Spruce-fir moss spider
|Spruce-fir Moss spider|
Crosby & Bishop, 1925
The Spruce-fir moss spider, Microhexura montivaga, is an endangered species of spider found at high elevations in the southern Appalachian Mountains. First identified in 1923, they inhabit moss that grows on rocks underneath the forest canopy.
This is one of the smallest mygalomorph spiders, with adults only measuring 3 to 4 mm. The coloration varies from light brown to yellow-brown to a darker reddish brown, with no markings on its abdomen. Their chelicerae project forward, and one pair of spinnerets is very long. They possess a second pair of book lungs, which appear as light patches behind the genital furrow.
They construct tube-shaped webs, apparently for shelter, for prey has never been found in them. They probably feed on the springtails that are abundant in the moss mats. The spiders can take as long as three years to reach maturity, due to the low temperatures and resulting slow metabolism.
The widespread death of Fraser fir trees has destroyed many habitats for the spiders, and they were listed as endangered in 1995. Many Fraser firs have died due to infestation with Adelges piceae, the balsam woolly adelgid, an insect pest introduced from Europe. The resulting thinning of the forest canopy leads to the drying of the moss mats that are essential for the spider's survival, as it requires climates of high and constant humidity.
This spider is known from Fraser fir and red spruce forests on mountain peaks at and above 1,650 m in the Southern Appalachian Mountains of North Carolina and Tennessee. They have been recorded from Clingmans Dome and Mount Collins (both very small populations), Mount Le Conte, Mount Mitchell (probably extirpated), Grandfather Mountain, and Roan Mountain.
The Tennessee population, located in Sevier County, was considered healthy up to 1989, but is now possibly extirpated. On two locations in North Carolina, there was only one spider found each in recent years. Only the population along the Avery/Caldwell County line in North Carolina seems to be relatively stable. This population appears to be restricted to the moss mats on a single rock outcrop and a few surrounding boulders.
- US Fish & Wildlife Service