BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.4. Gas Exchange

9.4.4. Insects, e.g., the locust

In insects, gas exchange is carried out through a system of tiny tubes known as tracheae. This system allows oxygen to pass directly from the air to the Tissues, eliminating The Need for Blood to transport it. This is a much faster method than the diffusion of dissolved oxygen through tissues; such a gas exchange system Supports a High Metabolic Rate.

Spiracles are paired openings located on the second and third thoracic segments and the first eight abdominal segments of the insect body, leading into air-filled chambers. Branching tubes called tracheae extend from these chambers (Fig. 9.15). Each Trachea is lined with epithelium that secretes a thin layer of chitinous material. Typically, this rigid layer is further reinforced by spiral and annular thickenings, which keep the air passages open even when the pressure within the tracheae drops below atmospheric (compare with the cartilaginous rings in the human Trachea and Bronchi). In each body segment, the tracheae branch into numerous smaller tubes called tracheoles; these further branch to permeate the insect's tissues and, in the most active tissues, such as flight Muscles, terminate blindly within individual Cells. The degree of tracheole branching can vary depending on the metabolic demands of the tissues.

Class="center">

Fig. 9.15. A. The tracheal system of a locust. B. The Structure of an insect trachea.

The tracheoles lack a chitinous lining. At rest, they are filled with an aqueous fluid (Fig. 9.16); during this time, oxygen diffuses through them to the tissues (and CO₂ in the opposite direction) at a rate entirely sufficient to meet the insect's needs. Under active conditions, increased metabolic activity in the muscles leads to the accumulation of certain metabolites, specifically lactic acid, which causes a corresponding rise in osmotic pressure within the tissues. When this happens, the fluid from the tracheoles is partially drawn into the tissues by osmotic forces, allowing more air—and consequently more oxygen—to enter the tracheoles. This oxygen is delivered directly to the tissues precisely when they need it most.

Fig. 9.16. Conditions within insect tissues at rest and during activity (function of the tracheoles).

The overall flow of air through the insect's body is regulated by a mechanism that closes the spiracles. Each spiracle opening is equipped with a system of Valves controlled by very small muscles. The edges of the opening are covered with hairs that prevent foreign particles from entering and guard against excessive moisture loss. The size of the opening is adjusted in response to the level of CO₂ inside the insect's body.

Increased activity leads to a higher production of CO₂. Chemoreceptors detect this, causing the spiracles to open. This same stimulus can also trigger ventilation body movements, particularly in larger insects such as locusts. When the dorsoventral muscles contract, the insect's body becomes flatter, which reduces the volume of the tracheal system and forces air out ("expiration"). Air intake ("inspiration") occurs passively when the body segments spring back to their original shape due to their elasticity.

Evidence suggests that the thoracic and abdominal spiracles open and close alternately. Combined with ventilatory body movements, this creates a unidirectional airflow that enters the insect's body through the Thorax and exits via the abdomen.

While the tracheal system is undoubtedly very efficient for gas exchange, It is important to note that in most insects, gas exchange relies entirely on the diffusion of oxygen through the insect's tissues. Diffusion, however, is only effective over short distances, which imposes strict limits on the maximum size insects can reach. These short distances over which diffusion remains efficient do not exceed 1 cm; therefore, although some insects may grow up to 30 cm long, their bodies cannot be more than 2 cm thick.