Except for a recently discovered parasite of salmon, all animals require oxygen for survival and expel carbon dioxide as a waste product.1 Because animals are so varied, many breathing systems have evolved throughout the animal kingdom, each one designed to work in an animal’s particular environment. Depending on its size and habitat, an animal may employ direct diffusion, gills, lungs, or other respiratory passages for gas exchange.
In higher-order animals, such as mammals, the respiratory and circulatory systems work together. Oxygen enters the body through the lungs where it attaches to hemoglobin in the blood and is then distributed by a matrix of vessels to cells and tissues. In the second half of the equation, carbon dioxide is collected from the cells and shunted back to the lungs where it is expelled by exhalation.
We’ve all seen pictures of tangled blood vessels that resemble the skeleton of a tree. The major vessels are thick like the trunk, and the branches start large and get progressively smaller as they stretch farther away. The vessels at the ends are downright tiny, but it’s these miniscule end pieces called capillaries that see all the action because that is where the gases are exchanged.
Right subject, wrong journal
Since this is the American Bee Journal — not the mammal journal — let’s take a look at the equivalent system in insects. Insects, too, have a respiratory and a circulatory system. Although the two systems work together to a limited extent, they remain separate. The respiratory system brings in oxygen and expels carbon dioxide while the circulatory system moves food in and waste out of the cells. But the insect circulatory system has no hemoglobin equivalent, so it has no ability to ferry oxygen from place to place throughout the body.
The parts needed to breathe
So how does a bug breathe without lungs or gills? In short, it has open ports on the integument that ultimately connect with the internal tissues, appendages, and organs. If you can envision an ocean liner with a series of portholes along its length, you’ve pretty much got the picture, except the bugs’ portholes allow air to enter 24/7. These holes, called spiracles, allow oxygen to move directly into the insect’s body.
In bees, as in most insects, each spiracle opens into a tube called a trachea. The tracheae are formed from ingrowths of the integument and are circled with chitin rings, the same tough material that comprises the integument.2 Looking like a spring, the chitin spirals around the outside of each tracheal tube.
This spiral design — very similar to the coil-reinforced tubing used for vacuum cleaners, radiator hoses, and jack-in-the-boxes — makes the trachea flexible, stretchable, kink resistant, pressure proof, and abrasion immune. The strong, failsafe tubes can withstand all kinds of bee use and abuse.
From big to small
Just like the tree of blood vessels in mammals, the tracheae divide into smaller and smaller tubes as the distance from the spiracle increases. At the very end, where the tubes contact the cellular tissues, the tracheae become super tiny and are called tracheoles. The tracheoles, just like the corresponding capillaries in mammals, are the place where gas exchange takes place. The oxygen inside the tracheole diffuses into the tissue, and the carbon dioxide — which has been building up inside the cell — diffuses from the tissues for its trip back to the outside world.
Most insects have ten pairs of spiracles, two pairs in the thorax and eight in the abdomen. In bees, however, the first three pairs appear on the thorax and the remaining seven pairs line the abdomen. The spiracles are one of the first things you can see on a developing larva, visible as black dots along each side of the larva’s body. Soon after the spiracles become visible, you can also see a network of tracheae beginning to form, looking like a white-on-white roadmap.
Balloon-like air sacs
Instead of lungs, honey bees have thin-walled air sacs at various places along the tracheae. The sacs are located throughout the bee’s body, including the head, thorax, abdomen, and legs. Those in the abdomen are especially large, while the others are smaller.
These sacs, similar to balloons or pillows, expand or contract along with the bee’s need for oxygen. Changing pressure in the air sacs helps to move the oxygen to where it’s needed. Muscle movements in the abdomen control the flow of air in and out of the sacs. A bee can contract her muscles dorsoventrally (from top to bottom) or along the length of the abdomen (from front to back).
Because the muscles attach to the inside of the rigid abdomen, and not to the sacs directly, the entire abdomen moves as she pumps air through her tracheal system. When the abdomen contracts, air is squeezed from the sacs, and as the muscles relax, air is sucked into them, much like a bicycle pump.
The muscle contractions appear to flow along the length of the abdomen in an accordion-like manner, causing the abdominal plates to slide over one another. New beekeepers often become concerned when they observe these contractions for the first time, describing them as “waves of convulsions,” “muscle spasms,” or “seizures.”
The amplitude of the muscle contractions changes with the bee’s oxygen demand. Usually, the contractions are small and barely visible, while at other times they are pronounced. The amount of muscle movement can increase due to oxygen depletion or carbon dioxide elevation.3
The miracle spiracle
Pressure in the tracheal system could not increase without sealing the openings. Similar to a check valve on a water system, each spiracle lets air in but not out. The presence of the valve means that when the bee contracts her abdomen, the air moves inward toward her organs and not back to the outside.
Easy access for mites
Of course, the bees’ system of breathing has its drawbacks. One of these is the tracheal mite, Acarapis woodi. Honey bee spiracles vary in size and the first pair — those just behind the head of the bee — are large enough to allow a tracheal mite to enter.
The microscopic female tracheal mite can move from bee to bee by simply sitting on a tiny bee hair and waiting to be brushed onto a new bee. She keeps transferring until she finds a nice young specimen to inhabit, the younger the better. Once satisfied with her host, she simply locates one of the large thoracic spiracles, saunters in, and begins laying eggs in the tracheal trunk — the largest section of the tracheal system.
