How does a housefly grow and change?

Answer 1

House flies are holometabolous insects, which means they undergo complete metamorphosis.

Starting from the beginning, adult females have mature ovaries about 5 days after eclosing from the pupa under ideal conditions (temperature and nutrition). If the female has mated, she will begin to lay eggs, which are fertilized as they are laid from sperm that is stored in the spermatheca. Within minutes the egg will contain a zygote, and within a day a maggot will hatch.

The maggot will moult two times before it becomes a pupa (like a butterfly cocoon). As a maggot, the house fly requires lots of bacteria as a food source and will continuously feed and increase in size and store fat so that it can later form a pupa. After two moults, the maggot will become a pupa, which is its dormant stage. At ideal temperatures, this can last about 3-5 days.

After the pupal stage, the adult emerges and starts the cycle all over again. Adult females can lay as many as 300 eggs over their ~20 day life span, and will often deposit them in clutches of about 30 if uninterrupted during the oviposition process.