Sight. The compound eyes of insects consist of thousands of identical units called ommatidia packed closely together on each side of the head. Each ommatidium consists of a lens system formed partly from a thickening of the transparent cuticle and partly from a special crystalline cone. This lens system concentrates light from within a cone of 20°, on to a transparent rod, the rhabdom. The light, passing down this rhabdom, stimulates the eight or so retinal cells grouped round it to fire nervous impulses to the brain. Each ommatidium can therefore record the presence or absence of light, its intensity, in some cases its colour and, according to the position of the ommatidium in the compound eye, its direction. Although there may be from 2000 to 10,000 or more ommatidia in the compound eye of an actively flying insect, this number cannot reconstruct a very accurate picture of the outside world. Nevertheless, the "mosaic image" so formed, probably produces a crude impression of the form of well-defined objects enabling bees, for example, to seek out flowers and to use landmarks for finding their way to and from the hive. It is likely that the construction of compound eyes makes them particularly sensitive to moving objects, e.g. bees are more readily attracted to flowers which are being blown by the wind.
Flower-visiting insects, at least, can distinguish certain colours from shades of grey of equal brightness. Bees are particularly sensitive to blue, violet and ultra-violet but cannot distinguish red and green from black and grey unless the flower petals are reflecting ultra-violet light as well. Some butterflies can distinguish yellow, green and red. The simple eyes of, for example, caterpillars, consist of a cuticular lens with a group of light-sensitive cells beneath, rather like a single ommatidium. They show some colour sensitivity and, when grouped together, some ability to discriminate form. The ocelli which occur in the heads of many flying insects probably respond only to changes in light intensity.
Sight. The compound eyes of insects consist of thousands of identical units called ommatidia packed closely together on each side of the head. Each ommatidium consists of a lens system formed partly from a thickening of the transparent cuticle and partly from a special crystalline cone. This lens system concentrates light from within a cone of 20°, on to a transparent rod, the rhabdom. The light, passing down this rhabdom, stimulates the eight or so retinal cells grouped round it to fire nervous impulses to the brain. Each ommatidium can therefore record the presence or absence of light, its intensity, in some cases its colour and, according to the position of the ommatidium in the compound eye, its direction. Although there may be from 2000 to 10,000 or more ommatidia in the compound eye of an actively flying insect, this number cannot reconstruct a very accurate picture of the outside world. Nevertheless, the "mosaic image" so formed, probably produces a crude impression of the form of well-defined objects enabling bees, for example, to seek out flowers and to use landmarks for finding their way to and from the hive. It is likely that the construction of compound eyes makes them particularly sensitive to moving objects, e.g. bees are more readily attracted to flowers which are being blown by the wind.Flower-visiting insects, at least, can distinguish certain colours from shades of grey of equal brightness. Bees are particularly sensitive to blue, violet and ultra-violet but cannot distinguish red and green from black and grey unless the flower petals are reflecting ultra-violet light as well. Some butterflies can distinguish yellow, green and red. The simple eyes of, for example, caterpillars, consist of a cuticular lens with a group of light-sensitive cells beneath, rather like a single ommatidium. They show some colour sensitivity and, when grouped together, some ability to discriminate form. The ocelli which occur in the heads of many flying insects probably respond only to changes in light intensity.
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