Fine structure of the compound eye of the fungus beetle Neotriplax lewisi (Coleoptera, Cucujiformia, Erotylidae)

In many ways, the apposition eye of the erotylid fungus beetle Neotriplax lewisi resembles that of chrysomelids: its 400-500 mostly hexagonal ommatidia are of the acone type and possess open rhabdoms, a tapetum is not present, and axons penetrate the basement membrane in distinct bundles of eight. The eye also shows some unusual features that, at present, defy clear functional interpretation. Firstly, the cuticle of the interfacetal areas stains differently from that of the corneal lenses and, secondly, the two rhabdom systems in each ommatidium (central and peripheral) both possess microvilli that are oriented in such a way as to permit e-vector discrimination. On the basis of comparisons with other open rhabdom eyes, it is postulated that vision in N. lewisi involves neither high resolving power nor superior absolute sensitivity. However, this beetle can distinguish illuminated from shaded areas, and seems specialized to make use of sky polarization (probably in the UV and green regions of the spectrum) and/or the position of the sun as a course-stabilizing function during flights.