Q.The respiratory structures of insects are

Different animal groups use different organs to exchange gases, depending on habitat and level of body organisation. Simple animals such as sponges, coelenterates and flatworms have no dedicated respiratory organ at all - gases diffuse directly across the body surface. Earthworms use their moist, vascularised skin. Insects have a system of branching tracheal tubes that carry air directly to tissues, bypassing the blood entirely. Gills serve most aquatic arthropods (like prawns), molluscs, and fishes among the vertebrates - because water holds far less dissolved oxygen than air, gill-breathing aquatic animals must ventilate much faster than land animals. Amphibians, reptiles, birds and mammals rely on well-vascularised lungs; frogs additionally use their moist skin alongside their lungs. In humans, air travels through the external nostrils, nasal chamber, nasopharynx, glottis, larynx, trachea, bronchi, bronchioles, and finally the alveoli, where gas exchange occurs; a thin flap called the epiglottis seals the larynx during swallowing to keep food out of the airway. Structurally, the pathway from the nostrils to the terminal bronchioles is the conducting zone (filters/warms/moistens air only), while the alveoli and their ducts form the respiratory zone, where actual gas exchange happens.