An apposition compound eye adapted for nocturnal vision in the moth midge Clogmia albipunctata (Williston) (Diptera: Psychodidae)

Abstract

Morphology and anatomy, dark/light adaptational changes and optics of the compound eyes of the nocturnal moth midge Clogmia albipunctata (Williston) are studied. Its apposition type of eye consists of approximately 260 well-separated ommatidia. Each ommatidium features a biconvex corneal lens covered by corneal nipples measuring around 17nm in height; a crystalline cone of the acone type; and an open (laterally fused) rhabdom formed by eight retinular cells (R1-R8). The corneal lens, whose biological significance is addressed, is composed of a thick yellow-coloured inner lens unit (ILU) surrounded by a thin, colourless outer lens unit (OLU). We identified two types of ommatidia: dorsally located T-type ommatidia and ventrally located P-type ommatidia. In the T-type ommatidia, the rhabdomeres of the retinular cells R7 and R8 are centrally located and are arranged in tandem with R7 above R8. In comparison, in the P-type ommatidia, only the R8 rhabdomere is central, whereas the R7 rhabdomere locates in the peripheral ring. Above the distal tip of the rhabdom, the crystalline cone and the PPCs form an aperture that dynamically changes its size under dark/light conditions, thus modulating the amount of light that reaches the photoreceptive layer. The Clogmia albipunctata eye has a low F-number of 1.2, a high interommatidial angle of 11° and a large eye parameter of 4.6μm·rad. The eye is characterized by relatively poor spatial resolution, but exhibits high absolute sensitivity.