I don’t know about you, but I love spiders. While here in South Carolina, I was lucky to find and photograph several common and attractive species. This is the first installment in a series of posts in which I’ll highlight these species for the enjoyment of all of you arachnophiles out there!
Lined Orbweaver, Mangora gibberosa
The Lined Orbweaver is a common spider that is widespread throughout the eastern United States. In the southeast, it is especially abundant in grassy pond pine (Pinus serotina) savannas like the one shown below.
Strung among the grasses throughout the savanna are small, white, silken rings that are conspicuously evident against the sea of yellowish-green vegetation. Upon closer examination, the white ring is at the center of a larger orb-web, the rest of which is invisible from a distance. The white part of the web is called the stabilimentum, a structure found in the webs of many species of orb-weavers (Araneidae).
The purpose of the stabilimentum has been debated among arachnologists for a long time. Hypotheses range from prey attraction to camoflauge for protection from predators to warning signals that keep megafauna from accidentally destroying webs. Phylogenetic work suggests that web-decoration with stabilimenta evolved several different times in many araneid lineages (Scharff & Coddington 1997).
My personal observations of the Lined Orb-weaver indicate that, in this species, one use of the stabilimentum is as a refuge from potential predators. When the web is approached closely or bumped, the spider quickly runs to the opposite side of the web from the intruder and hides behind the stabilimentum, which is sized perfectly to conceal the body of the spider.
How well this works, however, is open to debate. Bruce et al. (2005) tested the visibility of stabilmentum silk to birds and bees. They found that in one of their study species, the chromatic contrast between the spider and the silk was significant enough that the stabilmentum probably provided inefficient camoflauge from avian predators at close range. It’s also possible that, since they are visible to bees and other insects, the small, discoid stabilimenta of the Lined Orb-weaver may attract prey to the web by mimicking flowers.
Whatever their adaptive significance, the sight of dozens of these small, silken rings dotting the vegetation is a fantastic experience for the naturalist willing to look closely. The spider responsible for the web is pretty cool-looking as well!
For a nice review of the literature on this topic, check out Matt Bruce’s web site on web decorations.
Bruce, M.J., Heiling, A.M., Herberstein, M.E. 2005. Spider signals: are web decorations visible to birds and bees? Biology Letters 1: 299-302.
Scharff N, Coddington J.A. 1997. A phylogenetic analysis of the orb-weaving spider family Araneidae (Arachnida, Araneae). Zool. J. Linn. Soc. 120: 355–424.