Two weeks ago, as I began exploring the first of seven properties where I am conducting natural resource inventories this year for the Maine Coast Heritage Trust, I noticed a little ball of mud hanging from a goldenrod leaf. I was intrigued, but resisted the temptation to investigate it: I had lots of ground to cover, and I didn’t want to get distracted by minutia before I had accomplished anything. After I encountered a third one, however, I gave in and took some photos, then collected it.
The only one of these I remember seeing before was the one pictured on page 45 of Tracks & Sign of Insects…, which Noah photographed as we were wandering along a river in Pennsylvania. The paragraph accompanying that photo discusses how McCook (1884)* referred to the spider that makes this egg sac as Micaria limnicunae, and how it is unclear to what species that name refers. Platnick and Shadab (1988)**, in revising the genus Micaria (Gnaphosidae), listed this as an uncertain name:
The name M. limnicunae was applied only to egg sacs and spiderlings from Illinois, and no specimens identified by McCook can now be located in the collections of the Philadelphia Academy of Natural Sciences (Dr. James Newlin, in litt.); as Banks (1893) indicated, McCook’s description is “worthless.”
The spider may not even be a Micaria; McCook himself thought it might actually belong to the genus Herpyllus, which includes the eastern parson spider (H. ecclesiasticus):
Unfortunately, as far as I can tell, every known gnaphosid (ground spider) egg sac is an unadorned, disk-shaped object. Possibly some spiderlings will emerge from the egg sac I collected and shed some light on this mystery, but so far I have never succeeded in raising any kind of spider from egg to adult.
Here’s another observation that may or may not be relevant: Two years ago I collected this sand-covered egg sac, which was one of many found under some snake boards (big pieces of plywood lying on the ground) on Nantucket:
Some spiderlings emerged, but none lived to be more than about 1.5 mm long.
I would have had no clue what to call them, but Mandy Howe recognized them as liocranid sac spiders, probably in the genus Agroeca. Fortunately, Andrew McKenna-Foster has done extensive spider surveys on the island, and he told me that Agroeca is the only liocranid he has caught there. He has identified two species, A. pratensis and A. ornata; the former seems to be more common there.
It is well known that some European Agroeca species construct pendulous, dirt- or mud-covered egg sacs, but I haven’t found any references to American species doing this. Is McCook’s description of Micaria limnicunae so “worthless” that it might actually refer to an Agroeca? I haven’t checked, and I don’t know enough about spider identification to be able to judge, but I would be interested to hear an arachnologist’s take on this.
* McCook, H. C. 1884. A spider that makes a spherical mud-daub cocoon. Proceedings of the Academy of Natural Sciences of Philadelphia 36:151-153.
** Platnick, N. I. and M. U. Shadab. 1988. A revision of the American spiders of the genus Micaria (Araneae, Gnaphosidae). American Museum Novitates 2916:1-64.