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The salt marsh horse fly often called the greenhead fly actually describes two cryptic species Tabanus nigrovittatus and Tabanus contenninus.
The entire life cycle takes place on and around the salt marsh. The eggs are laid on salt marsh grass (Spartina spp.). Eggs are laid in masses of 100-1,000 eggs covered with a jellylike material on leaves, rocks, or debris overhanging water or on moist areas; greenheads often lay 3-4 layer masses
After hatching, larvae either crawl down, or get washed off the blade of grass onto moist soil. The larvae then burrow into the marsh and live there for 1-2 years. After that time the larvae come to the surface of the marsh to pupate. In general, the larvae of greenhead flies are carnivorous and cannibalistic, feeding on insect larvae, snails, earthworms, scullers, etc. whereas, larvae of deer flies feed on decaying organic matter (detritovores). Adult males and females feed on sugary substances (i.e. nectar- like) for energy. After laying their first set of eggs, females begin to look for a bloodmeal. Female greenhead flies require blood for the same reason that female mosquitoes need blood; the blood provides them with protein that is used to develop eggs. Unfortunately, female greenhead flies are present and searching for blood on the beaches and bay of Brigantine at the same time that most visitors are using beaches for recreation.
Developmental time (egg to adult) may require 3 months to 3 years. Adults live about 3-4 weeks, and produce 5-6 batches of eggs. Most species overwinter as larvae. The mouthparts are for tearing and lapping, not piercing. Both sexes feed on nectar, but females require a blood meal for egg laying.
Adults are strong fliers and are capable of flying over 31 mi (50 km). However they generally do not disperse widely, usually less than 2 mi (3 km). Moving and dark objects are most often attacked. A slight drop in temperature or increase in wind will greatly reduce the numbers flying, unless crews can be spotted on the bay.
Rowers are particularly good targets. Singles and doubles will usually catch the interest of the smaller observational sorties. The younger, less experienced flies attack body parts that the sculler can swat while losing only a single stroke. The wiser flies will dive for the softer, fleshier parts of the sculler's posterior. This tactic inflicts maximum pain, and blood loss, causing screams that will alert the main squadron. On occasion, an especially sadistic, elite squadron will attack a single or double with such precision and ferocity that the scullers will be swatting and screaming long after the boat has flipped and settled to the bottom. The squadron usually carries off a floating hat or water bottle as a trophy.
Octuples, Eights and Quads are the primary targets of the main squadron. When they are sighted, the sky blackens and the drone of wings is deafening. Most of these boats have their best times in their dash to safety; however, the times are not recorded because of the exterior propulsion of the flies. There have been confirmed reports of novice eights, crews furiously rowing and swatting, travelling at 30 knots at an altitude of 200 feet.
