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Reproduction

Mating often occurs in swarms, generally at landmarks such as hilltops. The season, time of day, and type of landmark used for mating swarms are specific to particular species.

Eggs are laid on stones or vegetation near water, in clusters of up to 1000, especially on emergent water plants. The eggs are white at first but darken with age. They hatch after about six days, with the emerging larvae using a special hatching spike to open the egg case. The larvae fall into the water or onto the moist ground below. Chrysops species develop in particularly wet locations, while Tabanus species prefer drier places. The larvae are legless grubs, tapering at both ends. They have small heads and 11 or 13 segments and moult six to 13 times over the course of a year or more. In temperate species, the larvae have a quiescent period during winter (diapause), while tropical species breed several times a year. In the majority of species, they are white, but in some, they are greenish or brownish, and they often have dark bands on each segment. A respiratory siphon at the hind end allows the larvae to obtain air when submerged in water. Larvae of nearly all species are carnivorous, often cannibalistic in captivity, and consume worms, insect larvae, and arthropods. The larvae may be parasitized by nematodes, flies of the families Bombyliidae and Tachinidae, and Hymenoptera in the family Pteromalidae. When fully developed, the larvae move into drier soil near the surface of the ground to pupate. In dry places a "remarkable" adaptation was discovered in the 1920s by W.A. Lamborn in Malawi (then Nyasaland). The larvae were discovered to tunnel in a spiral motion while the mud was still wet and plastic, forming a partitioned cylinder in the center of which the larva settled to pupate after closing the entrance; this adaption protects the pupae against mudcracks when the mud dries up, as a spreading crack would change direction when it hit the wall of the cylinder.

The pupae are brown and glossy, rounded at the head end, and tapering at the other end. Wing and limb buds can be seen and each abdominal segment is fringed with short spines. After about two weeks, metamorphosis is complete, the pupal case splits along the thorax, and the adult fly emerges. Males usually appear first, but when both sexes have emerged, mating takes place, courtship starting in the air and finishing on the ground. The female needs to feed on blood before depositing her egg mass.