It’s a quiet autumn evening. You’re enjoying some TV, when an unscripted movement catches your eye. A large house spider (Tegenaria domestica) is striding across the rug towards you. You make a sudden movement. The spider freezes. You reach for a paper to shoo it away, look back and it’s gone. What did the spider see? Was it afraid of you?
To a house spider we appear as a potential predator. Yet, despite having more eyes than us, most spiders don’t actually see much detail. Their world is sensed mainly by vibration, air currents, touch and taste.
Spiders usually have four pairs of eyes and there are two types: principal and secondary. Principal or direct eyes help the spider see detail; the photo receptor cells of their retinas are at the front. As in other animals they detect light and turn it into signals for the brain. In secondary or indirect eyes the photo receptor cells are inverted. These eyes are sensitive to movement, rather than detail and can give spiders an early warning of potential prey or predators.
About half of Britain’s 37 spider families spin webs to catch prey. Their eyes are usually arranged in two rows and of roughly similar size. They have poor eyesight and use vibration to sense where their food is.
However, in some spider families, vision is important. These are the spiders which hunt or ambush prey and they have much better vision than their web spinning cousins. In the UK, this includes crab spiders (with about 30 species), wolf spiders and jumping spiders (both have about 40 UK species). Wolf and jumping spiders have one pair of eyes much larger than the others. These principal eyes focus on prey and the smaller secondary ones detect movement. Many jumping spiders can wavelengths of light we can’t, such as ultraviolet. They use it to locate prey and in mating displays.
Crab spiders (Thomisidae), named for their flattened bodies and tendency to move sideways, live in meadows and gardens. They can detect movement with their principal eyes from a distance of up to 20 cm. They are ambush predators that use camouflage to blend into flower heads and catch unwary insects with their long front legs. Misumena vatia, another crab spider, takes this strategy further. Its base color is white, but it can gradually change to yellow. This broadens the number of flower species it can use to trap prey, and hides it from predators.
Net-casting spiders (Deinopidae) are called “ogre-faced” spiders because of their enormous eyes and angry looking appearance. They live in dark tropical habitats. Deinopis species have huge principal eyes whose lenses have a wide field of view. They can concentrate light more efficiently than a cat or an owl. Each night a light sensitive membrane is produced inside the eyes but destroyed at dawn as it is too sensitive to use in daylight.
Deinopus spinosa catches prey by making an expandable silken net. It deposits white faecal droppings onto a leaf as aiming points then waits above the net, head down. When an insect walks across the target area the spider opens the net and thrusts it down to enclose its prey.
Wolf spiders (Lycosidae) are ambush predators, also with enlarged principal eyes. Pardosa amentata scans its surroundings for movement with its secondary eyes, though it can only focus on prey up to a few centimeters away. As its target moves closer, a row of secondary eyes, which detect movement, is used to gauge distance. Wolf spiders also use vision for courtship. On encountering a female, a male stands on its back legs, waves its palps, and vibrates its front legs in time with them. A spider’s palps are appendages in front of its mouth. They can sense touch and taste and are also used in mating. Pardosa dances differ from species to species, enabling females to recognize males they can mate with.
Wolf spiders often hunt at night and their posterior median eyes shine in torchlight because they have a reflective membrane. This membrane acts as a mirror, so light passes their photo receptors twice which enhances low light vision. This structure is an example of parallel evolution as it is seen in nocturnal animals as diverse as cats and crocodiles.
Jumping spiders (Salticidae) are among the few invertebrates with large camera-like eyes like those seen in most vertebrates (including us). Their eyesight is sharper than any other spider and is comparable to that of pigeons or cats. Portia africanus can image up to about 75 cm, relying on shape and color to identify prey. Jumping spiders detect movement with their secondary eyes, which give a blurry image. The spider then turns and focuses its primary eyes onto the object. These give a sharp image. The spider then uses its lateral eyes to judge distance, and when close enough (2-3 cm) it jumps.
Source : https://studyfinds.org/how-does-the-world-look-through-spiders-eyes/
