Lined Orbweaver, Mangora gibberosa
Family: Araneidae

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.

Citations:

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.

As I mentioned a few posts back, I recently spent a week in the Catskills enjoying a wonderful river valley.  I’ll be sharing photos and nature tidbits from that trip in a series of posts.

The first really fun observation I would like to share is the experience of watching female Common Mergansers shepherding their large broods of chicks while the youngsters learned to fish.  During our stay, the several local merganser broods would travel up or down the river, passing in front of our cabin only when we were inside, or around the back not paying attention!  One female had a group of 14 chicks, while another nearby female herded only 6 or 7.  (The largest known clutch is 19).

This quiet stretch of river was a favored spot for Common Merganser fishing lessons. The cabin is visible on the far side of the river among the trees. The rock in the foreground is now called Eagle Rock, since a Bald Eagle used it as a staging area for its bath on our list visit. Photo (c) 2009 Matthew Sarver

At first, I was disappointed that I did not have the opportunity to photograph the merganser broods (a task that surely would have required me to build a blind by the river’s edge).  Later, though, I discovered that George Miksch Sutton had written this wonderful description of his encounters with merg broods on his trip down the Abitibi River with W.E. Clyde Todd in 1923:

The Abitibi was far more than a series of rapids, cataracts, and portage trails.  There were beautiful quiet stretches down which, aided by the strong current, we must have made up to five knots or more.  A notable feature of the birdlife were broods of half-grown common mergansers, each brood with its mother, all of them, mother and young alike, flightless, she in her late summer molt, they with their first major wing feathers only partly developed.  The close-knit companies dived when they saw the distant canoe approaching; when hard-pressed, however, they did not dive but rushed off half-standing, churning the water furiously, bodies weaving from side to side, wings not flailing the air but folded in tightly, thus protecting the stubby blood quills.  The noise of their sudden departure was startling, for it contrasted so sharply with the silence of the sequestered spots the birds so obviously enjoyed.  One fact about this truly common species, which the guides called the sawbill, impressed me greatly: not a single adult drake did we see during our descent of that mighty wild river!

-G.M. Sutton 1980. Bird Student: an Autobiography (p 125)

The Abitibi River at Iroquois Falls in northeastern Ontario. Photo by P199 on Wikimedia Commons. Used under a Creative Commons License (Attribution Share-Alike 3.0 Unported).

Common Mergansers nest in tree cavities, often using old Pileated Woodpecker excavations, and also sometimes in crevices among rocks.  The chicks leave the nest and follow their mother to the water.  Since young chicks can’t fly until they are more than 2 months old, it’s a long summer for merganser moms.

Females frequently brood parasitize other females of the same species by laying their eggs into the nest of the other female.  Another common, but poorly-studied phenomenon is the joining of several partly-grown broods under the care of a single female, knows as brood amalgamation.  While this happens frequently, it is unknown whether more aggressive females tend to “take over” other broods, or if the adoption of these chicks is simply the result of confusion on the part of the youngsters.

Because of these complications, it is impossible to know exactly how many chicks are actually the offspring of their apparent mother.  None of this is the male’s problem, however, since as Sutton observed, they are nowhere to be seen during this period.  The pair bond apparently ends at incubation, but where do the males spend their time during mid-summer?  Do they hang out in trees, enjoying the cool mountain shade?

According to A.C. Bent’s Life Histories:

The drakes desert the ducks and usually disappear from the breeding grounds entirely as soon as the eggs are laid, leaving the females to perform the duties of incubation and care for the young alone. In Newfoundland we saw only females on the lakes, where they were busy with family cares, but we saw plenty of males on the swift water rivers, playing in the rapids and fishing in the pools. Several observers in Maine have said that the males are not seen during the summer, but this may be due to the fact that the males are in eclipse plumage at this time and are very shy and retiring.

Can anyone out there shed some more light on the mystery of the missing merg males?