On the Trails: Woolly bears and spider webs

It’s early fall and we sometimes enjoy seeing fuzzy orange and black caterpillars trekking over the trails on their way to better foraging or a site for pupation. Although we usually call them “woolly bears” (and will probably go on doing so), that name is broadly applied to several different species.

At least one of them is similarly banded in orange and black, but others are yellowish. All of them are very fuzzy. Our local species is more specifically called the spotted tussock moth (Lophocampa maculata). Check out the long white hairs that add contrast to the black bands and look for some individuals that have a row of black spots down the middle of the broad orange band.

After hatching from an egg, the hatchling is very small, yellowish, and nearly hairless. The second instar is also yellow, but with a few hairs. The third instar is yellow with lots of hairs. Then the larva begins to get some orange and black tufts here and there, and the fifth and last instar is really fuzzy and has the familiar orange- and black-banded pattern with some long white hairs. Interestingly, there is extensive regional variation in coloration of the instars; a common variant substitutes yellow for orange. This species is not the only local one to have larval instars so different from each other. The rusty tussock moth (Ogyria antiqua; in a different taxonomic family) does it too, starting as small blackish larvae and ending up with a very colorful, tufted form.

My interest was recently caught by the contrast between these two examples and the common “loopers” that we encounter hanging on silk threads, gnawing on the capsules of sticky false asphodel, or looping their way over various garden plants. Many loopers look much the same as they pass from one instar to the next, just getting bigger, although some get darker.

What differences in ecology might be associated with having such different instars and such colorful and fuzzy last instars? So far, I have not found any good scientific consideration of this question.

So we can speculate: For instance, here’s one possibility: We might note that all the instars have to elude predators and maybe they do so in different ways, depending on size and coloration. Black and orange (or yellow) are often warning colors, signaling to a would-be predator that the bearer can be distasteful or dangerous. Also, the final instars of woolly bears, whatever color, are thickly covered with hairs that sting a bit or make a rash, perhaps making them unpleasant for a predator to handle. Perhaps the dramatic coloring and tuftiness of the rusty tussock moth’s last instar also has a deterrent function. Such possible protection allows the later instars to exploit more feeding sites than the younger instars, which might need to forage in more protected sites where they probably would be exposed to different dangers.

Of course, that leaves a fundamental question: Why do some species, even in different taxonomic families, have such distinctive instars while others do not? That’s a harder one to answer, needing some historical information.

On another topic: A morning walk on the dike trail discovered signs of spider activity. A few of the webs were classical orb webs: flat, with radiating spokes connected by strands circling the center of the spokes, and oriented more or less vertically. Far more common were tangly webs, mostly strung in the branching now-seedless seed heads of a grass or in the seed head of cow parsnip. From each dense focal tangle of silk threads extended an apparently disorganized set of lines and small tangles. The tangles probably provide shelter for the little predator. I saw only one tiny fly caught on these webs; maybe small prey were scarce or maybe the web-builder had already consumed some entrapped prey. The webs were festooned with condensed moisture droplets and caught the slanting light beautifully.

Spider silk is amazing stuff. Consisting mostly of protein, it is flexible and stretchy, but resistant to breaking. Spiders can make several types of silk and they use it in many ways. Prey can be captured not only by webs, but also (by some species) by portable nets dropped over prey or by lassos. Silk threads are used for shelters in a variety of situations, sometimes tucked into corners and crevices. They are a means of travelling, providing a line on which the maker can walk from one place to another or a way of becoming airborne on a breeze, the spider suspended on its thread. Certain species even make air sacs that allow them to breathe underwater. When prey is too scarce, spiders may eat their own silk, as a source of protein.

• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.