Fine Structure of the Eyes of Harvest-spiders - The Lens

You can see electron micrographs of the lens in the eye of Mitopus morio in the picture quiz for January.

The lens is a thickened patch of cuticle forming the outer surface of the eye, as you can see in a diagram of the eye.
Beneath the lens is a structure called, descriptively, the glassy body. Together, the lens and the glassy body form the dioptric apparatus of the eye, focussing the rays of light on to the sensory layer, the retina. In addition, the cells of the glassy body are responsible for building the lens by secretion - thus the cells which make up the glassy body are also called lentigen cells.
We can consider two aspects here: (a) the structure of the lens itself and (b) features of the lentigen cells associated with their function. (Biologists are obsessed with the close relationship between structure and function - and justifiably so!)

(a) Structure of the lens

At its outer edge, there doesn't seem to be any clear structure at all in the material of the lens, as you can see here.
Indeed, there's less than you'd think from this picture, as the darker stripe along its bottom edge is the outer edge of the underlying exocuticle.

Exocuticle of lens Looking at the exocuticle itself, you get the impression of parabolic microfibrils making up its structure. It is true that the cuticle derives its strength from fibrillar molecules of chitin embedded in a protein matrix, but they are not exactly paraboloid in form. In fact, the structure is quite amazing - essentially there are lamellae, plates in which the fibrils are parallel with each other but at a slightly different angle from those in adjacent lamella. You could liken it to how you'd construct a strong sheet of fibreglass, with the glass fibres in each layer arranged at a different orientation from those in other layers in order to give the sheet great strength but light weight. The cuticle of harvestmen (Opiliones) - like that of the true spiders (Araneae) and indeed of all arthropods (the other Arachnida, the crustaceans and the insects) - thus has great strength but good resilience (elasticity) which are just the properties needed for an exoskeleton providing both support and protection for the body.
You can take a closer look, if you wish.

(b) The lentigen cells of the glassy body

At its inner edge, we can see that the lens lies on top of specialised hypodermal cells which actually secrete the material of the lens - so they are called lentigen cells.

This image shows a slightly oblique section through the distal part of these cells. You can see the cuticular lens material at the top of the picture. The cells themselves here are relatively free from inclusions - but look closely. You can see small circular profiles which are round vesicles. At the cell membrane which directly underlies the lens, you can see that there are some of these vesicles close to the membrane and even some places where the vesicles have fused with the membrane and have opened to release their contents. This is the process of exocytosis.

Lentigen cell - proximal At the other end of these cells, their proximal part, we can see the nucleus (labelled n in this picture), which contains the genetic material which controls (or at least influences) the activities of the cell. Just above (labelled mit) and also to the right of the nucleus we can see mitochondria (singular: mitochondrion); these organelles are the main sites of energy metabolism in the cell.
Then, just to the right of this there is a little stack of membranes - the Golgi complex (labelled gc). Golgi was the remarkable scientist who first observed these organelles, by virtue of the special silver-based stain which he invented (also very good for showing up nerve cells).
Look closely - around and about the Golgi complex are lots of vesicles, the small clear ones which we also se in the distal part of the cell as well as some different ones, some of them larger and with denser contents. These organelles are part of the secretory, material-processing machinery of the cell and so are very important in terms of he lens-producing role of these cells.

There's not much else to be seen in these cells, certainly no pigment granules, which is consistent with their role if focussing light (together with the lens). For this function, the lentigen cells must be transparent.

Just below the cell is a layer of rather amorphous material, which makes up the pre-retinal membrane (labelled prm). This separates the dioptric apparatus (lens + glassy body) from the retina (with the photosensory rhabdoms) below.

Absolutely amazing...!

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This information is based on electron microscope observations described in:
Curtis D.J. (1968) Fine structural studies on the eyes of Phalangida. Ph.D. thesis, University of Liverpool.

For further details, see:
Curtis, D.J. (1970) Comparative aspects of the fine structure of the eyes of Phalangida (Arachnida) and certain correlations with habitat. J. Zool., Lond. 160: 231-265.