The fruit fly, Drosophila melanogaster, undergoes complete metamorphosis. This process, also termed holometabolism, describes the distinct life stages that separate the larval form from the winged adult. The journey from a tiny egg to the familiar adult fly is a rapid and complex cycle. This incredible reorganization of form is achieved through a precise sequence of developmental phases.
The Early Stages: Egg and Larva
The life cycle begins when a female deposits minute, oval eggs, typically on the surface of fermenting fruit or other decaying organic matter. These eggs are approximately half a millimeter long and hatch quickly, usually within 12 to 24 hours under optimal warm temperatures. The hatching creature is a larva, a soft, white, legless maggot whose primary function is to eat and grow.
The larval stage is divided into three growth periods called instars, separated by molts where the outer cuticle is shed. This entire feeding phase lasts about four days, during which the larva increases its mass by up to 200 times. Larvae use specialized mouth hooks to ingest the microbe-rich, decaying substrate. This continuous feeding behavior is necessary to accumulate the vast reserves of energy required for the next, non-feeding stage.
The Transformation Stage: Inside the Pupa
Once the larva reaches its maximum size, it enters the prepupal stage, crawling away from the food source to find a dry, elevated spot for pupariation. The third-instar larval skin hardens and darkens to form a protective, immobile, barrel-shaped casing called the puparium. Inside this shell, the process of metamorphosis begins, which takes about four days at 25°C.
This transformation involves biological demolition and reconstruction. A process known as histolysis involves the controlled breakdown and programmed death of most larval tissues, including the large larval muscles and the gut. The resources recovered from this tissue destruction are recycled and used as building blocks for the adult form.
The new adult structures are built through histogenesis, which occurs from small clusters of undifferentiated cells called imaginal discs. These discs have been present but largely inactive throughout the larval stage. They are precursors to all external adult parts, such as the wings, legs, antennae, and eyes. Under the influence of the steroid hormone ecdysone, the imaginal discs rapidly grow and differentiate, sculpting the adult fly from the remnants of the larva.
The Final Form: Adult Life and Reproduction
The culmination of the pupal stage is eclosion, the emergence of the adult fly from the puparium. The newly emerged fly is soft and its wings are unexpanded, but within a few hours, it hardens, and the wings inflate and dry. The adult fly is focused on mating and laying eggs to perpetuate the cycle.
The entire developmental cycle, from egg fertilization to the emergence of the adult, can be completed in as little as 7 to 10 days under ideal warm conditions. Females can begin mating and laying eggs within 24 hours of emerging, depositing hundreds of eggs over their lifespan. The rapidity of the life cycle ensures a continuous presence, allowing the fruit fly to exploit ephemeral food sources like overripe fruit.