In case you didn’t know, spiders usually have eight eyes (some have 6 or fewer). Spiders that spin webs usually have lousy eyesight because their food flies into their trap (silk web). Most are able to detect little more than light-dark intensity changes and rapid movement – enough to stimulate nocturnal web building, hunting or wandering activities, as well as to allow rapid reactions against daytime predators (e.g., by dropping from webs).
Spiders that hunt and stalk their prey however, have excellent vision, especially night vision.
Some spiders have median eyes that can detect polarised light and they use this ability to navigate while hunting. For most night-active spiders sight is unimportant compared with touch, vibration and taste stimuli.
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 and flower spiders, and the wolf spiders and net-casting spiders, more often seen by twilight or later at night.
TYPES OF EYES
Eight eyes are typically placed in two rows, on the front of the carapace. 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 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.
So to my friend Zoe who asked what were the two bright blue spots, they are the reflector eyes in front of the other row of black eyes. The arrangement of the eyes is giveaway for identification but I don’t know what kind of spiders they are. Please look at her fabulous photos here.
Source: Australian Museum Online; Smithsonian Museum