Once the eggs hatch, the developing mites pierce the tracheal wall with their mouthparts and, along with mom, feed on the honey bee hemolymph that surrounds the trachea. The babies grow into adults, mate with their siblings, and clog the trachea with offspring and lots of debris, causing the trachea to become blocked, scarred, and brittle. Damaged tracheae lose their elasticity until the bee can no longer breathe well enough to fly or even perform normal in-hive tasks. An outbreak of tracheal mites can quickly collapse a colony.
A built-in size restriction
Another drawback of insect respiration is its reliance on diffusion. Remember that although the tracheae conduct oxygen to the tissues, the actual transfer of oxygen into the cell is done by diffusion. This is slow compared to an active transport system that employs an oxygen carrier such as hemoglobin. This slower rate of gas transfer ultimately limits the size of insects. We don’t have bugs as big as elephants or even mice because the rate of oxygen transfer is too slow. So if you ever wondered why bugs are small, that is one major reason. All things considered, it’s probably just as well; murder hornets are big enough.
Some disagreement surrounds how carbon dioxide is removed from the bee’s body. Most sources agree it exits through the tracheal system, but not all agree on how it gets into the tracheal system. According to Snodgrass, Erickson, and Fahrbach in “The Hive and the Honey Bee,” “The carbon dioxide produced in cells cannot be directed into the tracheoles. It is mostly discharged directly into the surrounding hemolymph, from which it is carried off by diffusion through the tracheae or possibly through the thinner part of the integument.”4
In “The Bee: A Natural History,” Noah Wilson-Rich simply says, “Oxygen enters the body through spiracles … and reaches the cells directly through a tracheal system that delivers air through the body and returns carbon dioxide to the spiracles.”5
Edward Southwick, also in “The Hive and the Honey Bee,” posits that carbon dioxide diffuses more readily than oxygen through the bees’ tissues, so “carbon dioxide is lost also directly through the tissue and cuticle.”6
Based on these and several other sources, I suspect that carbon dioxide leaves the bee through some combination of tracheal delivery and direct diffusion, but we don’t have a complete answer.
The circulatory system
Although the honey bee circulatory system doesn’t play a major role in transporting oxygen and carbon dioxide, it has many functions that mirror our own circulatory system, including the distribution of glucose, nutrients, and hormones to the cells, removal of waste materials, and maintenance of the immune system. With only two major parts — a heart and an aorta — the system is simple and effective.
The tube-shaped heart, which resides in the dorsal section of abdominal segments three through six, has four chambers and a series of one-way valves, all of which float in the pool of hemolymph that fills the abdomen. When the heart relaxes, hemolymph enters the heart through the series of valves called ostia, and when the heart contracts, the valves close and hemolymph is pumped forward through the aorta that leads directly to the head and brain.
The aorta floods the bee’s head with fresh hemolymph before muscles of the diaphragm pump it back to the thorax and then the abdomen. Except for the aorta, hemolymph is not confined to vessels but flows unconstrained throughout the body cavities, bathing the cells with supplies and picking up metabolic waste products.7
In order to get extra hemolymph to small but energy-demanding parts such as the legs, wings, and antennae, a series of vesicles at the base of these features assist in pumping hemolymph into the tight spaces. Vesicles are small membrane-enclosed pouches that store and help transport substances from one area to another. When squeezed by the bee’s muscles, they become mini-pumps for hard-to-reach places.
The only other roadblock to hemolymph flow occurs between the thorax and the abdomen at the petiole or “wasp waist.” Although the aorta directs fresh hemolymph forward through the petiole, a set of muscles called the ventral diaphragm helps move the hemolymph through the small petiole and back to the abdomen.
Overall, you can picture the heart pumping hemolymph vigorously from the abdomen to the head. After that, the hemolymph flows gently from the head back to the abdomen with a few mechanical assists along the way.
Nutrient exchange in the abdomen
Hemolymph sloshing around in the abdomen floods two other vital organs, the Malpighian tubes and the ileum. The Malpighian tubes are analogous to our own kidneys, filtering waste materials from the hemolymph and sending them to the excretory system.
The ileum comes next and is comparable to our own small intestine. Here, nutrients from digested food are passed to the hemolymph. In turn, the hemolymph, aided by the bee’s movements, will eventually transfer this food to all body tissues by way of simple diffusion and the rudimentary circulatory system.
Honey Bee Suite
- Yahalomi D, Atkinson SD, Neuhof M, et al. 2020. A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. Proceedings of the National Academy of Sciences 117(10):5358-5363.
- Snodgrass RE, Erickson EG, and Fahrbach SE. 2015. The Anatomy of the Honey Bee. In JM Graham (Ed.) The Hive and the Honey Bee (pp 149-151). Hamilton, Illinois: Dadant & Sons, Inc.
- Southwick E. 2015. Physiology and Social Physiology of the Honey Bee. In JM Graham (Ed.) The Hive and the Honey Bee (p 169). Hamilton, Illinois: Dadant & Sons, Inc.
- Snodgrass RE et al. p 151.
- Wilson-Rich N. 2014. The Bee: A Natural History (p. 169) Princeton, New Jersey: Princeton University Press.
- Southwick E. p 169.
- Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland, Oregon: Beargrass Press.