Where Do Maggots Come From? Some Arrive Already Hatched

In the middle of July, the Medici court physician Francesco Redi sealed a snake, some fish, eels from the Arno and a slice of veal in four large flasks, and left four more open. Flies went in and out of the open ones, which soon turned wormy, while in the sealed ones he “did not see a worm.”

Sealing also shut out air, so he tried again with meat and fish in a large vase “closed only with a fine Naples veil,” set inside a frame covered with the same net. He “never saw any worms in the meat,” though flies landed on the outer net now and then, some leaving six or seven worms at a time.

Redi published these tests in 1668 in Experiments on the Generation of Insects. They are quoted here from Mab Bigelow’s 1909 translation of the 1688 edition. He was testing “the dictum of ancients and moderns, and the popular belief” that rotting flesh breeds worms.

Where do maggots come from?

Maggots are fly larvae. A female fly lays her eggs on moist, decaying matter such as carrion, food waste or animal droppings, and maggots hatch from them; blow fly eggs hatch in about 24 hours. Many flesh flies skip egg-laying and deposit live maggots instead. Rotting food never turns into maggots on its own.

House flies lay batches of about 100 to 150 eggs in “warm, moist, organic materials such as manure, garbage, lawn clippings, decaying vegetables and fruits,” according to UC IPM, the University of California’s pest program.

Blow flies can arrive within 10 minutes

The first insects at a dead body are usually blow flies, the shiny green or blue flies of carrion, and in some cases they have “been known to turn up within just 10 minutes,” London’s Natural History Museum reports. Each female lays masses of about 50 to 200 eggs, and several females laying together can leave thousands on one body, University of Florida factsheets report.

A fly that lands on carrion tastes it through its feet. Blow fly eggs “look like small grains of rice,” says NC State Extension’s blow fly factsheet, and are typically concentrated on open wounds or in crevices. In another of Redi’s trials, six open boxes of meat, from two snakes to a sheep’s heart, “all became wormy in little more than twenty-four hours.”

Some flies deposit live maggots

Some maggots arrive already hatched. Many flesh flies “deposit maggots directly on a corpse instead of eggs,” a 2018 review of forensically important flesh flies explains, and a female can set down 20 to 40 at one sitting, according to the University of Wisconsin-Milwaukee’s field station.

“It is true that some kinds of flies bring forth live worms and some others eggs, as I have proved by experiment,” Redi wrote.

Maggots turn into flies inside their own skin

A maggot feeds and molts through three larval stages, then converts its last larval skin into a puparium, “a hardened shell within which the pupa develops,” UC IPM explains, and the adult “pops off the end of the puparium and emerges.” Heat speeds every stage: under ideal temperatures, UC IPM says, blow flies can go from egg to adult in as little as 7 days.

Stage	What happens	Blow fly timing
Egg	Laid in batches on carrion, typically on wounds or in crevices	Hatches in about 24 hours
Maggot	Feeds and molts through three larval stages	About 1 week
Pupa	Develops inside the puparium, the hardened last larval skin	About 8 days
Adult fly	Emerges; females need a protein meal before they can mature eggs	Roughly 16 days after laying

The timings are NC State’s, totaled in the last row, and run longer in cool weather. The museum gives the blow fly Calliphora vicina about 19 days from egg to adult, and says half of that time can be spent in the puparium.

A closed lid is not always a barrier

Maggots in a closed bin still came from a fly, though the fly may never have got in. Fruit flies, a University of Kentucky factsheet warns, lay eggs under the lid of a container that is not well sealed, “and the tiny larvae will enter the container upon hatching.”

Redi noticed the occasional maggot on the paper covers of his sealed flasks seeking “some crevice by which to enter.”

The reverse surprise, flies with no maggots ever seen, has the same cause. Maggots “may go unnoticed until after they have pupated and emerged as adult flies,” NC State notes, and a sudden swarm of blow flies indoors often traces back to a dead animal in a wall void, crawlspace or attic.

Spontaneous generation, from Aristotle to Kircher

The belief Redi was testing is at least as old as Aristotle, whose History of Animals, from the fourth century BC, says some insects “are not derived from living parentage, but are generated spontaneously,” in dew, mud, dung, timber, hair and “the flesh of animals.” He gave eels a parentless origin too.

Aristotle knew that flies mate and that their mating produces grubs, but he wrote that “from these issues the parent-species is never reproduced.” In his account, the grubs that became new flies arose in the dung on their own: flies, in D’Arcy Thompson’s translation, “grow from grubs in the dung that farmers have gathered up into heaps.”

In Redi’s century, the Jesuit scholar Athanasius Kircher described breeding flies from dead flies soaked in honey-water, which, Redi replied, “only serves to attract the living flies to breed in the corpses of their comrades.”

Hunters and butchers knew better, Redi noted, and covered meat in summer with white cloths. For microbes, spontaneous generation stayed in contention until the work of Louis Pasteur and John Tyndall in the mid-19th century.

Redi still believed oak trees made insects

Redi’s experiments are “regarded as a key contribution to the downfall of belief in spontaneous generation,” Emily Parke wrote in a 2014 paper. Parke argues he treated living hosts as a separate question: whether insects can come from living things that are not their parents. He had once “believed, or rather suspected” that a fly slit a young oak twig in spring and hid an egg there. Then he noticed that “every gall or growth has its own peculiar kind of worm, gnat, or fly, which never varies,” and credited the insects to “that soul or principle which creates the flowers and fruits of living plants.” He thought tapeworms arose inside living bodies the same way.

His first guess was the better one. Oak gall wasps “lay their eggs on oak tissues, and this action induces the formation of a gall structure around the eggs,” a 2024 article in Plant Physiology explains.

No worm reached Redi’s veiled meat, though only because he removed the ones that “succeeded at last in penetrating the fine meshes.” Some of his flies did not even wait for the net: they “dropped them in the air before reaching the net.” Every maggot comes from a fly, and some leave their mother already hatched. Strictly FYI.