Spider Vision

 

Nursery Web Spider, Pisaurina mira

Evolution makes dynamic matches between creatures and their circumstances. At Halibut Point we find various spiders with many different visual adaptations to their niches. 

Most spiders have four pairs of simple (camera) eyes, arranged in different configurations to suit their lifestyles. The camera construction parallels that of us vertebrates, each eye with a single lens that focuses light onto a retina. It's entirely different from the compound eyes of insects that merge information from hundreds or thousands of lenses into a single image.

Spider eyes, detail of photo above

Web-building spiders possess relatively poor vision and rely on vibrations from their webs to sense and catch prey. They try to avoid sharper-eyed predators by being camouflaged, secretive, and nocturnal.

Goldenrod Crab Spider, Misumena vatia

Flower spiders have good daylight vision. They specialize in actively ambushing insect pollinators during the day.

Thin-legged Wolf Spider, Pardosa sp.

Wolf spiders have also developed excellent eyesight used to advantage with speed and agility for catching insect prey on the run. They are uniquely gifted with a reflective membrane behind the retina that enhances their vision for hunting in low-light conditions.

The Thin-legged Wolf Spider's eye configuration

The location of the Wolf spider's eyes facilitates binocular vision useful in the precise tracking of prey. Three rows of eyes help with versatility. The first row has four small eyes, the second has two large ones, and the third has two medium-sized ones.

Dimorphic Jumping Spider, Maevia inclemens

The reigning champions of 8-legged, 8-eyed vision are the jumping spiders. Their goggle-like principal eyes are constructed like telescopes to focus precisely for leaps of up to 50 times their body length in tackling prey or escaping danger. But the long tubular arrangement of lenses comes at the cost of a very narrow field of vision. 

Jumping spiders' eyes, brain, and body take a team approach to vision. Their secondary eyes are positioned and designed to scan nearly 360˚. When something interesting attracts their attention you'll notice that they hop and spin to position their laser-like principal eyes for a sharper look, governed by retinas at the back of two independently movable tubes each controlled by 6 muscles. Meanwhile the wide-angled secondary eyes keep monitoring the environment for significant information in planning its next moves, and safety.

White-jawed Jumping Spider, Hentzia mitrata

Adding to their sight sophistication, jumping spiders have true color vision to a degree probably unmatched among arachnids and insects, and rare in the vertebrate world. Their ability to see blue, green, and red colors gives them a variety of advantages, and they also have ultraviolet receptors in their retinas. These spiders discern more colors than humans. 

It is a marvelous creature that can integrate these abilities effectively with complex other sensory stimuli into a tiny body with acrobatic capabilities.  I think I read somewhere that the brains of a jumping spider tend to expand into its 'non-cranial' spaces. 

You may enjoy these appreciative observations of Ximena Nelson, from a website of her own name. 

Visual processing in a unique modular system 

 

The challenge of perceiving the visual world is an ancient design problem. There are three distinct functional requirements for any effective system: animals must detect objects in a large area of the environment, they must achieve sufficient resolution to identify them, and they must reduce the visual information to a level that matches the capacity of their brain to handle it.

Familiar vertebrate eyes like our own are but one solution. Remarkably, at least 10 distinct designs have evolved. These can be broadly grouped into two classes, each accommodating the competing tasks of high-resolution vision and motion detection. Some animals have large eyes that address both functional requirements in a single structure; this obliges them to allocate a relatively large proportion of their neural resources to the massive processing load that such a system generates. The alternative is to adopt a design that segregates the tasks of motion and form processing into separate, dedicated, systems. The resulting output makes much more modest computational demands, suitable for animals with small brains.

The seemingly impossible feat of achieving high acuity and large field of view with only modest computational power has been achieved in only one group. Jumping spiders have evolved a unique ‘modular’ design. Four or six secondary eyes function as motion detectors, functionally analogous to peripheral vision in vertebrates. These combine to produce a field of view of almost 360 degrees. When something interesting is detected, the spider spins around to bring its two large forward-facing primary eyes (analogous to a fovea) to bear. These animals have a body length between 2-12 mm and brain only half the size of a honeybee’s, yet their sophisticated visual system attains a visual acuity that is almost an order of magnitude superior to their closest insect rivals and approaches that of primates. This is a stunning demonstration of evolutionary design and miniaturisation that, were it understood, would make our best robotics engineers weep.