Ultrastructure of dorsal rim area of compound eyes of the male the red palm weevil, Rhynchophorus ferrugineus (Olivier) (Curculionidae: Rhynchophorinae): in relation to habitat

Abstract

The compound eyes of a male red palm weevil, Rhynchophorus ferrugineus (Olivier), were investigated using scanning techniques, specifically, the ommatidium in the dorsal rim area was studied by light and electron microscopy. Each eye contained 3000-3500 mostly hexagonal facets. No ocelli, corneal nipples or interommatidial hairs were found. The eye was of the acone and apposition types, with 8 retinular cells per ommatidium (two central, six peripherals).The peripheral distal and proximal rhabdome were composed of six separated rhabdomeres, with the separation between them increased by interretinular spaces. The central distal rhabdom consists ofr7, while the central proximal rhabdomeres was fused by R7 and R8. The microvilli of the peripheral distal rhabdomeres R2, R3, and, the central proximal R7, R8 were arranged in a three-dimensional pattern, where as distal R4 was warped around itself. The distal R7 and the proximal R1-R6 were possessed microvilli oriented in such away as to permit e-vector discrimination. The difference between the central and peripheral rhabdomeres in the two rhabdom regions. With regard to the microvillar arrangement implied the occurrence of the rhabdomeric twist. It was encountered only at the distal region, where the rhabdomeric organization displayed an effective strategy to improve absolute sensitivity, at the expense of the polarization of sensitivity, to overcome the particular problem of vision in dim habitats. A possible function in the perception of host related visual cues is proposed and discussed in relation to the plausible roles of the dorsal rim area in host location by male weevils.