A comparison between the larval eyes of the dimly luminescent Keroplatus nipponicus and the brightly luminescent Arachnocampa luminosa (Diptera; Keroplatidae)

Abstract

Larvae of the weakly blue-luminescent fungus gnat Keroplatus nipponicus possess on either side of their heads a small black stemmatal eye with a plano-convex lens approximately 25 μm in diameter. In total, 12-14 retinula cells give rise to a centrally fused rhabdom of up to 8 μm in diameter. The rhabdom's constituent microvilli, approximately 70 nm in width, are roughly orthogonally oriented, a requirement for polarization sensitivity. Screening pigment granules are abundant in the retinula cells and measure at least 1 μm in diameter. In comparison with the stemmatal eye of the brightly luminescent Arachnocampa luminosa, that of K. nipponicus is considerably smaller with a poorer developed lens and a rhabdom that is less voluminous, but possesses wider microvilli. Although the larval eye of K. nipponicus can be expected to be functional, as the larvae react to light with a behavioural response, the eyes are probably mainly involved in the detection of ambient light levels and not, as in A. luminosa, also in responding to the luminescence of nearby conspecifics.

Keywords: bioluminescence; fungus gnat; photoreception; retina; vision.