Functional aspects and eye organization of black and white morphs of the New Zealand beach beetle Chaerodes trachyscelides (Tenebrionidae)

Abstract

The tenebrionid beetle Chaerodes trachyscelides occurs in two distinct colour varieties in New Zealand: dark beetles are found on black sandy beaches, white beetles on white sandy beaches. No differences were encountered between black and white beetles with regard to eye size, facet dimension, ommatidial number and gross anatomical organization. The 38 μm-thick cornea is biconvex with outer and inner radii of curvature being 35 and 12 μm, respectively, and the eye being of the acone, apposition type. There are 8 retinula cells per ommatidium (2 central, 6 peripheral). Each retinula cell is 88 μm long, of which 40 μm are occupied by the rhabdom. Eyes of both black and white beetles contain large amounts of electron-dense screening pigment grains in primary and secondary pigment cells as well as in the retinula cells, where they are distributed evenly from distal tip to basement membrane. Differences were found with regard to intracellular organelles and the organization of the rhabdom microvilli. Eyes of black beetles displayed greater signs of light- or darkness-induced microvillar disruptions, whereas eyes of white morphs contained subcellular structures and organelles indicative of increased metabolic activity. Both kinds of sand, but in particular the black one, strongly attenuate impinging light right across the entire spectrum within the uppermost few millimetres. Electroretinogramme recordings revealed the great absolute sensitivity of the beetles' eyes to white light when compared with that of a similarly tested fly and also showed a broad sensitivity peak in the bluegreen-green part of the spectrum. Secondary peaks were not obvious and flicker fusion frequency did not exceed 20 Hz. It is concluded that the dark-sand environment not only has led to the dark-coloured variety of beetle, but has also influenced the degree to which retinula cells tolerate light despite the fact that gross structural organization and eye morphology had remained seemingly unaffected.