The story behind these elegant spider webs

One fine, cold day in late January, I took a stroll on crunchy snow. In a couple of places, I ventured off-trail a little way, to check out some tracks or a lichen garden. As I did so, I ran into occasional dangling silk strands decorated with frost. In warmer seasons, when spiders are busily making webs, I often find it convenient to follow a taller person, who conveniently clears the webs from the trail.

 

The single silk strands I saw on that wintery day may be been used, last fall or summer, by spiderlings (baby spiders) to “balloon” away from their birthplace to a place of their own — a risky but common means of dispersal for some species. Most adult spiders are thought to be too heavy for this mode of transportation, but small adults of some species do it. When spiders are ready to balloon, they stand on tiptoe with their rear ends elevated. Then they spin a thread of silk that — as it elongates — catches a breeze that wafts the little spider away, maybe only a few feet or maybe many miles.

Those single silk strands might also have been an adult spider’s attempt to start a new web. The first step in web-building (for a spider that builds the familiar orb web) is to cast a line of silk across a gap between two twigs, in hopes of it catching on to an anchor point across the gap; it may take several tries. Once that basic line is established, the rest of the web is built below it. The “mis-fires” are left to dangle. Or perhaps an adult spider dropped down its safety line to escape some perceived danger.

Those elegant orb webs take some hours and a lot of silk to build. After the anchor line is firm, construction starts with the radiating “spokes” of the orb, followed by the spiral across all the spokes.

Much of the spiral is sticky, so flying insects get trapped. Their struggles alert the patient spider lurking nearby. The waiting spider then darts out and immobilizes the prey in a silk wrap, to be hauled off and eaten or temporarily stored nearby. Webs can accumulate lots of small fragments of debris or little tears, but rather than repairing dirty or slightly damaged webs, some orb weavers eat their old webs after a day or two, thus recycling all the proteins of the silk.

Some orb weavers add a patch of thick, conspicuous strands to part of the web. This has been thought to make the delicate web visible to birds, which can then avoid flying into the web and destroying it. Or maybe it just makes the rest of the web less readily visible to potential insect victims.

Some decades ago, a researcher accidentally discovered that orb-weaving spiders that consumed various psycho-active drugs (caffeine, amphetamine, etc.) could no longer build normal webs. Instead, their webs were incomplete or seriously distorted, in some cases no longer functional at all.

Another kind of spider deliberately makes tangled webs of various forms; this group includes the common household spiders making cobwebs in secluded corners. Those spiders are mildly venomous but seldom bite humans; they could be considered to be helpful to us, because they consume insect pests in our houses. It’s another story with the black widow spiders, the females of which are highly venomous. Tangle-web spiders generally hang out somewhere in their messy webs, waiting for some hapless insect to stumble in.

Way down under, the Australian funnel-web spiders build their silk-lined burrows in the ground or in tree holes. The mouth of the burrow is typically a funnel made of silk; from the top of the funnel, silken trip-lines radiate outward. The spider sits just inside the burrow, with its legs touching the trip lines. When an insect or a small lizard or other crawling critter touches a trip-line, the waiting spider dashes out to nab its prey. Females generally stay in their burrows, but males wander around more, as they look for females. Some of these spiders reach a body size of about two inches, and they are renowned for their venom.

A group of other spiders, known as funnel weavers, occurs worldwide. They are not related to the notorious funnel-web spiders and they are not nearly as venomous. They make a sheet-like web in front of their funnel-shaped retreat; this web is not sticky but its fibers snag the body parts of passing insects, holding them for the lurking predator. Some African species build communal webs, hunt cooperatively, and even raise their young communally.

A variety of spiders build sheet webs, flat mats of entangling fibers that trap the legs and bodies of unwary insects. One species in New Zealand builds enormous (compared to its small body size) sheets up to a meter across. A common one in parts of the U.S. is called the bowl-and-doily spider. Its web is complex, consisting of a silken bowl above a skimpy flat sheet, with a tangle of trip-lines above the bowl. The spider hangs upside down under the bowl, ready to reach through it to grab an insect that falls in. The “doily” is thought to protect the waiting spider from predators. Females and sometimes young males build these webs; fully-grown males are too busy looking for females.

Among the most intriguing web-making spiders are the net-casters, found mostly in the tropics and subtropics. They have enormous eyes that are very sensitive in dim light. They hang from a silken framework, waiting for prey, holding a small web between the front legs. When the spider spies a passing prey insect, or the unwary insect happens to touch a strand of silk, the predator stretches out the web and pounces, entangling the prey in the expanded net, which folds up around the prey.

There are many thousands of species of spider, and all can make silk. But not all spiders make webs. Jumping spiders and wolf spiders hunt using their excellent vision for 3-D precision in locating prey. Jumping spiders often spin a safety line when they jump, so they can retreat to their perch if they miss their strike. One jumping spider species claims the spider record for high elevation — it lives on Mount Everest at 20,000 feet and more. Wolf spiders often wander around when hunting, but some prefer to sit near their burrow and wait in ambush. Trapdoor spiders live in silk-lined burrows, behind a slightly open door disguised by various available materials; passing prey insects are ambushed as the door is flung open and the spider lunges out. Crab spiders don’t make webs, either. Some sit in flowers, mimicking the flower’s coloration, in hopes of ambushing some flower-visiting insect, while others perch on leaves or bark.

That’s just a sampling of what spiders can do!


• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.


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