Spider Sunday: Twobanded Antmimic

You would think that by virtue of the fact that spiders are venomous they have few enemies willing to tangle with them. Such is not the case. Some spiders have therefore evolved to look like other animals that are even more distasteful or hazardous to potential predators. Many spiders resemble ants, for example, and one common example is the Twobanded Antmimic, Castianeira cingulata.

The only reason I was able to get clear images of this particular female specimen was because she had climbed to the top of a doorknob and seemed perplexed as to where to go next. She actually “danced” in place, raising her abdomen and making choreographed movements with her legs. While antmimic spiders in the family Corinnidae are adept hunters, they do not see quite as well as wolf spiders and jumping spiders. I am not sure whether she detected my presence or not, though it seems likely that she did.

At only 7-8 mm in body length for females, and 6-7 mm for males, these are not imposing creatures. Their size does match that of carpenter ants, though, and so does their coloring and overall body shape. The two pale bands on the abdomen may serve to give the impression that there are actually three body segments instead of two. Even more astonishing is that the spiders behave like ants, even appearing in the company of ants to reinforce their disguise. They move reasonably slowly when prowling, bobbing their abdomens and even waving their front legs like a pair of antennae.

Why bother mimicking ants? Carpenter ants in particular are pugnacious creatures that can bite fiercely and also emit formic acid, a very repulsive chemical compound. Few other insects, or spiders and other predators want to take on such an aggressive dynamo, so looking and acting like an ant has its advantages.

Castianeira cingulata ranges from the northeastern U.S. and adjacent southern Canada east to South Dakota and south to Arkansas and Florida. It inhabits woodland habitats, scouring the leaf litter on the forest floor for insect prey. The spiders are active both day and night. The spiders can overwinter in dense silken sacks that they spin in sheltered situations such as rock crevices and the recesses of decaying logs. It is suspected that they can potentially live several years, at least in the southern reaches of their range.

Sources: Kaston, B.J. 1978. How to Know the Spiders (Third Edition). Dubuque, Iowa: Wm. C. Brown Company Publishers. 272 pp.
Gaddy, L.L. 2009. Spiders of the Carolinas. Duluth, Minnesota: Kollath+Stensaas Publishing. 208 pp.
Weber, Larry. 2003. Spiders of the North Woods. Duluth, Minnesota: Kollath+Stensaas Publishing. 205 pp.

House Centipedes

One of the most confounding, and arguably creepiest, creatures I am asked about at AllExperts.com has to be the “house centipede,” Scutigera coleoptrata. Neither insect nor arachnid, it is variously described to me as spidery, an animated feather, a speeding, ghostly apparition, and plenty of other epithets born out of both fear and fascination.

This species seems to be genuinely domestic, occurring mostly in and around human habitations where these venomous predators prowl in search of other invertebrates to eat. It is thought to have originated in the Mediterranean region, but unless one starts speaking Italian or Greek, you could fool me. International commerce has now succeeded in transporting the house centipede to virtually all inhabited corners of the globe anyway.

Related species in the genus often live in caves, and indeed the long legs and antennae of this animal are characteristic of habitats where visual acuity is much less important than a sense of touch. The lanky build of these creatures also makes them appear much larger than the 25-35 mm body length of the average specimen. Couple that with the incredible speed at which they can travel, and you have the heebie-jeebies come to life. I confess they can even freak me out at times because they can climb walls and scurry across ceilings at about Mach 7.

Fortunately, house centipedes are totally harmless to people and pets (well, if you have a flea circus I guess you’d better be careful). I encourage folks who encounter house centipedes to just let them continue their pest control patrol, like this one is doing, or usher the centipede into a container and release it outdoors in a woodpile, rock wall, or other sheltered situation where it will be equally happy hunting for food and mates. Those approaches are certainly preferable to "Honey, can you get me a shoe? A really big shoe?"

Giant ichneumon wasps

Prowling around a pile of cut logs near my temporary residence in South Deerfield, Massachusetts on the evening of June second, I almost literally stumbled upon one of the most spectacular wasps on the continent. Female giant ichneumon wasps in the genus Megarhyssa may look menacing, wielding a whip-like “tail” that can exceed their own body length, but they cannot sting and in fact are quite shy, easily startled by sudden movements. It is a pity that people are often so alarmed by these insects, as they have an extraordinary life cycle.

That long, streaming appendage at the end of her abdomen is not a stinger, as the uninitiated tend to assume, but an egg-laying organ called an ovipositor. What she does with it is truly remarkable. She is seeking the grubs of another stingless wasp, the “pigeon horntail,” Tremex columba. Horntail grubs bore in the solid wood of dead, dying, or severely-weakened trees. Along comes a giant ichneumon wasp. The ichneumon somehow “divines” the location of the horntail grub inside the tree or log, then arches her back to pull her ovipositor into position to drill into the wood to reach the horntail grub. Whether she actually penetrates the solid wood, or winds her thread-like organ through existing cracks and crevices is still a mystery, but she manages to deliver her nearly fluid egg to its target host. The ovipositor is actually three filaments: two form a sheath for the actual ovipositor, bracing the animal’s abdomen and allowing the ovipositor to work smoothly without undo bowing.

Once her mission is accomplished, and her egg is attached to the horntail grub, the adult female ichneumon slowly withdraws her ovipositor and begins the process over again, walking the log or tree with antennae outstretched like divining rods.

Meanwhile, her egg eventually hatches, and the larval ichneumon attaches to the exterior of the horntail grub. It will wait patiently for its host to reach a large, nearly mature size, then begin consuming it. The horntail grub is doomed, destined to yield an adult ichneumon rather than a horntail wasp.

Look for giant ichneumons around dead or dying trees at all times of the day. While ovipositing, females are vulnerable, and one can sometimes see where a wasp was taken by a predator. A hair-like filament protruding from a log is all that is left of the wasp in such cases.

Male giant ichneumons like this one can also indicate the presence of females as they vie for mating opportunities. Note that they almost resemble a different species of wasp! Many males may gather at the site where a female is about to emerge. It is quite a spectacle.

Four species of giant ichneumons collectively occur over most of North America. The ones shown here are probably the species Megarhyssa macrurus. Females can vary considerably in size depending on the size of their host as larval wasps. Smaller individuals tend to have reduced spotting on the wings (or no spots at all). Enjoy looking for these insects and observing their amazing behavior.