The Spruce-Fir Moss Spider: At the Peak of Unique

The high-elevation forests of the Blue Ridge provide habitat for a number of species not found at lower elevations, or anywhere else in the world. Among these unique species is the spruce-fir moss spider.

M. Montivaga photo by Dr. Marshal Hedin, SDSU

At full size, the spruce-fir moss spider (Microhexura montivaga) measures about 1/8”, making it the world’s smallest tarantula-like spider. This rare arachnid only occurs in the high-elevation red spruce-Fraser fir forests of the Southern Appalachian Mountains, with six recognized populations occurring on the mountain peaks of Virginia, Tennessee, and North Carolina above 5,300 feet. In its peak condition, the spruce-fir ecosystem has a dense canopy and moist understory. The spruce-fir moss spider (SFMS) takes advantage of this quality by living below damp, well-drained moss and liverwort mats on heavily shaded north-facing rock outcroppings.

Limited-edition Spruce-Fir Moss Spider shirt, available at the Spring MRNR.

The tiny spruce-fir moss spider constructs thin-walled tube-shaped webs between moss mats and rock surfaces. No food source has been discovered within these webs, suggesting that the SFMS likely feeds on springtails (collembolans) in the moss.  In June, the SFMS female lays 7-9 eggs in a thin-walled, translucent egg sac. She remains with the egg sac until her spiderlings hatch out in September.

In 1995, the spruce-fir moss spider was federally listed as an endangered species due to widespread loss of the endangered Fraser fir (Abies fraseri) trees, which also only occur in the Southern Appalachian Mountains. This loss is a result of an infestation of the balsam woolly adelgid (Adelges piceae), a wingless insect whose toxic saliva kills Fraser firs. The balsam woolly adelgid was introduced into the United States from Europe around 1900 and is responsible for the loss of 80% of the Fraser firs across the tree’s range, greatly reducing SFMS habitat. When the forest canopy is thinned, the moss mats become exposed to sunlight and dry out. This reduces the amount of habitat available to the SFMS, which requires high and constant humidity within healthy, well-shaded moss.

Blue Ridge Discovery Center is proud to feature the rare and endangered spruce-fir moss spider in our 2019 Species Shirt Collection. These limited-edition shirts will be released at the 45th Annual Spring Mount Rogers Naturalist Rally May 10th-12th. Our 2019 collection also includes three other unique, high-elevation species: Gray’s lily, the northern pygmy salamander, and the northern saw-whet owl.