The arachnids, which include scorpions, spiders and ticks, are an extremely ancient group of invertebrates that initially evolved in the sea but which became one of the first animals to emerge onto the land, way back in the Silurian period over 430 million years ago.
The spiders evolved from a scorpion-like common ancestor and are distinguished from other arachnids by their extensive use of silk. The earliest spider we know about is Attercopus fimbriunguis, found in New York and dating to the middle Devonian period about 385 million years ago. But it has silk apparatus that is different from all the other spiders, and is likely not in the direct line of ancestry.
Silk was an important innovation, though, and it set the spiders off on an evolutionary pathway of their own. The earliest spiders did not build aerial webs like today’s familiar species but instead lived in silk-lined burrows and hunted on the ground. Today, the group known as Mygalomorphs, which includes the tarantulas, funnel web spiders, and trapdoor spiders, still live a similar lifestyle and are considered to be the most primitive of the living spiders. Unlike the “true spiders”, the Mygalomorphs have fangs which move from front to back like a pair of folding knives, rather than from side to side like a pair of scissors. They also have a primitive breathing system consisting of two pairs of “book lungs”, in which the spider’s blood absorbs oxygen directly from the air: most modern spiders have only one pair of book lungs but distribute oxygen through their bodies using a series of internal tubes called “trachea”.
The familiar tarantulas are the best-known of the Mygalomorphs, and they are found in most of the tropical or desert world, including across the American West. In the temperate rainforests of the Pacific Northwest, however, there are no native tarantulas: instead, that ecological niche is filled by the Folding-Door Spiders of the family Antrodiaetidae. Most of these are found only in California, but two species, Hexura picea (which has no common name, though some people call it a “Dwarf Tarantula”, even though it is not technically a tarantula) and the much more common Pacific Folding-Door Spider (Antrodiaetus pacificus) are found in the Portland area and up into Canada. In a biogeographic oddity, there are also two species in this family, Antrodiaetus roretzi and Antrodiaetus yesoensis, which are found only in Japan—a relic of the time when North America and East Asia were once joined together in a single continent.
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Pacific Folding-Door Spider, about an inch and a half legspan. Unlike tarantulas, which are very hairy, Folding-Door Spiders are mostly bald.
The mouthparts and fangs look fearsome, but the spiders are not dangerous. They are much smaller than tarantulas.
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The Folding-Web Spiders are best-known for their elaborate living quarters. Like other Mygalomorphs they live in burrows, which are usually set at an angle next to a rock or tree trunk and can go as much as 18 inches deep. The entire interior is lined with a thick layer of silk to form a sort of tube which protects the spider from predators and environmental conditions.
At the top of the tube, at ground level, are a number of silken triplines which radiate outwards from the entrance. Here, the spider will spin a thin “curtain” of silk, camouflaged with bits of twig, leaf litter and dirt, and with a slit in the middle. When nocturnal prey such as an insect wanders over the tripline, the spider senses the vibrations, pushes through the flexible sides of the curtain and quickly drags the food inside, where it is paralyzed by venom. (The closely-related Trapdoor Spiders use a similar strategy, but they differ in having a thick and solid lid like a manhole cover, which has a silk hinge to quickly open and close like a submarine’s deck hatch.)
Here’s a YouTube vid I found demonstrating how the folding-door works in action:
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Mating occurs in the late summer. The males emerge from their burrows and wander in search of a female, who they can track through scent. Upon approaching a female’s burrow, the male must carefully tap a signal on the ground which tells her that he is there to mate, and if she is receptive she will open the lid and let him approach. A few weeks later she will lay a clutch of eggs which she will enclose in a silken eggsac and guard faithfully until the spiderlings hatch out. In the spring when the youngsters are big enough to be on their own, they will emerge from Mom’s burrow and wander off to dig a burrow of their own. Females can live for over 15-20 years, while the males typically die after just a few years.
This one does not appear to have the enlarged “boxing gloves” on the end of the pedipalps that breeding male spiders always have, and in any case it is way early for breeding season. So it is either an immature male or a female (and from the size I’m assuming it is a full-grown adult, so probably a female). I have no idea why (s)he would be wandering around in the open daylight like this. Maybe she got driven out of her burrow for some reason? In any case, she was taking a serious risk—outside of their burrow they are very vulnerable to predators like birds or small mammals.
I gave some serious thought to taking her home with me (I used to keep a much larger Red Trapdoor Spider from Africa many years ago), but decided not to, since it’s technically illegal to bring a non-native invertebrate into Florida (there are no Folding-Door Spiders in Florida, though we have a couple native species of Trapdoors). In any case these spiders spend all their time inside their burrows and are pretty un-entertaining pets. So I left her to wander to her fate. If she’s fortunate she’ll find another place to dig a new burrow. If she’s unfortunate she’ll be lunch for a Crow or something.
And now it is your turn—what’s burrowing in your neck of the woods?
(And now, just in case Admin wants to give me more petty bullshit about “cite your sources!” … Most of this information comes from my book here:
The Tarantula: An Owner's Guide to a Happy, Healthy Pet, by Lenny Flank, 1998, Howell Book House.
www.amazon.com/…
I’ve kept Mygalomorphs for many years, and during that time I have learned … um … a few things about them.)
We are always looking for new people to produce Daily Buckets, where we practice "backyard science" by reporting and recording what we have seen in the natural world around us (and no matter where you are--even in the middle of the city--there is nature around you). You don't need any science degrees or training or in-depth experience to be a backyard scientist, and you don't need any expensive camera equipment or photo-editing software to do Daily Buckets. All you need is a pair of eyes and ears, a phone with a camera, and a lot of curiosity and enthusiasm.
