Spider Sunday: Longlegged Sac Spiders

Some spiders seem to be much more common indoors than outside. Chief among those are the “longlegged sac spiders” in the genus Cheiracanthium. They belong to the family Eutichuridae, a recent (2014) change. They were formerly in the family Miturgidae, known collectively as “prowling spiders;” but before that they were classified in the sac spider family Clubionidae.

Indeed, telling Cheiracanthium from Clubiona is not at all easy. Cheiracanthium lacks a distinct groove in the center of its carapace (the top portion of the cephalothorax), and there is no obvious tuft of curved setae (hairs) on the front edge of the abdomen. Clubiona does possess those characters. More to the point, you are not as likely to find Clubiona indoors.

The sac spiders are named for the silken retreats they spin. They do not construct silken snares like many spiders, but hunt “on foot” for small insects they can simply overpower. Their tarsal claw tufts, composed of densely-packed hairs, help them grip even slick surfaces. They easily run across the ceiling, defying gravity and giving the average homeowner the heebie-jeebies.

Cheiracanthium are mostly nocturnal, so you are likely to find them resting during the day, inside those retreats. Look for their “sleeping bags” at the juncture of a wall and the ceiling. They seldom, if ever, reuse a retreat, so you may find more than one. Females also deposit their eggs in a sac within a retreat, and the spiderlings that hatch may return to the retreat after minor explorations. The spiderlings will disperse after their next molt, though.

Because these spiders usually stick to the higher reaches of the average house, they don’t often come into contact with the human occupants. All the same, there has been a good deal of historic controversy as to whether longlegged sac spiders are dangerously venomous. Rumors of necrotic bites similar to those sometimes caused by recluse spider bites have largely been put to rest.

Spider bites in general are very rare events, contrary to what you might think from media sensationalism and internet exaggerations. One truly serious threat that is understated or omitted in the discussion of spider bites is the possibility of secondary infection that can turn any minor puncture wound into something much, much worse. Always flush any kind of abrasion or wound thoroughly with potable water. Use antibiotic ointments at your discretion, bearing in mind that overuse of antibiotics has contributed to the rise of “superbugs” that are increasingly resistant to the drugs we throw at them.

Back to the spiders at hand. There are two species of Cheiracanthium in North America, known commonly as “yellow sac spiders” or “agrarian sac spiders.” C. inclusum, is native to the U.S. and occurs coast to coast except the northernmost states. C. mildei is native to Europe but was introduced to North America decades ago. It now has a very widespread distribution in the states, including New England and Washington state. This is the species most often encountered indoors. Both species are medium-sized, the body length of mature individuals varying from 4-10 millimeters, males on the smaller end of the spectrum.

Mature and penultimate males (specimens that have only one molt remaining before adulthood) can be recognized by a “leggier” appearance and the modified pedipalps that make the spider look like it is wearing boxing gloves (see image above).

There are some very good internet resources on this genus of spiders, including this fact sheet from Penn State University, and this one from Washington State University. Do go offline, though, and see if you can’t find one of these spiders in your own home, or possibly empty retreats that the spiders once occupied.

Housemates

Besides the human and canine residents at 24 Sugarloaf Street, our house is also occupied by a variety of arthropods, mostly spiders. Crystalyn says she is not fond of spiders, so I’m hoping she doesn’t read my blog, at least for the sake of the health of these arachnids living with us.

The first spider to meet my acquaintance was this very gravid (egg-laden) female “prowling spider” of the family Miturgidae, on the ceiling of the bathroom. She is most likely a member of the genus Cheiracanthium, known also as “longlegged sac spiders.” The genus used to be in the family Clubionidae, but was moved to the Miturgidae in 1997, a decision still being debated in arachnological circles.

These spiders are mostly nocturnal, prowling during the night then spinning silken retreats in which they spend the day. This girl spun a bivouac as I watched.

Later I would find her mate in the kitchen. Well, it was an adult male at least, evidenced by the modified palps. They appear as the dark “boxing gloves” near his mouth. The palps (or pedipalps) are leg-like mouthparts that double in the male as intromittent sex organs. He deposits sperm on a mat of webbing, then draws it into each palp. These are complex appendages, designed to fit like a key in a lock, the lock in this case being the female’s sex organ called the epigynium. Like the female, the male also “sacks out” during the day.

Cheiracanthium had long been considered a genus of spiders at least mildly venomous to humans, but research has proven that this is a myth. So, I let them roam and carry out pest control activities. We definitely have too many mosquitoes here!

Crane flies of the family Tipulidae are very abundant now, some species being attracted to lights at night, and inevitably one strays indoors. This one has patterned wings. It is on the bathroom ceiling, too, but I don’t know whether one of the spiders got it. Crane flies are often mistaken for giant mosquitoes, or are referred to as “mosquito hawks,” but the adult insects probably do not feed, let alone take prey. Their size is deceptive as they are fragile insects. Look at one cockeyed and a leg will fall off. Their lifespan as winged insects is brief, just long enough to find a mate and reproduce. Crane fly larvae live in an incredibly variety of habitats, with diets that vary with the genus to which they belong.

A more permanent guest is yet another spider. This is what I imagine is a juvenile long-bodied cellar spider, Pholcus phalangioides, because adults are much larger. This one is well on its way, though, especially if it continues catching prey in the bathroom doorjamb like this. Cellar spiders are totally harmless to people and pets. While they do build tangled webs, they will also stray from those webs into the webs of other spiders and kill and devour them. Pholcids are frequently referred to as “daddy longlegs,” but that colloquial name is also given to harvestmen, arachnids of the order Opiliones (not spiders), and, ironically, to crane flies, especially in England. A crane fly would be more than this little one could handle.

Crystalyn is so busy that she sometimes forgets to turn the oven off, let alone observe the room for roaming arachnids. Her dog Ruby did see one, once, in the dining room, and drew our collective attention to it as it crossed the ceiling out of reach.