Factors Influencing Larval Color in the Simulium vittatum Complex (Diptera: Simuliidae)

Abstract

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 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. Additional key words: black flies, pigments, aquatic insects, stream The ability of immature and adult insects to change color has been investigated frequently in terrestrial environments (Raabe 1983) but rarely in aquatic habitats. The larval odonates Leucorrhinia dubia (VAN DER LIND) and Ischnura verticalis (SAY) can change color to match green or brown backgrounds (Moum & Baker 1990; Henrikson 1993). Air sacs of the phantom midge Corethra plumicornis change color in response to background (Wemhoner & Weber 1973), and larvae of two species of anopheline mosquitoes undergo color change when exposed to different backgrounds (Benedict & Seawright 1987). In each of these cases, predator avoidance is suggested as the evolutionary significance of color change, along with avoidance of detection by prey in the case of the odonates. Black fly larvae are stream-dwelling filter feeders common throughout the world (Crosskey 1990). Larvae of the Simulium (Psilozia) vittatum ZETTERSTEDT complex show a strong correlation between larval pigmentation and the color of the background to which they attach; larvae on green vegetation are pale with distinct head spots, whereas those on rocks are dark with confluent head spots (Adler & Kim 1984). This apresent address and author for correspondence: Department of Biology, Clemson University, Clemson, SC 296341903. E-mail: jzettle@clemson.edu relation holds for all members of the subgenus Psilozia and might be a defining synapotypy for the taxon (Adler, unpubl. data). The implication is that these larvae might have the ability to change their color. To gain insight into the relation between larval color and substrate in the S. vittatum complex, we designed our study to address two basic questions. At the proximate level, we asked what factors influence larval color. To address this question, we tested the hypotheses that larval color is influenced by food, by background color, by ultraviolet (UV) radiation, and by sex, and that larval color is seasonal. At the ultimate level, we asked whether larval color provides a selective advantage against predators, and tested two hypotheses: (1) risk of predation for larvae on a light or dark background depends on body color, and (2) larvae select a background matching their body color.