How Do Insects Breathe?

The Respiratory System of Bugs

Diving beetle larva.
Getty Images/Oxford Scientific/Larry Crowhurst

Insects, like people, require oxygen to live and produce carbon dioxide as a waste product. That, however, is where the similarity between the insect and human respiratory systems essentially ends. Insects do not have lungs, nor do they transport oxygen through a circulatory system in the manner that humans do. Instead, the insect respiratory system relies on a simple gas exchange that bathes the insect's body in oxygen and expels the carbon dioxide waste.

Insect Respiratory System

For insects, air enters the respiratory systems through a series of external openings called spiracles. These spiracles, which act as muscular valves in some insects, lead to the internal respiratory system which is comprised of a densely networked array of tubes called tracheae.

To simplify the concept of the insect respiratory system, think of it like a sponge. The sponge has small holes that allow water inside to moisten it. Similarly, the spiracle openings allow air into the interior tracheal system bathing the insect's tissues with oxygen. Carbon dioxide, a metabolic waste, exits the body through the spiracles.​

How Do Insects Control Respiration?

Insects can control respiration to some degree. They are able to open and close their spiracles via muscle contractions. For example, an insect living in a desert environment can keep its spiracle valves closed to prevent moisture loss. This is accomplished by contracting muscles surrounding the spiracle. In order to open the spiracle, the muscles relax. 

Insects can also pump muscles to force air down the tracheal tubes, thus speeding up the delivery of oxygen. In cases of heat or stress, insects can even vent air by alternately opening different spiracles and using muscles to expand or contract their bodies. However, the rate of gas diffusion—or flooding the inner cavity with air—cannot be controlled. Due to this limitation, as long as insects continue to breathe using a spiracle and tracheal system, in terms of evolution, they are not likely to get much larger than they are now.

How Do Aquatic Insects Breathe?

While oxygen is plentiful in the air (200,000 parts per million), it is considerably less accessible in water (15 parts per million in cool, flowing water). Despite this respiratory challenge, many insects live in water during at least some stages of their life cycles.

How do aquatic insects get the oxygen they require while submerged? To increase their oxygen uptake in water, all but the smallest aquatic insects employ innovative structures—such as gill systems and structures similar to human snorkels and scuba gear—to pull oxygen in and force carbon dioxide out.

Insects with Gills

Many water-dwelling insects have tracheal gills, which are layered extensions of their bodies that enable them to take in greater quantities of oxygen from water. These gills are most often located on the abdomen, but in some insects, they are found in odd and unexpected places. Some stoneflies, for example, have anal gills that look like a cluster of filaments extending from their hind ends. Dragonfly nymphs have gills inside their rectums.

Hemoglobin Can Trap Oxygen

Hemoglobin can facilitate the capture of oxygen molecules from the water. Non-biting midge larvae from the Chironomidae family and a few other insect groups possess hemoglobin, much like vertebrates do. Chironomid larvae are often called bloodworms because the hemoglobin imbues them with a bright red color. Bloodworms can thrive in water with exceptionally low oxygen levels. By undulating their bodies in the muddy bottoms of lakes and ponds, bloodworms are able to saturate the hemoglobin with oxygen. When they stop moving, the hemoglobin releases oxygen, enabling them to breathe in even the most polluted aquatic environments. This backup oxygen supply may only last a few minutes but it's usually long enough for the insect to move to more oxygenated water.

Snorkel System

Some aquatic insects, such as rat-tailed maggots, maintain a connection with air on the surface through a snorkel-like structure. A few insects have modified spiracles that can pierce the submerged portions of aquatic plants, and take oxygen from air channels within their roots or stems.

Scuba Diving 

Certain aquatic beetles and true bugs can dive by carrying a temporary bubble of air with them, much like a SCUBA diver carries an air tank. Others, like riffle beetles, maintain a permanent film of air around their bodies. These aquatic insects are protected by a mesh-like network of hairs that repels water, providing them with a constant air supply from which to draw oxygen. This airspace structure, called a plastron, enables them to remain permanently submerged.

Sources

Gullan, P.J. and Cranston, P.S. "The Insects: An Outline of Entomology, 3rd Edition." Wiley-Blackwell, 2004

Merritt, Richard W. and Cummins, Kenneth W. "An Introduction to the Aquatic Insects of North America." Kendall/Hunt Publishing, 1978

Meyer, John R. "Respiration in Aquatic Insects." Department of Entomology, North Carolina State University (2015).