Blow Fly Biology and Maggot Therapy for Healing Wounds
What Are Blow Flies?
Blow flies are interesting and often colourful insects that are common in some areas. Many species are famous for their habit of feeding on dead bodies and decaying or injured animal tissue. The name "blow fly" (or blowfly) comes from the phrase "fly-blown meat". This is a very old term that refers to meat containing fly eggs.
Maggots are the worm-like larvae of certain flies, including blow flies. The maggots of some blow fly species can be surprisingly helpful to humans when they’re used in a medical setting. When they're placed on a wound that isn't healing by a doctor, the maggots may feed on dead and dying tissue and remove it. The process is known as debridement. When the larvae are detached from the wound, they leave healthy tissue behind. This healthy tissue then repairs the wound.
Blow Fly Classification
Order Diptera (Flies)
Family Calliphoridae (Blow Flies)
Adult Blow Flies
Blow flies belong to the family of flies known as the Calliphoridae. Their body is frequently colourful and often has a metallic sheen. Blue bottles, green bottles, screwworms, and cluster flies are all blow flies.
Blue bottles have a deep blue colour and green bottles are green, as their names suggest. Both insects are noisy fliers and produce a loud buzzing sound as they travel. The species of screwworms that most commonly affect human lives are green in colour and have black stripes. Screwworms get their name from the action of their larvae, which burrow into the flesh of living or dead animals.
Unlike the blow flies described above, cluster flies have a dull instead of a shiny body. I think they're still attractive insects, though. They have a dark grey thorax with yellow hairs and a checkerboard pattern on their abdomen.
Cluster flies have different behaviour from other blow flies. Adults feed on nectar and spend the winter in buildings. The larvae are parasites of earthworms.
Like other insects, blow flies have a tough exoskeleton over the outer surface of their body. The exoskeleton protects the fly and provides an attachment site for the muscles inside the body.
The body consists of three sections—the head, the thorax, and the abdomen. The large, compound eyes on the head are very noticeable. The head also bears two antennae. The fly has three pairs of legs and one pair of wings, all attached to the thorax. As in other members of the phylum Arthropoda, the legs are jointed.
The ancestors of blow flies had two pairs of wings. In the modern blow fly, the hind wings have been reduced to a pair of stalked structures known as halteres. Each haltere has a knob at its tip. Halteres vibrate during flight and act as stabilizers.
The antennae of a blow fly are sensitive to smell, taste, and touch. Other parts of the body can also detect these stimuli. For example, the mouth and the bottom sections of the legs (the tarsi) have taste receptors.
The antennae are also sensitive to vibrations. They contain an organ known as the Johnston's organ. This organ can detect sound vibrations and air currents. It's also sensitive to deflection due to gravity.
The compound eyes contain many lenses. The insect's brain combines the information from each lens to make a single image. The image is less detailed than the one that we would see, but the blow fly's eye is much better at detecting movement.
Respiratory and Circulatory System of Flies
Blow flies don't have lungs. Instead, they have a network of tubes called the tracheal system. The tubes transport oxygen and carbon dioxide through the fly's body. Gas exchange with the atmosphere takes place through a series of small openings on the sides of the body. These openings are called spiracles.
Blow flies do have a heart that pumps blood, although it has a very different structure from our heart. A fly's heart consists of a series of expanded chambers in a blood vessel.
The blow fly's circulatory system is classified as an open system instead of a closed one. In an open system, the blood travels through the body cavity for much of its journey around the body instead of travelling inside blood vessels. The cavity that it travels through is called the hemocoel. Insect "blood" is technically known as hemolymph. It doesn't contain red blood cells or hemoglobin as our blood does and is either colourless or a pale yellow or pale green colour.
Nervous, Digestive, and Excretory Systems
Blow flies have a brain, although it's relatively simple compared to the brain in more advanced animals. It's sometimes referred to as the cerebral ganglion or the supraesophageal ganglion.
An insect neuron or nerve cell has a cell body, which contains the nucleus, and an extension called an axon. The cell bodies of a group of neurons are found in the cerebral ganglion and the axons extend along the body to form nerves. The central nervous system of an insect contains a chain of ganglia.
Digestion occurs in the alimentary canal, which runs from the mouth to the anus. It's made of a foregut (also called the stomatodeum or the stomodeum), a midgut or mesenteron, and a hindgut or proctodeum. As in us, the insect's digestive system breaks down food with enzymes and absorbs nutrients. Undigested food is released from the anus as a fecal pellet.
Malpighian tubules remove metabolic wastes from the insect's body. Kidneys do this job in humans. The tubules are connected to the pyloric valve at the end of the midgut where it joins the hindgut. They extend into the hemolymph and remove nitrogenous waste substances from the fluid. They then convert the substances to uric acid, which enters the hind gut and is excreted in the fecal pellet.
Eggs and Larvae
Adult blow flies feed on carrion (dead and decaying animal flesh). They release digestive enzymes into the food and then absorb it. The flies also drink plant nectar.
A female blow fly deposits her eggs on the bodies of dead animals, animal feces, garbage, or even wounds on living animals, especially if the wounds are bloody. The eggs may also be laid on uninjured but damp areas of an animal's body, such as regions where hair is soaked with feces or urine.
The female lays around 150 to 200 white or yellow eggs, which look like grains of rice. The eggs hatch quickly—sometimes in less than a day—and release white, wriggling larvae (maggots) that resemble worms. The larvae form a group known as a "maggot mass", which is shown in the video below. When the maggots find food, they grab hold of it with a hook-like structure and secrete digestive enzymes into the food. The liquified food is then drunk.
