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.