From: jim.speirs@canrem.com (Jim Speirs)
To: dannys@iis.ee.ethz.ch (Danny Schwendener)
Subject: (De)Mystifying Spiders



Article #R93.
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(De)Mystifying Spiders
Ben Kruser
The Leader, May 1985


I remember reading a news story about a crisis in a southern
California town. Far away in the foothills, a forest fire raged, but
it wasn't the smoke or flames that panicked these people. It was
something they found more mysterious and threatening. In an effort to
escape the inferno, millions of spiders had cast their silk to the
wind and were parachuting onto parks, streets and homes.

>From the innocent spoiler in Little Miss Muffet to the menacing
mechanical "hound" in Fahrenheit 451, spiders have carried the
unwarranted reputation of being sly and arcane. Quite the contrary,
they are shy and timid creatures, and their special ability to catch
legions of noxious insects makes them fascinating subjects for study.


Natural History

In the fossil record, spiders first appeared 400 million years ago.
They are arthropods, closely related to such species as lobsters,
crabs and scorpions. True insects have six legs and a body of three
sections; head, thorax and abdomen. Spiders have eight legs and only
two body sections. The head and thorax section, called a cephlothorax,
attaches to the abdomen.

Spiders also have eight eyes, but only non-web spiders that stalk
their prey have excellent vision. Those that use a web to catch food
have poor sight and use the sense of touch to detect web movements
made by a struggling insect.

Flying insects are a spider's main source of food. After catching an
insect in one of a variety of traps. the spider paralyzes its victim
with a poisonous bite. The toxin also liquifies the prey's soft body
tissue so that the spider can suck out its insides. Sometimes a spider
will wrap the prey in a silk cocoon and hang it for a later meal.


Spider Silk

Greek mythology tells the story of a Lydian girl who challenged the
goddess Athena to a spinning contest and, as a result, was changed
into a spider. The girl's name was Arachne, from which the class name
for spiders, Arachnids, is derived.

Whereas caterpillar spinnerets are found on the lower lip, spiders
produce gossamer, or silk, from two or three pairs of spinnerets
located on the abdomen. They spin two basic types of silk. One is dry
and inelastic. It's used to make anchorlines from which to hang the
web and "lifelines" when the spider falls or needs a quick escape. The
other is sticky silk - elastic and covered with fine drops of
adhesive. The spider uses it to catch and secure unwary prey.


Spiders are careful to walk only on the dry silk. They, too, can get
stuck to their glue.

Spider silk must be strong because it has to hold large insects.
Although a gossamer strand averages only .005 mm in diameter, it is
stronger than steel wire of the same size.

The silk also helps spiders disperse to distant habitats. Newly
hatched spiders, or spiderlings, cast out gossamer strands until wind
friction is enough to lift them by their "parachutes". Pilots have
spotted them floating at altitudes of 14,000 feet, (4,270 m) so it's
easy to see how the jet stream could carry spiders and other insects
to remote ocean islands.


Webs

All spiders produce silk, but only some weave a web. Just as a bird's
nest identifies the owner, a web configuration is characteristic of a
particular spider species.

The orb web, also called the cartwheel web, is perhaps the most
symbolic and interesting in design. It's made by the garden spider,
who starts by casting silk into the wind until it touches something
solid, then pulls the strand taut. After securing several lines in
this manner, the spider picks a center and begins to spin outward
radiating spokes. Finally, it connects the spokes with spirals of
sticky silk and takes a position either in the center or to one side
of the circular web.

Garden spiders are very industrious and build a new web every 24
hours. In order to use the protein components of the silk, they eat
the old web before beginning the new. Some orb webs are very large -
in one reported case, 3 metres (10') in diameter.

These spiders wrap their eggs in brown silk sacs which you can find
hanging in secluded spots. During winter, the young hatch inside the
case where healthy spiderlings feed on their weaker siblings until,
come spring, the survivors emerge and disperse.

When dew covers low brush and grassy fields, you can see the funnel
web of the grass spider. It's a web of interlacing sticky silk with a
funnel on one side. Here the spider sits, patiently waiting for
something to land in its net. The funnel has a rear door as well and,
when frightened, the spider quickly exits to hide in the grass.

Old barns and window sills are common places to find the dull grey cob
web, the work of the house spider. This spider is a relative of the
funnel spider, but its web is more loosely constructed so that it will
hinder the flight of any fly that blunders into it. The web also
quickly collects dust and becomes very unsightly.

Some of the other kinds of webs you might find include the flimly
dome, the triangle, and constructions resembling the nests of tent
caterpillars.


