Search

The apposition acone eye of Labidura is relatively small--550-600 facets--with a thick corneal lens and shallow retina. The retina cell columns are each formed of six peripheral cells plus two central cells, a partially fused rhabdom, and dense pigment in two or three cell types. Upon adaptation from light to dark, the most striking photomechanical response is a proximal broadening of the cone cells, which results in a 38-fold increase in cross-sectional area of the aperture. While longitudinal rhabdom movement is small, microvillar diameters swell in response to light and contract in the dark. Irregularities of facet pattern and shape, and in ommatidial alignment were found, particularly towards eye margins. Three types of interommatidial sense organs on the eye surface are described, one of which has not been previously reported. An argument is presented to explain how the field of view and sensitivity are both apparently decreased in the acone eye by exposure to light.

The eye of Dytiscus (Coleoptera) has rhabdomeres at three different levels. The crystalline threads stretch from the ends of the crystalline cones only as far as the distal layer of rhabdomeres. There is one distal rhabdomere per ommatidium, and in this system the ommatidia are anatomically separate. Between the distal rhabdomere and the rhabdomeres of the next six retinula cells is a wide clear zone in which light entering by one facet could possibly reach deep rhabdomeres of a different ommatidium. Of the six proximal rhabdomeres, four have rhabdomere tubules which lie horizontal with reference to the normal posture, the other two having vertically oriented tubules. The eighth cell, with nucleus near the basement membrane, has a small rhabdomere. All eight retinula cells have axons and there is no other class of axons in the eye.

Ectodermal sensory cells of the statolith each bear one nonmotile kinocilium, and in some hydromedusae bear stereocilia which are defined as distinct from microvilli by having fibrillar rootlets. The progressive elaboration of stereocilia can be traced. A hypothesis in which jellyfish statocysts evolve from vibration receptors avoids the difficulty of the uselessness of incipient gravity organs. Comparative study suggests that the kinocilium or its basal body is the transducer.

An electron microscope study of the compound eye of the termite Mastotermes darwiniensis shows that the ommatidia have both primitive and specialized features. The ommatidia are of the apposition type with eight similar retinular cells, a fused rhabdom, irregularly orientated rhabdomere tubules and no tracheae. The retinula cells contain lipid. The cone cells have unusual processes, which thicken towards the basement membrane and contain rough endoplasmic reticulum, granules and dense bodies. These processes may act as a primitive transport system for nutrients.