Occhio composto / Ocello , ommatidi.
Struttura dell'occhio composto nei dipteri, mosche
Gli occhi composti sono i fotorecettori più complessi, preposti alla percezione delle immagini. Hanno forma e sviluppo vario e i generale si localizzano nella parte dorso laterale del capo, talvolta fondendosi in un'unica struttura in corrispondenza del vertice. In alcuni insetti sono sostenuti da brevi processi. Strutturalmente sono composti da un insieme di unità elementari, dette ommatidi, in numero elevato, fino a diverse migliaia.
Ogni ommatide (o ommatidio) si presenta come un elemento prismatico formato dai seguenti componenti (dall'esterno all'interno): la cornea, il cristallino, la retinula. La cornea è una lente, prodotta da apposite cellule, meno spessa rispetto a quella degli ommatidi; immediatamente sotto è presente un corpo rifrangente, il cristallino, in genere composto da quattro cellule. La retinula è composta da 4-8 cellule sensoriali e da un asse centrale a forma di bastoncello, detto rabdoma, composto dalle terminazioni nervose. Nel rabdoma si concentra il recettore chimico (retinene). La struttura dell'ommatidio può variare in relazione alla genesi e alla consistenza del cristallino, al rapporto che sussiste fra cristallino e retinula, ecc. Tali differenze possono incidere più o meno sensibilmente sulla funzionalità della percezione visiva.
Sezione schematica di un ommatidio per apposizione. A: cornea; B: cristallino; C, D: cellule pigmentarie; E: rabdoma; F: cellula sensoriale della retinula; G: membrana basale; H: neuriti delle cellule sensoriali.
L'elemento differenziale più importante è tuttavia quello che distingue gli insetti diurni da quelli crepuscolari e notturni: negli insetti diurni ogni ommatidio è otticamente isolato da un rivestimento di cellule pigmentate (iride), mentre negli insetti notturni le cellule dell'iride isolano solo la porzione superiore dell'ommatidio. Questa differenza di struttura implica una differenza funzionale che si ripercuote sulla qualità della percezione visiva:
Gli insetti diurni hanno una percezione per apposizione: gli ommatidi percepiscono solo i raggi paralleli al loro asse e l'immagine si compone a mosaico. Questa funzionalità permette la percezione di un'immagine nitida ma solo in condizioni di elevata luminosità.
Gli insetti notturni hanno una percezione per superposizione: gli ommatidi percepiscono anche i raggi obliqui grazie alla continuità ottica nella parte inferiore e l'immagine si compone per sovrapposizione. Questa funzionalità permette la percezione di un'immagine sfaccettata ma in condizioni di scarsa illuminazione.
The Eyes
Compound Eyes
Compound eyes are so named because the cornea is composed of a number of
individual facets or lenses (called ommatidia), rather than a single
lens as in ocelli (or our own eyes). The number of separate visual
elements or ommatidia varies greatly between species as well as between
the larger taxa, so that while worker ants of different species may have
between 1 (Ponera punctatissima) and 600 ommatidia per single eye, adult male Odonata may have more than 28,000 per single eye.
This
creates a considerable difference in the presentation of light stimulus
to the insect brain, however the ability of insects to navigate the
world by means of visual stimuli suggest that they have overcome the
problems inherent in this multi-faceted perception. The physical
differences between single-lens and multi-lens perception are
excellently shown at the
Bee Keepers Home Page in the section on "the world through the eye of a bee".Much like our eyes, the eyes of insects, can be divided into four basic parts: the supportative material that keeps all the parts together; a light gathering part (the lens and the auxilary lens called a 'crystalline cone'); a light receptor that converts the recieved light into electrical energy; and the nerves that carry the electrical impulses to the brain for analysis. In the compound eyes of insects these parts are repeated numerous times side by side in a space saving hexagonal pattern. See above.
The lens is formed by a transparent and colourless cuticle and it is usually biconvex. Beneath this is the crystalline cone (which is comprised of four cells called 'Semper cells' after the man who first described them). Normally this functions as a secondary lens.
The
receptive parts of an insect's eye are the 'retinula cells'. Each
ommatidium normally has eight retinula cells arranged to leave a central
core space in the centre of the ommatidium, into which each retinula
cell projects a series of microvilli (like very small fingers). These
microvilli are the actual light detecting part of the cells and are
collectively referred to as the rhabdomere (think cornea). The eight (or
occasionally 7 or 9) rhabdomeres (sets of microvilli) form a rhabdom.
The corneal lens is supported by 'primary pigment cells' and the retinula cells and associated rhabdoms are supported by 'secondary pigment cells'. The retinula cells are connected to axons at the base of the eye, it is these which carry the information collected by the lenses and converted into electrical impulses by the rhabdom to the brain, thus allowing the insect to see.
Simple Eyes o ocellus
Ocelli are present in most insects to some degree, though as with all
aspects of insect anatomy there is a great deal of variety in form and
even in relative function. Generally they consist of five separate
parts the 'cornea', the 'corneagen layer', the 'retina', the
'pigment cells', and the 'central nervous connections'.
1)The CornealLens this is a thickened area of generally transparent cuticle to the outside of the ocellus which serves as a lens.
2)The Corneagen Layer this is a single layer of specialised transparent and colourless epidermal cells which secrete the cornea.
3)The Retina this is a group of primary sensory cells which convert light into an electrical stimulus and transfer it to the; the cells are called 'retinula' cells and they are arranged in circular groups with each member of the group contributing to its portion rhabdomere to the group rhabdom. The rhabdom is the light sensitive pigment, or the part of the ocellus that converts the light into an electrical stimulus.
4)The Axon, which is the nerve link to the 'protocerebrum' and hence to the 'Corpora pedunculata' (the brain) which in turn allows the insect to use the information the ocellus produces.
5)The Pigment Cells this is a group of highly pigmented (coloured) cells variably distributed around the ocellus whose main roll would appear to be the exclusion of light from parts of the ocellus other than the cornea.
The function of the corneal lens is obscure, although it does project an image into the ocellus this image forms below the level of the light-sensitive cells, or rhabdom. Therefore the ocellus can generate no image information, however it is very sensitive to low levels of light and to changes in light intensity and scientists believe that the ocelli are useful in allowing the insect to detect the horizon, to respond quickly to changes in light intensity.
Two different forms of ocelli have been described for insects, Dorsal ocelli and Lateral ocelli.
Dorsal ocelli occur mostly in adult insects and are situated on the front of the insects face in the area of the 'frons' and or the 'epicranium', lateral ocelli generally occur on the sides of the insect head and are the form of eye most common in larval forms; there are a number of concrete differences between the two forms which can be be found explained in any competent entomological text book such as Imm's 1984. See also the Book Reviews for other texts
