What are the primary defining characteristics of the class Insecta?

The primary defining characteristics of the class Insecta include having a segmented body and three pairs of legs. Insects are distinguished by their body plan, which is divided into three distinct sections: the head, thorax, and abdomen. This segmentation is a key feature of insects and plays a crucial role in their anatomy and function.

The thorax, in particular, is where the three pairs of legs are attached - a fundamental characteristic of insects. Each pair of legs is adapted for specific functions such as walking, jumping, or swimming, depending on the lifestyle of the species. This anatomical structure not only aids in locomotion but also supports their various adaptations across different environments.

The other characteristics presented in the choices do not accurately define Insecta. For instance, insects do not necessarily have a dorsal spine, are not limited to aquatic environments, and they possess an exoskeleton made of chitin, which is vital for supporting their bodies and protecting them from environmental challenges.