Factors influencing larval color in the Simulium vittatum complex (Diptera: Simuliidae)

We investigated factors that influence larval color in black flies of the Simulium vittatum complex. In the laboratory, larvae were exposed to different light sources (ultraviolet and fluorescent "cool white"), background colors (black and white), and diets (periphyton, Spirulina, and fish food). Larvae were significantly darker when exposed to ultraviolet radiation, when reared in black containers, and when fed periphyton. Larval color was independent of sex. In the field, larvae on rocks were significantly darker than those on in-stream vegetation. Larval color also varied among seasons. Pigment analyses suggested that field-collected larvae contain a carotenoid. To investigate the adaptive significance of larval color, visual predators (Gambusia holbrooki) were introduced to a recirculating flume that contained dark larvae on black and white backgrounds. The predatory fish consumed significantly more dark larvae on white squares than expected by chance. Larvae, however, did not select a background that matched their body color. Our investigations show that a significant component of color is under environmental control.