What Are the Functions of the Spiracles?

The blowhole of whales is an example of a spiracle.
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Insects do not have lungs like vertebrates. They still have to breathe to obtain oxygen for metabolic processes, but their bodies use a very different system to do so.

They use openings called spiracles and a special system of tubes called tracheae (singular: trachea) for respiration.

What Is a Spiracle?

A spiracle is an opening found on the outside of an insect’s exoskeleton that is used for breathing. There are numerous spiracles on the body of an insect, typically paired and present on the thorax and abdomen.

Different orders of insects may have different numbers of spiracles, but there is no set pattern. The maximum number of pairs of spiracles that an adult insect can have is 10.

Spiracles are protected by hairs and valves with flaps, and are surrounded by spines, folds and ridges. They are controlled by muscles that open them when oxygen is needed. Then they close partially while insects rest. Spiracle size can also be adjusted in insects that live in dry climates, to prevent losing moisture.

The closing mechanism of a spiracle, or valve, contains a protein called resilin that allows for contraction and expansion. Hairs keep dust from entering the spiracle. A spiracle leads to a trachea, or air tube.

Tracheal System in Insects

Insects do not have a respiratory system connected to a circulatory system. Instead, there is a tracheal system in insects that delivers oxygen throughout their bodies, and that removes carbon dioxide during respiration.

Following the spiracle to the tracheal tube leads to tracheoles, special cells used for gas exchange. These tiny branches, about 0.1 micrometer in diameter, are filled with liquid and extend to most of the insect’s cells. Oxygen is brought through the tracheal tubes, dissolves in the liquid and passes through to the cells.

Insect Respiration and Body Size

Simple diffusion is used in the tracheal system in insects that are small in size. Larger insects such as cockroaches, beetles, locusts and grasshoppers require more pumping action. This helps them when they are active or when they are undergoing stress due to heat.

Larger insects open and close separate spiracles with their abdominal muscles to move air throughout their bodies. Because of the nature of these tubes and their sensitivity to pressure, it is thought that they are the reason insects did not evolve and grow into larger organisms like many vertebrates.

Pesticides and Cockroach Anatomy

Cockroaches are pernicious pest insects that prove to be difficult to eradicate. They wreak havoc on food supplies and homes. By studying at cockroach anatomy, pesticide makers can find new approaches to try and fight them more effectively.

Looking at cockroach anatomy reveals that cockroaches bear 10 pairs of spiracles, the most an insect can have. Some spiracles can be found on the thorax between dorsal parts of the legs. Others run along the sides of the abdominal segments or between them.

Thoracic spiracles use externally closing lids attached to muscles. The largest spiracle on the cockroach is the first thoracic spiracle. Most of the abdominal spiracles are similar in size, with the exception of the last abdominal spiracle. It is larger and shaped differently, with a D-shaped opening.

Insecticide makers can formulate chemicals that inhibit the breathing of cockroaches by using powders, liquids or gases. Insecticides that are gaseous work by attacking the tracheal system of cockroaches.