Phototactic Responses to Ultraviolet and White Light in Various Species of Collembola, Including the Eyeless Species, Folsomia candida

Previous observations have indicated homology in the cellular components between Collembolan eyes and the compound eyes of insects. However, behavioral or physiological studies indicating similarities in function are lacking. Collembolan eyes were examined from three species in the Family Isotomidae using scanning electron microscopy. Collembolan eyes are arranged dorsally and laterally on each side of the head in two species, Proisotoma minuta with eight eyes on each side of the head and Folsomia similis with one eye on each side of the head. In both of these species the eyes were located just posterior to the postantennal organ. In Folsomia candida, no external eye structures were detected. These three species were assayed for a series of behavioral preferences using ultraviolet (UV), white light and dark, and temperature conditions. The tests demonstrated that over 76% of all three species, including the eyeless F. Candida, chose white over UV light, over 69% preferred dark over UV, and over 77% favored dark over white light. The results demonstrated that all three species detect both UV and white light and avoid it, preferring cool, dark habitats. From the results of this study, it is hypothesized that F. candida may, in fact, be only "lensless" and may be able to detect light by having internal, non-ocular photoreceptors. Further histological studies are needed to investigate this possibility.