Housefly

Housefly eggs are about 2.5 mm in diameter.

An excerpt from the article 22 facts about Housefly
They are most commonly laid in pig feces, where up to 15,000 larvae may hatch in one kilogram of a substrate. After 24 hours, the larvae hatch from the eggs and feed on the liquid fraction of the substrate. The quality and freshness of the substrate affect the growth rate of these insects.