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I Spy: Houseflies Always Seem To Be Nearby

Updated: Jan 3

I spy something that can fly and transform through metamorphosis and likes to hang out near people. Look around, and you’ll probably spy one too.

A closeup of a housefly.
A housefly. (Photo via Shutterstock)

Any guesses? It’s a housefly! Summer and flies go hand in hand, so it seems like there’s one (or two or 10) nearby at all times. They like our warm homes, tasty picnics and especially our garbage.

Did the metamorphosis clue make you guess butterfly? Butterflies do metamorphose, or change, from a baby insect into an adult insect. During their life cycle, they change from egg to caterpillar to chrysalis to butterfly. Flies also metamorphose, but their life cycle stages are different — and faster! A fly completes its cycle from egg to adult in a week or so in warm weather.

Stage 1: Egg

(Illustration via Shutterstock)

White eggs resemble small grains of rice. Females lay 75 to 100 eggs at one time, usually on soggy food waste (garbage) or manure (animal poop). Being born on a pile of something rotten may sound gross to you, but it’s perfect for the flies. The eggs need to stay moist, and lots of things ooze as they rot.

Stage 2: Larva

Eggs hatch in just eight to 20 hours! Out comes a tiny, wriggling larva called a maggot. The hungry maggot first eats its own egg. Remember how its egg was laid on an oozing pile of rotten stuff? That’s the perfect feast for a maggot. Imagine sitting at a pizza desk or sleeping in a bed of nachos and taking a nibble whenever you feel hungry! Maggots have it made, at least in the snacking department!


Words to know

Chrysalis: The pupa of a butterfly as it transform from caterpillar to adult.

Larva: The active immature form of an insect.

Metamorphosis: The process of transformation from an immature form to an adult form.

Pupa: An insect in its immature form between larva and adult.


Maggots shed their skin three times as they eat and grow. Each time they shed their skin is called an instar. All of that eating, growing and shedding happens in just three to five days! Caterpillars (butterfly larva) shed too, but usually four or five times over two to five weeks.

Stage 3: Pupa

A hard pupal case forms after the last larval shed. First, it’s a yellowish color, but it darkens to black as it gets older. Older meaning only three to six days from start to finish!

Inside the case a maggot transforms into a fly! When the transformation is complete, the fly pounds its way out using a special sac on its head called a ptilinum. The ptilinum shrinks and swells like squeezing a stress ball.

The hard pupal case is supposed to protect the maggot as it transforms. But danger still lurks. Another insect lays its eggs on the perfect food source. That perfect food source isn’t garbage, like flies, though. It’s actually … fly larva! Some wasps lay their eggs inside the pupal case. When the wasp larva hatches, it eats the fly pupa. It’s an insect eat insect world out there!

Stage 4: Fly

Flies emerge from the pupal case fully developed. They reproduce after only two to three days, starting the life cycle for the next generation of flies. If you’ve been doing the math, you realized that only about 12 days after a fly egg is laid, it starts laying its own eggs! Human babies can’t even see clearly when they are 12 days old, but flies are already parents by then!

Female flies will lay about 500 eggs in batches of 75 to 100. It only takes them three or four days to lay all those eggs. Let’s say half, or 250, of the maggots in those eggs are females. In one or two weeks, they will each lay 500 more eggs, which equals 125,000 flies. And half of them will lay another 500 eggs, equaling 31,250,000 more flies. And on and on!

Ten to 12 generations of flies can be born in a single year. It’s no wonder it’s so easy to spy a fly!


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