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.

 

Spider Silk

 

Marbled Orbweaver, Araneus marmoreus

All spiders produce silk though not necessarily webs. This orbweaver looks like an eight-handed pianist playing music in cords rather than chords, barely visible until they trap dewdrops and startle the landscape.

Orb-woven cobwebs

Most of this intricate work is accomplished under protection of darkness. Its bejeweling at dawn temporarily defeats the lethal secrecy of the web.

A grass spider Agelenopsis pennsylvanica approaches a lasioglossum bee caught in its web

Funnel weavers create sturdy if less exquisite tangle webs that include a hiding tunnel where they can escape the attention of both predators and prey.

Enshrouded goldenrod, the web scaffold

Consisting of mainly protein, silks are about a sixth of the density of steel. A strand long enough to circle the Earth would weigh about 4.4 pounds. *

Enoplognatha, a Space-filling Web Builder

The silk is produced in glands as a liquid gel that travels down a tube and out through valves that control its thickness and flow. It hardens in contact with air. Most spiders have 6 spinnerets capable of producing silk with different properties for different purposes.

Most silks, in particular dragline silk, have exceptional mechanical properties. They exhibit a unique combination of high tensile strength and extensibility (ductility) which defines toughness‒the ability to absorb a large amount of energy before breaking.

A Common Stretch Spider Tetragnatha extensa consuming a dragonfly

Swathing bands of silk in combination with immobilizing venom preserve prey in the spider's larder.

Among the versatile complexities of web construction are adhesive variations enabling the spider to bind the pattern at intersections, add glue-like properties for capturing prey, and weave in non-sticky threads for its own travel lines.

A Tuftlegged Orbweaver, Mangora placida

The term "spinning" is misleading because no rotation of any component occurs. Silk production is similar to extrusion, with the subtle difference that the force is induced by pulling at the finished fiber rather than being squeezed out of a reservoir. It must first be attached to an anchor point. Then depending on its various uses‒and the chemistry imparted by different silk glands‒the spider can balloon out on a lifeline into space or weave intricate patterns.

Spider silk is of great interest to research scientists trying to unlock its secrets for commercial, military, and medical applications. It continues to be available as a natural wonder for people of all ages.

 

* I am indebted to a Wikipedia article by this name for factual material.

Splendid Spiders

Intrigued by the fascinating 8-legged creatures of Halibut Point I sent away for a copy of Common Spiders of North America by Richard A. Bradley.

Hentzia palmarum, a jumping spider

The book takes a different approach to cataloging its subject than most insect identification guides which are organized by genetically-based anatomy. Much of Common Spiders is based on behavioral and habitat groupings as presented with my photos below. This can prove helpful in field observations.

Mecaphesa asperata, Northern Crab Spider

Unlike insects that have three main sections of the body, spiders’ bodies are condensed into two main sections. In the front its fused head and thorax are typically fused, comprising the eyes, fangs, stomach, legs, and brain. At the rear its abdomen holds the spinnerets used for making webs. 

Every spider has 48 knees, one for each of the 6 joints on its 8 legs.

ORB-SHAPED WEB GROUP

Leucauge venusta, Orchard Spider

These spiders produce webs in a pattern of sticky lines spiraling out from the center of a flat frame, supported by a series of spoke-like radial strands.

FUNNEL-SHAPED WEB GROUP

Agelenopsis sp., Grass Spider

Hiding within a tubular retreat, these spiders run out to capture prey trapped in or walking on the web.

SHEET WEAVERS GROUP

Florinda coccinea, Black-tailed Red Sheet Weaver

Sheetweb weavers await their prey hanging upside down directly under or beside the web rather than in an adjoining retreat.

UNDER DEBRIS GROUP

Dictynidae sp. - Mesh Web Weaver

'Under debris' spiders are usually discovered by turning over logs, boards or rocks, where they have formed this sheets of webbing.

SPACE-FILLING GROUP

Enoplognatha sp.

These spiders make three-dimensional tangles of lines to entrap insects passing through an area. 

