Today, let's weave a spider web. The University of Houston's College of Engineering
presents this series about the machines that make our civilization
run, and the people whose ingenuity created them.
Tennants of an Old Farm is the title of
this 1884 book by Henry McCook. He was a Congregationalist minister who did
field work in entomology each summer. Many of the fine steel plate illustrations
were done by Daniel Beard who later founded the Boy Scouts of America. Beard
began as an illustrator, and he was good.
The book's narrator visits a farm where the owner tells him,
"We are not the only tenants." Then he takes our visitor on a walk to visit
all the many insect tenants who're usually ignored. We meet wasps,
ants, bees, caterpillars, and the creature that's spun its web around McCook's
mind, the spider.
One chapter is titled, Argonaut and Geometer. Argonaut refers to the
affinity between spiders and freshwater boats. No surprise, since small flying
insects abound over water. But I'm drawn to the the spider as Geometer.
We follow the complex geometry in the structural engineering of an orb weaver
spider building her web.
First she swings downward from a twig on a single strand of thread until she lands
on another twig below. She then finds her way back up, dangling the thread behind
her, and she attaches it to another part of the first twig. Now a single catenary
strand hangs like a support cable on a suspension bridge.
At this point, McCook has our visitor joined by an assistant to the Roeblings,
whose Brooklyn Bridge had just been finished with its spider web of cables. Together
they watch, fascinated, as the spider climbs down to the bottom of her strand, then
drops to the lower twig with a new strand and tightens it all to shape the letter
Y. Next she returns to the middle
and spins out new strands, anchoring each to create the spokes of a wheel. These
are all non-sticky strands, and so is the one she spins next. She returns to the
center and begins to spin a spiral of Archimedes upon the spokes -- moving outward,
around and around.
Now she surprises us. She begins spiraling back in, laying down strands coated with
glue, while she eats the previous structural spiral. (She'll recycle that strand
to make her next web.) When we look very closely at this last sticky spiral, we
find that surface tension has pulled the glue into a string of beads. They are what
catch and hold any errant flying insect.
Here McCook recalls a legend from the life of Robert the Bruce, 14th-century Scottish
king. Robert has fled from the British and hidden in a deserted cottage. He lies on
the straw, watching a spider trying to swing his thread from one rafter to another.
On the thirteenth try he makes it, and Bruce realizes he too must keep trying. He
springs up and returns to action. Beard illustrates that message with a wonderful
drawing of a huge spider lecturing the young king like a stern schoolmaster.
That makes a rather typical nineteenth-century moral fable. But it seems much less
saccharine as we digest what that remarkable spider has accomplished in the interests
of its own survival.
I'm John Lienhard at the University of Houston,
where we're interested in the way inventive minds
H. C. McCook, Tenants of an Old Farm: Leaves from the Note-Book of a Naturalist.
(Philadelphia, George W. Jacobs & Co., Revised Ed., 1902. Originally published in 1884.)
The Beard cartoon sketch is from this source. All photos by J. Lienhard.
Here is a nice movie showing construction of the structural portion of the web.
See also the Wikipedia articles on spider webs
and on Henry C. McCook.
A spider web among the Douglas Fir on the Oregon Coast
The Engines of Our Ingenuity is
Copyright © 1988-2009 by John H.