What characteristic distinguishes insects as invertebrates?

Insects are indeed classified as invertebrates primarily due to their exoskeleton. This hard outer structure provides support and protection but does not afford the internal bony framework characteristic of vertebrates, which possess backbones. Invertebrates like insects rely on this exoskeleton made of chitin, which enables them to have the necessary strength and flexibility for movement, as well as serving critical functions such as preventing water loss and providing a barrier against predators.

The presence of a backbone defines vertebrates, so insects, lacking this feature, are categorized as invertebrates. Additionally, characteristics such as having a warm-blooded physiology or lungs are related to different classifications within the animal kingdom. Insects typically utilize a different system for respiration, involving spiracles and tracheae rather than lungs, which further highlights the distinctions between them and vertebrates.