Tiny aero­nauts and gos­samer bridges at dawn

Rappahannock News - - NATURE • FIRE & RESCUE - PAM OWEN wil­[email protected]

This time of year I en­joy hav­ing my morn­ing cof­fee out on my deck, which over­looks a small copse of trees. I of­ten start when light has yet to reach this side of the hol­low, and be­fore me is a sub­tle sea of green. I see some birds flit­ting around in the trees, but vir­tu­ally no in­sects. But, on a sunny day, as soon as the sun crests the moun­tain on the other side of the hol­low, it re­veals thou­sands, of tiny, fairy-like crea­tures float­ing and fly­ing in the air among a lat­tice­work of silken bridges — all back­lit by the ris­ing sun. In this ethe­real light, I get an en­tirely dif­fer­ent view of the nat­u­ral world un­fold­ing be­fore me.

Some of the tiny aero­nauts I see be­fore me are mere bits of fluff rid­ing the breeze, re­sem­bling seeds from an ash or cot­ton­wood tree. But their in­sect bi­ol­ogy is re­vealed as they change course, fly­ing against the wind. Oth­ers larger, stronger in­sects with pro­nounced wings mover more as­sertively through the air — per­haps preda­tors end­ing or just start­ing their daily hunt­ing. With the light be­hind them, it’s hard to tell.

While the num­bers of in­sects in the air is stun­ning, the most as­ton­ish­ing thing is the amount of silk fil­a­ments that cre­ate a lat­tice­work through­out the for­est, bridg­ing one tree or branch to another. The early morn­ing dew mag­ni­fies each strand and makes it glis­ten. These bridges are the work of spi­ders, who jump into the wind to sail through the gaps, an­chored by a thread of silk they at­tach to a tree or branch. Many times on a for­est trail I’ve been star­tled by these bungee jumpers when they land on me from out of the sky.

In a Q&A in the science sec­tion of the New York Times a few years ago (ny­times.com; July 7, 2012), science writer C. Clai­borne Ray ex­plained how spi­ders ac­com­plish this amaz­ing feat. Spi­ders that build orb-shaped webs usu­ally start with a sin­gle “su­per­strength” strand, known as a “bridge thread” or “bridge line,” he writes. Spi­der silk is five times stronger than steel, and the strength of this line is be­lieved to come from its tele­scop­ing pro­tein struc­ture. Ray de­scribes how the spi­der stretches this thread from one point to another:

“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.”

One Mada­gas­car arach­nid, Dar­win’s bark spi­der ( Caerostris dar­wini), regularly bridges rivers through kit­ing. And kit­ing is not just used for bridg­ing gaps when spi­ders con­struct webs. Spi­der­lings (the young) of some species also use this method to dis­perse from the nest. The thin strand of silk they spin to do this is usu­ally re­ferred to as “gos­samer.” By lit­er­ally cast­ing their fate to the wind, they’ve been able to pop­u­late re­mote lo­ca­tions, even is­lands in the oceans.

Charles Dar­win noted in his jour­nal that the HMS Bea­gle, the ship he trav­eled on in his ex­plo­ration of the fauna of the Pa­cific, in­ter­cepted clouds of spi­ders sev­eral times while in the bay of the Rio de la Plata. The ship was cov­ered with gos­samer, Dar­win wrote, with the spi­ders low­er­ing them­selves from the rig­ging to the deck on the strands.

As I’m writ­ing this, na­ture seems to be try­ing to bring the les­son home. A tiny spi­der­ling just rap­pelled down from the roof eave above me on its own silk thread, pass­ing within an inch of me and land­ing on the deck. It ran up the leg of the chair I’m sit­ting in, ap­par­ently thought bet­ter of it, and dou­bled back, dis­ap­pear­ing un­der the deck.

The back light­ing at dawn also also dis­torts the forms of crea­tures walk­ing along tree limbs, mak­ing them seem larger and stranger. On a cou­ple of morn­ings, what seemed like a huge spi­der with thin legs was am­bling along a tulip­tree limb not far from where I was sit­ting. Its man­ner of walk­ing — slowly ex­tend­ing one leg af­ter another in a lum­ber­ing grace that reeked of the primeval — re­minded me of the taran­tu­las that I've seen mov­ing across the Mo­jave Desert. The in­ver­te­brate near my deck was not nearly so bulky and hairy but just as an­cient. With light be­hind it and the mois­ture in the air, its size could be de­cep­tive.

In re­vis­it­ing the deck this morn­ing, the same limb was host to a meet­ing of other smaller leggy arach­nids, this time two har­vest­men (in the or­der Opil­iones, also known as “daddy longlegs”). Their meet­ing was brief, with each touch­ing each other’s feel­ers be­fore con­tin­u­ing on their re­spec­tive jour­neys.

I usu­ally re­visit the deck in the evening and, while pleas­ant in the shade and with the evening bird song this time of year, I see no sign of the ethe­real fairy world of dawn. The in­sects that emerged then are now seek­ing shel­ter for the night. And sun­light no longer il­lu­mi­nates what’s left of the spi­ders’ lat­tice­work, which they will reweave again af­ter the sun goes down.

© 2015 Pam Owen

The ship was cov­ered with gos­samer, Dar­win wrote, with the spi­ders

low­er­ing them­selves from the rig­ging to the

deck on the strands.


Left: Spi­ders build webs be­tween trees, of­ten yards apart, by jump­ing into the wind, at­tached by a sin­gle, strong fil­a­ment of silk from their start­ing point. Above: Two har­vest­men meet at dawn. Be­low: The dawn light cap­tures a web about 20 feet off the...

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