JUMPING SPIDERS GROUP

Zygoballus sp.

When hunting or trying to escape a predator, jumping spiders are able to make very agile movements and jump multiple times their body length. This is possible due to an internal hydraulic system. Jumping spiders can alter the pressure of fluids in their legs resulting in a springing motion that propels the spiders forward.

CRABLIKE SPIDERS GROUP

Misumena vatia, Goldenrod Crab Spider

Many crablike spiders wait in ambush within a flower, arms outstretched to catch insect pollinators.

SURFACE HUNTERS GROUP

Pisaurina mira, American Nursery Web Spider

Rather than constructing static traps the surface hunters wander over landscape features, plants and streams in search of prey.

GROUND HUNTERS GROUP

Cesonia bilineata, Two-lined Stealthy Ground Spider

Similarly, ground hunters pursue prey with vision and speed superior to many other species.

Sergiolus capulatus, Variegated Ground Spider

Spiders may be colorful not only on the outside. They have pale blue blood because its oxygen is carried by hemocyanin containing copper, unlike the hemoglobin found in humans that gets its red color due to using iron.

Beauty in Photography (6) - The Sublime

Unmoored by beauty, carried to the sublime, we find ourselves in a life-stopping place. It's not a  place outside ourselves. We never stay transfixed there more than a moment. The sublime is an ultimate end, a rapture. We have to move on or expire.

In sublimation a solid substance transmutes directly into vapor without melting. As in frost crystals on a cold day returning to the atmosphere with no intermediary droplets or puddles, dematerializing to the ether.

Mystics, mothers, and soldiers take sublime risks to engender life. Techniques of breath sustain their achievements. Beauty unites both creation and sacrifice.

The Beauty that concerns me is that of Form. Beauty is, in my view, a synonym for the coherence and structure underlying life.

Robert Adams, Beauty in Photography

[It] can only be found within a framework that is larger than we are, an encompassing totality invulnerable to our worst behavior and most corrosive anxieties.

How is it that the subjects of these pictures, absorbed in ordinary life moments, can represent grand experiences when captured just so in a photograph?

Exterior images lead the photographer to memories, revelations, hopes and dreams.

Art simplifies. It is never exactly equal to life. In the visual arts, this careful sorting out in favor of order is called composition.

Innocence and gravity reveal the beauty of the uncontrived.

William Carlos Williams said that poets write for a single reason‒to give witness to splendor. It is a useful word, especially for photographers.

Beauty depends on discovery. It's just a feeling about what is‒what already is, all by itself.

The looking and the feeling call for response. The photographer finds ways to make it his own.

I have...learned, however, that the word beauty is in practice unavoidable. Its very centrality accounts, in fact, for my decision to photograph.

There appeared a quality‒Beauty seemed the only appropriate word for it‒in certain photographs and paintings that opened my eyes, and I was compelled to learn to live with the vocabulary of this new sight, though for many years I still found it embarrassing to use the word Beauty, even while believing in it.

The sublime stops us in our tracks. It takes our breath away, pulls us to unity with beauty. But we have to be ourselves. We have to keep breathing and moving, or become ethereal.

Time is movement toward the sublime.

Beauty in Photography (5) - Human Moments

 

This is our world. We are of it, dependent, sometimes transfixed.

Its rhythms demand grace and strength and fluent attention.

Its beauty is as likely to unfold on a boisterous path as on a gentle one,

as likely to be favored by grace as by endeavor.

Its soft boundaries support the resolve of becoming.

Prizes won and given away multiply the shared moments of a day.

Words of life make unspoken testimony on human and near-human faces.

The faces prepare the ground for words, and for photographs.

Every human photograph portrays the possibilities of a story.

Our myriad facial muscles relate those stories directly from the center of our beings.

Beauty in photography captures the complexity of those stories written on faces.

The complexities flowing back to apparent simplicity reflect universal harmonies, wrinkled as calligraphy on the face of camaraderie. 

Recording moments of people being fully present, photography memorializes beauty.