Eggs of horse flies are laid in masses of 25 to 1,000 on vegetation. The eggs hatch in 5 to 12 days and the larvae drop off. It was long thought that swamps and other wet areas were necessary for larvae development. It has been discovered however, that the forest floor is the larval home of several species. The larvae live in mud, water or soil for varying amounts of time depending on species.
All horse flies overwinter as larvae.
Pupation occurs in drier soil and last 1 to 3 weeks. When adults emerge they mate and the females seek blood meals. We have many species of horse flies in Mississippi although we don't have them all at the same time. Horse flies occur from late March through September with various species occurring at very predictable times of the year. This regularity is caused by their development time and by the fact that they all overwinter as larvae - which sets their clock, so to speak.