Respiratory (Tracheal) System

Overview

Cockroaches, like other insects, respire by means of a tracheal system — a network of air-filled tubes that carries atmospheric oxygen directly to every tissue and removes carbon dioxide, entirely bypassing the blood. This is why cockroach haemolymph carries no respiratory pigment: gas exchange is the tracheal system's job, not the circulatory system's.

Spiracles

Air enters and leaves the body through small external openings called spiracles, arranged in a row along each side of the body. The cockroach has 10 pairs of spiracles — 2 pairs on the thorax (mesothorax and metathorax) and 8 pairs on the abdomen. Each spiracle is guarded by a small valve controlled by tiny muscles, which can open and close the pore — this valve action conserves water by limiting spiracle opening to when gas exchange is actually needed, and can also exclude dust, parasites and excess moisture.

Tracheae

From each spiracle, a short atrium leads into a trachea — a tube whose wall is a thin cuticular lining thrown into spiral thickenings called taenidia, which keep the tube open (like the wire coil in a vacuum-cleaner hose) even when it is bent. The tracheae branch repeatedly inside the body, forming a rich, tree-like network that reaches almost every organ and tissue; at several points the tracheae are also dilated into thin-walled air sacs, which act as bellows-like reservoirs of air, improving the volume of air moved with each ventilatory stroke.

Tracheoles

The finest terminal branches of the tracheal tree are called tracheoles — extremely fine, thin-walled tubules (only about a micrometre or so across) that penetrate between and even into individual cells, especially in actively respiring tissues such as flight muscle. Unlike the larger tracheae, tracheoles have no taenidia and their tips are typically filled with fluid; oxygen dissolves into this fluid and diffuses the last short distance directly into the adjoining cells, while carbon dioxide diffuses out along the reverse path. This arrangement means that gas exchange in insects occurs almost entirely by diffusion through air-filled tubes reaching deep into the tissues, rather than through a blood-gas transport system as in vertebrates.

Ventilation …