According to Lyon: "Pupation occurs in a cocoon, open at one end. Adults emerge in two to three days when the water is warm. They are capable of immediate flight and mating. The entire life history spans about four to six weeks, depending on species, water temperature, available food, etc. There may be four generations per year." (Lyon, 2000) IV. The LARVAE SPINS an UNDERWATER COCOON

According to the Agriculture and Agri-Food Canada report entitled "Black Flies (Simullidae)" various species of black fly larvae "may be found in every type of flowing water, from minute seepages in which the flow is scarcely detectable, to the largest rivers and waterfalls. Each species seems to have its preferences for streams of a certain width, velocity, and character; springs and seepages have their own particular black fly species, whereas large rivers and water falls support a different fauna. Most species seem rare; in contrast, some are so abundant that their larvae carpet thousands of square meters of river bottom." (Black Fly Larvae, 2003) These larvae may remain in one place however they may also change positions. When the...

The cocoon is attached to underwater objects and may be even located in the silt on the bottom of the body of water. Prior to the emergence of the adult the pupa becomes filled with gas and then the adult appears and expands its wings and the pupa floats to the surface while be completely "surrounded by this protection film of gas." (Black Fly Larvae, 2003)
Bibliography

Lyon, William F. (2000) Black Flies HYG-2167-97 Entomology - Ohio State University Fact Sheet. Online available at http://ohioline.osu.edu/hyg-fact/2000/2167.html.

Black Flies (Simullidae) (2003) Agriculture and Agri-Food Canada Online available at http://sci.agr.ca/ecorc/diptera/bf11-dp11_e.htm.

Featured Creatures (1998) University of Florida Institute of Food and Agricultural Sciences. Online available at http://creatures.ifas.ufl.edu/livestock/bfly.htm

Black Fly Larvae