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Spiders usually have eight eyes but few have good eyesight.
Spiders usually have eight eyes (some have six or fewer), but few have good eyesight. They rely instead on touch, vibration and taste stimuli to navigate and find their prey. Most are able to detect little more than light-dark intensity changes which stimulate nocturnal web building, hunting or wandering activities and rapid movement to allow quick reactions against daytime predators (e.g. by dropping from webs). Some spiders have median eyes that can detect polarised light and they use this ability to navigate while hunting.
However, for a few spiders, good vision is vital for hunting and capturing prey and for recognising mates and rivals. They include the day active jumping spiders (Salticidae) and flower spiders (Thomisidae), and the wolf spiders (Lycosidae) and net-casting spiders (Deinopidae), more often seen by twilight or later at night.
Types of eyes
Eight eyes (E) are typically placed in two rows, on the front of the carapace. The rows are described as anterior (A) or posterior (P) and then the position within the row is lateral (L) or median (M). The AME or direct eyes, differ markedly in structure from the other indirect eyes (ALE, PLE, PME). The direct eyes appear dark, whereas the indirect eyes usually have a layer of light reflecting crystals, the tapetum, behind the light sensitive retina, giving these eyes a silvery appearance. The tapetum increases visual sensitivity because light entering the light sensitive retinal cells is immediately reflected back through them, so intensifying the image. These indirect eyes are adapted for seeing at low light intensities and their lenses are often enlarged in spiders with good vision. Spider eye lenses are better than photographic lenses in terms of their image brightness (very low F-numbers). However, because most spider eye retinas have relatively coarse-grained mosaics of receptor cells, their resolution of these images is much poorer than in the human eye.
Most wolf spiders (Lycosidae) hunt in the dimmer light of dusk and moonlight. Their four large posterior eyes have well-developed tapeta which help them spot prey movement in such low light conditions. At night, wolf spiders can be easily spotted because the tapeta in their large eyes shine brightly in torchlight.
Net-casting spiders (Deinopidae) have eight eyes, but in one genus, Deinopis, two of the rear eyes (PME) are enormously enlarged. Their great, curved lenses face forward like twin search-lights, giving the spiders a rather menacing appearance (the 'ogre-faced spiders').
The two biggest eyes are specialised for providing outstanding low-light night vision. They have enormous lenses that give a wide field of view and gather available light very efficiently. The lenses have an F number of 0.58 which means they can concentrate available light more efficiently than a cat (F 0.9) or an owl (F 1.1). Each night a large area of light sensitive membrane is manufactured within these eyes (and rapidly destroyed again at dawn).
This remarkable combination of large, powerful lenses and the nightly production of new light-sensitive membrane, enables net-casting spiders to accurately track and 'net' their prey at night. Interestingly, they do this without the help of a tapetum, the reflecting layer present in other spiders with highly sensitive indirect eyes. White faecal spots on a leaf below the net are aiming points placed there by the spider.
Daylight hunters with multi-purpose vision
Jumping spiders (Salticidae) are most active during the day. They have excellent vision which they use to hunt prey and recognise mates and enemies. Jumping spiders can leap more than 20 times their own body length, propelled by their back legs. However, when pouncing on their prey, they make shorter, more accurate leaps.
When hunting, the eyes of jumping spiders see in three different ways, using three different sets of eyes:
- The spider first senses movement of distant prey with the side eyes (PLE), which provide a blurry wide-angle image.
- Once movement is detected, the spider turns in that direction and locks onto the moving prey with the large, middle front eyes (AME). These eyes provide a clear, focussed telephoto image, probably in colour. The spider can track moving prey both by body movements and by using muscles to internally swivel the elongated eye capsules so that the light sensitive retina of each eye remains locked on the prey.
- While the spider stalks closer, it uses the side front eyes (ALE) judge the distance to the prey. When it judges the prey to be close enough (about 2 cm - 3 cm), the spider leaps.
They are the only animals which use silk in almost every part of their daily lives.