Concept:Insects do not have lungs; they breathe through a network of air tubes called tracheae that open to the outside via small pores.Explanation:Oxygen enters the insect’s body through these external openings.These openings are known as spiracles.Air passes from spiracles into the tracheae, which carry oxygen directly to the tissues.Thus, the structure used for respiration in insects is the spiracle.Answer:D. Spiracles