Chapter 9 - Respiratory Systems

Abstract

This chapter describes the physiology of the respiratory systems in insects. Although there are many striking similarities between the physiological systems of vertebrates and those of insects, one conspicuous difference is in the way that oxygen is brought to the cells. It was Malpighi in 1669 who first identified the series of branching tubes that bring oxygen directly to tissues, a departure from the system in vertebrates where air is regularly and often visibly drawn into the body to oxygenate body fluids that are then circulated to all cells. The hemolymph of the insect circulatory system is responsible for bathing all cells and allowing them to exchange nutrients and metabolites, but the transport of oxygen to the cells occurs through a tracheal system where external surfaces are invaginated into the body cavity to provide an oxygen pipeline from the outside. Although the tracheal system provides an enormous surface area that is permeable to both water and oxygen, water loss by this route is minimal because the system is only open to the outside at the small area that the spiracles present to the environment. Cutaneous respiration occurs in some apterygotes, aquatic insects, and endoparasites. Rather than the continuous ventilation expected by diffusion, a periodic cycle of gas exchange occurs in many insects when they are resting, known as discontinuous gas exchange cycles. Aquatic insects evolved from terrestrial ancestors, and various adaptations have been necessary for them to return to the water, such as cutaneous respiration, plastron respiration, spiracular gills, development of tracheal gills, and evolution of hydrofuge surfaces in insects that retained terrestrial tracheal system.