How do insects primarily respire?

Insects primarily respire through spiracles and tracheae, which are specialized structures that facilitate the exchange of gases directly with their tissues. Spiracles are small openings located along the sides of an insect's body that allow air to enter and exit. From these spiracles, a network of tracheae—internal tubes—distribute oxygen directly to the cells and tissues, allowing for efficient gas exchange without the need for a circulatory system to transport oxygen.

This method of respiration is highly efficient for the small size of insects, as it enables rapid delivery of oxygen and removal of carbon dioxide. The tracheal system also maintains relatively low weight and minimizes water loss, which is especially advantageous for terrestrial insects. This adaptation is crucial, as insects have high metabolic rates that require a constant supply of oxygen.

In contrast, other forms of respiration mentioned in the other choices are not applicable to insects. For example, gills are used by aquatic organisms to extract oxygen from water, lungs with alveoli are found in mammals, and while skin diffusion occurs in some animals, it is not the primary means of respiration in insects. The tracheal system is distinctly suited for their physiology and ecological niches.