Tracheae - Respiratory Organs
Tracheal respiration is characteristic of insects. Onychophorans, diplopods, arachnids, and chilopods as well use tracheal system for respiration. In this type of respiration air is carried directly to the tissues without the intervention of blood. Tracheal system commonly consists of a large number of interconnected small tubes, the tracheae. These open outside through minute pores called spiracles that are located on either side of the body. Air is pumped into and out of tracheae by these spiracles by the ventilating movements of the body-and gaseous exchange occurs directly in the individual cell. Single spiracle may serve for both inspiration and expiration; but generally there are numerous spiracles, some of them for inflow and others for outflow of the air. The pattern of the internal tracheal system is variable but there exists a pair of longitudinal trunks along with cross connections. The tracheae are supported through the thickened spiral rings of the cuticle, the taenidia. The rings resist compression (that is preventing collapse) but permit stretching of the tube. The tracheae divide and redivide to form-minute branches termed as tracheoles. These ramify by the different tissues of the body. In some insects the tracheal tubes are dilated to form air-sacs.
A number of tracheoles may be made by a single tracheole cell. In flight muscles of a number of insects, the tracheoles even push into the fibrils. The tracheole cuticle is not shed throughout molting as is the case of tracheae and after molting new tracheae are joined to old tracheoles. Exchange of gases by the tracheae is known to occur primarily by diffusion. However, spiracles remain closed most of the time and exchange possibly takes place as a result of both diffusion and ventilation. Studies have illustrated that the spiracles open very briefly and not all at once in response to a localized reduction in hemocoel pressure. The spiracle is literally sucked open and a "gulp" of air is taken in. The pressure drop results from inter-segmental muscle contraction and is under the control of nervous system that in turn may be regulated by the oxygen or carbon dioxide tension of the blood. More spiracles are hence open during flight compared with the insect at rest. Ventilating pressure gradients result from body movements, largely abdominal movements that bring about compression of the air sac and the longitudinal extension and contraction of trachea. Ventilation is facilitated by the sequence in which specific spiracles are opened and closed. Gases are exchanged by diffusion down a concentration gradient. Tracheoles are permeable to liquids, and in most insects their tips are filled with fluid. This fluid seems to be included in the final transport of oxygen. Some of the small insects like collembolans and proturans which live in moist surroundings lack tracheae and gas exchange takes place over the general body surface. Some immature aquatic insects also lack tracheae, specifically during early stages of development. Tracheae are as well usually present in adult insects which live in water. The adults merely use air from air bubbles or films held against the body surface by special "unwettable" (hydrofuge) hairs. But the nymphs and larvae of specific groups may possess special adaptations for gas exchange in water.