The life cycle of a fly is a process of complete metamorphosis, consisting of four distinct stages: egg, larva, pupa, and adult. This biological journey is rapid, especially for the common house fly (Musca domestica). Under warm conditions, the entire transformation from a newly laid egg to a reproductive adult can be completed in as little as seven to ten days. This speed allows for multiple generations to occur over a single warm season.
The Egg Stage
The life cycle begins when a female fly deposits tiny, white, oval-shaped eggs, measuring approximately 1.2 millimeters in length. A female is capable of laying multiple batches of 75 to 150 eggs, often totaling 500 or more eggs in her lifetime. Eggs are always placed on material that will serve as an immediate food source for the young.
The female seeks out moist, decaying organic matter, such as animal manure, garbage, or compost piles, to ensure offspring survival. Moisture is required for the eggs to hatch, and the warmth generated by decomposition accelerates the process. The egg stage is the shortest phase, with hatching typically occurring within 8 to 24 hours after being laid.
The Larval Stage (Maggot)
The organism that emerges from the egg is the larva, commonly called a maggot, which is a legless, creamy-white, worm-like creature. This stage is dedicated to feeding and growth, as the maggot efficiently consumes the surrounding decaying material. Maggots grow rapidly, reaching 7 to 12 millimeters in length before development is complete.
The larval stage is characterized by three distinct growth periods known as instars, separated by molting. During molting, the larva sheds its outer skin to accommodate its increasing size, driven by constant nutrient consumption. The entire larval phase typically lasts three to five days under optimal warm conditions, but it can be extended in cooler temperatures. After reaching full size, the maggot enters a transitional, non-feeding stage, crawling away from the food source to find a drier location for the next transformation.
The Pupal Stage
Once the maggot has reached its final size and found a suitable location, it enters the pupal stage, a period of internal reorganization. The larval skin shrinks and hardens to form a protective, barrel-shaped casing known as the puparium. This casing initially appears yellowish, but quickly darkens to a reddish-brown or black color as it ages.
The puparium serves as a stationary, non-feeding shelter while the larva undergoes metamorphosis, transforming its body structure into that of an adult fly. Inside this shell, the insect develops wings, legs, and other adult features. This stage typically lasts three to six days, though the duration depends heavily on ambient temperature.
The Adult Stage and Reproduction
The pupal casing eventually splits open, and the fully formed adult fly emerges, initially soft and unable to fly. The newly emerged fly uses an inflatable sac on its head, the ptilinum, to push its way out of the hardened puparium. Once fully emerged, the fly’s exoskeleton hardens and its wings expand, marking the beginning of the adult stage.
The adult fly’s main purpose is reproduction; females are capable of mating and laying eggs within a few days of emergence. The average lifespan is relatively short, typically ranging from 15 to 30 days. Despite this short lifespan, a female can produce hundreds of eggs. Temperature is the greatest factor influencing the speed of the entire life cycle, with warm conditions allowing the process to complete in about a week.