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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Carp diem: Fish and blow flies, Calliphoridae


True gourmands are blow flies. When not feasting on rotting flesh or excrement, blow flies can be found pollenating beautiful flowers.


Warning – This episode contains graphic scenes of writhing maggots consuming dead fish. If maggots creep you out, please do not watch the video accompanying this story. 

I know, I know, right now you are thinking carpe diem not carp diem, “seize the day”, not “fish day.” But really, this episode is kind of a fishy story about carp and eaters of their flesh, blowflies.  Last week on a particularly spectacular bike ride along the C & O Canal trail, my journey was interrupted by a trail washout at Culvert 82 between Point of Rocks and Brunswick. Torrential rains that had devastated Ellicott City and flooded parts of the mighty Potomac breached a culvert and washed away the trail. This breach also allowed water in the canal to escape, stranding aquatic animals both small and large in low-lying pools of water. Canal dwelling creatures, including carp, apparently collected in pools sustained by above average rainfall in late spring. But as rainfall diminished over recent weeks, these pools eventually dried up, leaving dozens of fish to perish as their habitat disappeared. In one such pool-of-death, some thirty large carp became a feast for tens of thousands of blow flies and their larvae.

Within four minutes of placing piece of meat on my picnic table a blow fly appeared to lay claim to the feast.

Blow flies are regular residents in our landscapes and forests where they cruise about ever vigilant for rich organic resources of food such as carrion. While living animals are of little interest, cadavers release wonderful aromas irresistible to blow flies. A blow fly’s strategy in life is to rapidly locate recently deceased animals, lay eggs before the arrival of competitors such as burying beetles or raccoons, and monopolize the rich nutrients provided by decaying flesh. To see just how quickly blow flies could find a dead thing, I placed a piece of meat on my picnic table one sunny morning. The first blow fly appeared and began to slurp the tasty juices in less than four minutes. If a carcass is suitable, the female blow fly will deposit a few score to more than 100 eggs on the flesh. Wounds and orifices usually are the first places eggs are laid. Eggs hatch within a day into tiny larvae called maggots. Their sole purpose in life is to eat the liquefied decaying flesh and avoid being eaten by predators such as the margined carrion beetle, Oiceoptoma noveboracense, we met in a previous episode. Maggots molt two times before leaving the carcass to pupate in the soil nearby. After several days, new adult flies emerge to mate, feed on nectar and other liquid foods, and search for more dead things.

While the image of a carcass writhing with a mass of maggots may be repulsive, imagine what a vile and smelly place the world would be without Mother Nature’s sanitary force of blow flies. They are the heroes of repurposing flesh. Blow flies serve several other useful purposes as well. During the American Civil War, surgeons noticed that blow flies often infested the grievous wounds of soldiers. Physicians were surprised to see wounds of maggot-ridden soldiers healing more rapidly and with fewer complications than injuries of soldiers without maggots. Many blow fly larvae consume dead and dying tissues rather than healthy ones. Furthermore, they secrete potent chemicals that kill harmful bacteria and aid in the healing process. Using blow fly larvae to treat wounds is called maggot therapy. By placing aseptic maggots of the blow fly Lucilia sericata in infected, necrotic tissues, doctors have successfully used maggot therapy to treat serious illnesses ranging from gangrene to osteomyelitis. Some species of blow flies such as the screwworm, Cochliomyia macellaria, infest wounds of livestock and may be important pests.


Receding waters of the C & O canal stranded more than thirty carp in a pool-of-death. Their flesh became food for tens of thousands of blow flies and their larvae, some of Mother Nature’s most efficient and beautifully repulsive recyclers of dead things

For those of us addicted to the genre of forensic dramas on television such as Bones and CSI, Phaenicia sericata and Phormia regina are household names. For many years, forensic entomologists have cataloged the seasonal activity, behavior, and habitat patterns of several species of Calliphoridae. The relationships between temperature and development are well known for many species of blow flies and their carrion-consuming relatives. By collecting fly larvae from the body of a crime victim, identifying the species and developmental stage of the fly, and correlating these facts with local weather information, detectives can accurately estimate the post-mortem interval (PMI) and determine when a body was placed at a crime scene. This information has been used to convict numerous perpetrators of homicides. While testimony of humans can be sketchy, prosecutors know that flies tell no lies.

Now that we have extolled the virtues of the blow fly, we would be remiss if we failed to point out one small issue associated with some questionable eating habits of these marvelous flies. Although a feast of decaying flesh is superb, blow flies like other liquid and semi-liquid cuisine such as doggie droppings and excrement from most other creatures. They feed by slurping nutrient laden liquid with sucking-sponging mouthparts. Unfortunately, part of the eating process of a blow fly also involves the regurgitation of liquid from its former meal along with saliva to aid in the digestion of its next supper. As you enjoy your steak or chicken at the next picnic, consider where the blow fly might have dined just before it visited your plate. I always recall the lyrics of the old song “shoo fly, don’t bother me” and conduct my shooing with extreme prejudice. Blowflies are my least favorite picnic guest.


We thank our gruesome friends Amy Ludwig and Ian Scheibel for providing the inspiration for this Bug of the Week. Information for this episode came from Herm’s Medical Entomology by James and Harwood, and from the fascinating article “Maggot debridement therapy as primary tool to treat chronic wound of animals” by Vijayata Choudhary, Mukesh Choudhary, Sunanda Pandey, Vandip D. Chauhan, and J. J. Hasnani.