Science Q&A

How can spiders spin such incredibly wide webs?

New York TimesJuly 21, 2013 

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Spiders that build the familiar orb-shaped web usually start with a single superstrong strand.

MARK CHAPPELL — AP/SCIENCE

Q: I found single spider-web filaments between bushes that are 4 feet apart. How did the spider spin them over such a distance? And what happened to the rest of the web?

A spider relies on the wind to carry the filaments across wide intervals. But a mere 4 feet is a minor accomplishment for spiders. One recently discovered species from Madagascar – Darwin’s bark spider (Caerostris darwini) – habitually bridges rivers.

Spiders that build the familiar orb-shaped web usually start with a single superstrong strand called a bridge thread or bridge line. The telescoping protein structure of this silk is believed to gives it its strength.

First, the material for the bridge thread emerges from one of the spider’s specialized silk glands and is formed into a strand by its spinnerets. The loose end is drawn out by gravity or the breeze and allowed to blow in the prevailing wind, a process called kiting or ballooning.

If the strand does not make contact with something and attach to it, the spider may gobble up the strand and recycle its proteins, then try again. If the gap is bridged, the spider reinforces the strand and uses it to start the web.

A single bridge thread may be left in place overnight to mark a spider’s territory and a desirable starting spot for building a web the next day.

Q: I recently removed a tick from behind my ear, a common feeding place. How does the tick know to travel from ankle to head?

The ear is such a popular spot for ticks to feed that one species – Rhipicephalus appendiculatus – is called the brown ear tick. The mammalian ear usually has thin skin, making it an ideal site for feeding on blood.

Whether a tick feeds on humans or animals, the guidance system for reaching such an ideal attachment point is built into the tick by evolution.

Many ticks lie in wait for their hosts in the questing position, holding onto vegetation with their hind pairs of legs and stretching out the front pairs, ready to latch on to a passing person or animal. Depending on the species, a tick can detect carbon dioxide in the host’s breath; odor; heat; moisture; vibrations; or even a shadow.

Some ticks attach right away, while others go on an upward quest for thin, unprotected skin.

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