Non-Web Spiders

Daddy longlegs are common and comical spiders that stalk prey rather
than building a web. Sometimes you'll hear them called Harvestmen or
Haymakers, probably because of their appearance at haying time.

You can easily recognize Daddys by their round, compact body and long,
stick-like legs. Nocturnal creatures, they eat flies, leafhoppers and
dead plant material. They are also very clean creatures that regularly
run each seven jointed leg through their mandibles to remove dirt and
debris.

Daddys are one of the few spiders that hibernate. Large colonies
gather to winter in a sheltered area, intertwining their legs for
added insulation. A Canadian Wildlife Service study found the
temperature of Daddys wintering in a Kootenay cave to be 3 degrees C
while, outside, the thermometer read -8 degrees C.

Once upon a time, when children were sent to fetch in the herd, they'd
first stop to find a Daddy and say, "Grandfather Greybeard, tell me
where the cows are," hoping the spider would show the way. Having
spent many hours rounding up stray cattle in the bush. I can attest
that a spider pointing in eight different directions isn't much help!

The crab spider is also known as the flower or goldenrod spider.
Proportioned like a crab, it hides in tubular flowers and waits for
insects to land on the corolla. The crab spider can change colour from
white to yellow. In spring, it favours white flowers and, later on,
moves to yellow ones. The choice of white for spring is probably not
accidental .

The white petals of such early flowering plants as the Prairie
Pasqueflower form a deep cup which becomes a miniature solar disc.
Using small heat-measuring instruments, researchers have found inner
flower temperatures to be consistently 14 to 18 degrees (F) warmer
than the surrounding air.

There are many advantages to this arrangement. The heat reflected onto
stamens and carpels enables seeds to mature as much as 25% faster so
that flowers growing in northern climates can add several days of
development time to a short growing season. Coldblooded insects are
attracted to these flowers for warmth and, in turn, serve as
pollinators. As a result, the plant is saved from spending precious
energy on such insect-attracting schemes as scent or nectar
production.

It seems quite likely, then, that the crab spider adapted to using
white flowers in the spring both for their warmth and their ability to
attract insect prey.


Spider Defences

Spiders are prey for larger predators such as birds and frogs and have
developed numerous means of self defence to outsmart these foes.
Garden spiders are yellow and black to resemble wasps for example.
Some spiders even buzz like a bee when disturbed to fool predators
that tear being stung. Both the crab and house spider use camouflage
to blend into their surroundings, and Daddy longlegs is reported to
give off a malodorous scent when attacked. If a Daddy loses a leg in a
struggle, a new one quickly grows to replace it.


Wasps are the most efficient spider predators. Ichneumon wasps land on
the spider's back and deposit an egg. The hatching maggot feeds off
the spider and, soon after killing it, pupates and emerges as an adult
fly. Pompilus wasps attack and sting spiders with a paralyzing venom.
The wasp carries its victim to a pre-dug nest burrow and places it
with a wasp egg. Again, the wasp larvae need living flesh to eat and
the spider faces an unpleasant demise.


Spider-Related Activities

Looking for spider webs is enjoyable detective work. Search in shady
places near rock walls, low brush or grass, and around the corners of
buildings. To avoid being seen, flying insects prefer cool, shadowy
places, and spiders know it.

You can bring "invisible" webs to view with a fine water spray from a
plant mister. Be careful not to spray too close because you may break
the web. Keep applying the mist until the web appears as water
droplets bead on the sticky silk.

When the water hits a web, the spider may emerge to check what is in
his trap. The weight of the water will make the web sag, and you can
observe the spider pulling in the slack to keep the silk taut.

With your group, ask how the web is made and what it does. Older boys
can try to calculate how long the silk strand would be if they unwound
an orb web.

On a dewy morning, count the number of webs you find in a small plot
and then try to figure the number of spiders in an acre or hectare. In
one field, a British naturalist reportedly found there were two
million spiders per acre.

Orb web painting is another activity you can try. Chase the spider to
safety, then use white spray paint to coat its web with a fine layer
of paint. Hold a dark piece of construction paper behind it and catch
the web against the paper. When the paint dries. you'll have the web
preserved tor posterity.

Spiders are an important part of any ecosystem. Perhaps it's fitting
to pay one last respect to the person who supposedly started it all
with this poem by Elizabeth Akers.

Here shy Arachne winds her endless thread
And weaves her silken tapestry unseen. 
Veiling the rough-hewn timbers overhead
And looping gossamer festoons between.