A blow fly has six stages in its life cycle—the egg, three larval stages, the pupa, and the adult. The time at which each stage appears depends on the environmental temperature, but in general the process is as follows.
- The egg hatches in about 24 hours.
- The first two larval stages each exist for about 24 hours before they molt (lose their old exoskeleton to reveal a new one underneath) and change into a larger larva.
- The third larval stage exists for about five days and then becomes a pupa.
- The pupa exists for about a week.
- The adult emerges from the pupa at the end of the week.
The pupa is a sac-like structure covered by a thick, brown skin. The early pupa moves away from the food source to a hidden place—usually in the soil—where it completes its development. Inside the pupa, the blow fly larva changes into an adult.
Blow flies are sometimes useful in forensic science. The length of time needed for each life stage at specific temperatures is known. If investigators find animal or human remains containing blow flies, they may be able to estimate the time of death. This estimation is based on the stage of the blow fly life cycle discovered and the recent environmental temperatures in the area where the remains were found.
The screwworm is an especially annoying and potentially dangerous example of a blow fly. The larva can cause serious injuries to animals and even kill them. Adult secondary screwworm flies—the type that is most likely to be seen in North America—are green with three longitudinal black stripes on the back of their thorax. Like other blow flies, they have a shiny appearance. Also like other blow flies, their larvae are legless and worm-like.
Screwworm larvae are important in veterinary and human medicine because they cause myiasis, which is the infestation of a living animal. The larvae of the secondary screwworm, or Cochliomyia macellaria, feed only on dead tissue of their host. The larvae of the primary screwworm, or Cochliomyia hominivorax, feed on living tissue of the host, however.
A screwworm infection begins when a female fly lies her eggs in a surface wound on an animal. Examples of these wounds include minor cuts, insect bites, and the naval of a youngster that has just been born. The eggs hatch into larvae, which feed on the animal's tissues.
Cochliomyia hominivorax can infect humans. Its species name—hominivorax—is Latin for "man-eating". At the moment, this species isn't generally found in the United States, but it's sometimes discovered in Florida. It usually attacks other animals, but it does feed on humans at times.
The video below introduces maggot therapy. It shows a deep, open wound and living maggots in the wound.
Maggot Debridement Therapy for Treating Human Wounds
Maggot Debridement Therapy, or MDT, is an old form of wound treatment that has made a modern comeback. It uses specific types of blow fly larvae that can be helpful for humans. The discovery of antibiotics caused maggot therapy to fall into disfavour, but now that many antibiotics are losing their effectiveness doctors are taking another look at the treatment.
Dead tissue in a wound can be removed surgically. The use of maggots is cheaper, however. Maggot therapy removes only the damaged tissue and none of the healthy tissue, unlike surgical debridement. The feeding process isn't painful for the patient because the maggots don't eat living tissue.
The maggots secrete digestive enzymes into the wound. The enzymes break down and liquify injured cells and bacteria. The insects then feed on the liquid. Maggots are placed in the wound as tiny creatures and enlarge very quickly. The secretions of the maggots are thought to be antibacterial and to actually stimulate wound healing.
Using Maggots to Heal Wounds
Only a few species of blow flies are used for wound debridement. The maggots are produced in medical laboratories. They are shipped in a disease-free condition and in sterilized containers. The insects are generally corralled in a cage of some type when they are placed in a wound. The cages are made of wire mesh, gauze, or another material, which restricts the movement of the maggots but allows them to breathe and feed.
Maggots are generally used in cases where wounds have refused to heal for a long time and where amputation of a body part will likely be needed if a successful treatment isn't found soon. Examples of these types of wounds include certain ulcers that develop as a result of diabetes and some wounds that follow surgery.
How Successful Is Maggot Therapy?
They have been glowing reports of the success of maggot therapy, but unfortunately some of these have come from a laboratory that sells medical maggots. That doesn't mean that the claims are false, but scientists would like to see results from other sources as well.
There are some positive results from medical sources unconnected to laboratories. One investigation found that 17 of 23 diabetic patients with serious wounds experienced either an improvement in their condition or a cure after receiving maggot therapy. Another survey found that maggots debrided significantly more of a wound during the first week of treatment than a conventional treatment did. At the end of two weeks there was no significant difference between the two types of treatment. The researchers concluded that since the maggots worked more quickly during the first week they should be used during this time. A different treatment should be used during the second week.
It's wonderful that maggot therapy has helped to heal some people's wounds. It will be interesting to see what else researchers discover about the action of maggots in a therapeutic setting.
A Maggot Therapy Poll
Would you accept maggot therapy if you had a serious wound that wouldn't heal?
- Blow Fly facts from the Missouri Department of Conservation
- Common green bottle fly life cycle, forensic importance, and medical use from the University of Florida
- Secondary screwworm information from the University of Florida
- Insect digestive and excretory systems (plus other insect information) from North Carolina State University
- Marineau ML, Herrington MT, Swenor KM, Eron LJ. Maggot Debridement Therapy in the Treatment of Complex Diabetic Wounds. Hawaii Medical Journal . 2011;70(6):121-124.
- Maggot therapy for wound debridement from the Journal of the American Medical Association
- Maggot therapy might be useful in a war zone from The Guardian newspaper
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
Questions & Answers
© 2012 Linda Crampton