from predators and environmental hazards. Its composition, including waxes in terrestrial arthropods, also helps prevent desiccation, vital for survival outside of aquatic environments.
The exoskeleton facilitates movement, serving as a direct attachment point for muscles. Unlike vertebrates where muscles overlay bones, arthropod muscles attach to the inner surface of their rigid external shell. This allows muscles to contract and pull against the hard plates, facilitating precise movements for locomotion, feeding, and other activities. Though rigid, the exoskeleton is segmented with flexible membranes at the joints, enabling a wide range of motion.
Growth and the Exoskeleton
Growth poses a challenge for animals with rigid exoskeletons, as the outer shell cannot expand. To overcome this, these creatures undergo molting, also known as ecdysis. During molting, a new, soft cuticle forms beneath the old exoskeleton, secreted by underlying epidermal cells. Enzymes then digest the inner layers of the old shell, separating it from the new one.
Once the new exoskeleton is developed, the animal sheds its old covering, often by inflating its body to split the old shell along lines of weakness. After emerging, the new exoskeleton is initially soft, pale, and flexible, making the animal vulnerable. Over several hours to days, this new shell expands and hardens through sclerotization, where proteins within the cuticle cross-link, providing rigidity and protection. This periodic shedding allows the animal to grow in size and progress through life stages.