Why do insects have an external skeleton instead of an internal skeleton?

Insects possess an exoskeleton rather than an internal skeleton because it serves as a highly efficient, multifunctional system for small animals. This rigid, chitinous envelope provides several critical evolutionary advantages:1

• Protection and Support: The exoskeleton acts as a protective armor, shielding soft tissues from the external environment. Its cylindrical design is structurally efficient, resisting bending and buckling with minimal material weight.21
• Waterproofing: For terrestrial insects, the cuticle includes a waxy, water-resistant layer that prevents dehydration, a vital adaptation that has allowed them to colonize diverse land habitats.234
• Muscular Attachment: The exoskeleton provides a large surface area for the attachment of muscles, functioning as a lever system that enables movement. In some insects, hardened internal ingrowths of the cuticle, called apodemes, even provide internal sites for muscle attachment.41

While the exoskeleton is highly effective for small organisms, it imposes limits on body size, as a massive external skeleton would be too heavy to support and would risk collapse during the molting process.1