BLACK FLY 



Siwulium spp. 



These insects belong to the family, Simuliidae, a small family which 

 is classified in the fly order Diptera. The importance of this insect 

 to the hydraulic factors of irrigation systems involves the pupal 

 encasements, or cocoons, that are laid down on submerged canal and 

 flume linings. These slipper-shaped encasements create extensive areas 

 of roughened surfaces that increase resistance to waterflow in a canal. 



The adult fly is a small gnatlike insect that is seldom more than 

 5 millimeters long, varying in color from gray-brown to black (insert 1). 

 These insects are completely aquatic in all stages of their life cycle, 

 except the free-flying adult which may be present in some areas in such 

 great numbers as to be almost unbearable to a visitor. The "black fly," 

 "buffalo gnat," or "no-see-ums" is well known for the persistent 

 manner in which the female pursues and bites warm-blooded animals. 



In its life history, the adult fly deposits eggs on vegetation or other 

 solid substrate just under the surface of swift water, especially where 

 the current is broken. Overwintering sometimes occurs in the egg 

 state. The eggs hatch below the water surface to produce larvae which 

 attach themselves to a solid submerged substrate. The larval stage may 

 last from 2 to 6 weeks. During the last stage of development, the larvae 

 construct silken cocoons in which the insects develop into pupae, a 

 growth stage preceding development and emergence of the adult. The 

 cocoons are firmly cemented to the substrate and may be in the shape 

 of a pocket, slipper, or vase (insert 2). Hiis is the growth stage that 

 creates roughened surfaces on a canal lining. The pupae (insert 3A) 

 are oval and enlarged at the upper end, and are yellow to red-brown 

 in color. They have small abdominal hooks by which they remain 

 attached to the cocoon (insert 3B). At the head end are two groups 

 of long branched filaments extending out of the cocoons that perform 

 as respiratory organs. The pupal stage usually last from 2 to 8 days prior 

 to emergence of the flying adult. The pupal cases generally are quite 

 persistent following emergence of the adult insects, and often require 

 mechanical removal from canal linings. 



Identification of the genera and species of these insects is 

 somewhat difficult, usually requiring detailed study of various portions 

 of the insect body. These insects are widespread, but have been 

 reported most commonly on irrigation structures in the Northwestern 

 and the Rocky Mountain States. 



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