by Kate Redmond
Bug o’the Week Lined Orbweaver Spider
When the BugLady spotted this small spider on its horizontal web (while she was officially censusing butterflies and dragonflies), she thought it might be one of the sheet-web spiders. Fortunately, she has a Spider Guy, and he set her straight (thanks as always, BugFan Mike).
Turns out that it’s a small orbweaver called the Lined orbweaver (much has been written in BOTW about some of the larger species in the orbweaver family Araneidae).
Orbweavers, famously, spin circular/orb-shaped trap webs (the silk that makes up the radii isn’t sticky but the silk in the spiral is). The spiders often hang from the web’s center during the day, or they hide in a nearby retreat. They monitor the vibrations of the web, and when an insect sticks and struggles, they’re all over it. Harmless prey is bitten, stunned, and wrapped for later consumption https://bugguide.net/node/view/1565567/bgimage, but prey that bites back or stings is wrapped and immobilized before the coup de grace is delivered. One source said that the orbweavers are the only spiders that chew their food.
Many orbweavers (but not this one) spin their webs at night and eat the day-old web, thereby recycling the proteins. They have eight eyes and poor vision, and they communicate via vibration and chemicals (pheromones). According to an article entitled “Orb-weaver spider uses web to capture sounds” on a Cornell University College of Arts and Sciences Animal Behavior website, “Orb weaver spiders are known to make large webs, creating a kind of acoustic antennae with a sound-sensitive surface area that is up to 10,000 times greater than the spider itself” (another article said that they “outsource” their hearing). They both can and will bite (not dangerously) if mishandled, so — don’t.
There are more than 3,100 species of orbweavers throughout the world.
THE STABILIMENTUM RABBIT HOLE
Some orbweavers, especially those that are active in the daytime, that spin webs in the open, and that leave the webs up for a few days (like spiders in the genus Argiope), take the time and energy to produce a heavy silk and to weave it into a non-sticky, thickened area in the web called a “stabilimentum” https://bugguide.net/node/view/1125407/bgimage. Why? Short answer – no one knows for sure, but stabilimenta might serve different purposes for the various species that deploy them. Nocturnal spiders and those with unobtrusive webs don’t make them.
Originally, scientists believed that these structures strengthened the web (hence the name), but the silk is only loosely attached, and the web fares just fine if the stabilimentum is removed.
In other hypotheses, the stabilimentum:
- provides camouflage for a spider that’s sitting in the middle of the web https://bugguide.net/node/view/1585298/bgimage;
- fools potential predators into thinking the spider is bigger than it is https://bugguide.net/node/view/1884530/bgimage;
- reflects UV light, like flowers do, and therefore attracts insect prey (but – the silk isn’t sticky, and an insect that flies into it won’t get stuck);
- makes the surrounding web less noticeable by comparison;
- attracts the male of the species when the female is receptive;
- is part of the spider’s thermoregulatory strategy;
- and/or, protects the web by making it more visible to birds that might blunder through it (though some spider predators have learned to search for stabilimenta).
Addenda: In experiments, some researchers have noted that webs with stabilimenta catch 30% fewer insects, presumably because they are more visible, but other equally reputable scientists say that webs with stabilimenta catch up to 41.6% more prey. Sated spiders seem more likely to make stabilimenta. Some species change the shape of the stabilimentum as they age.
The BugLady thinks it’s just grand that these things haven’t been figured out yet.
THE LINED ORBWEAVER (Mangora gibberosa) is the most common of the seven members of its genus that occur north of the Rio Grande (gibberosa is from a Latin word “gibber” meaning “hump-backed” and “osa” meaning “full of” or “extremely”). Another 180 or so genus members live in Central and South America. Lined orbweavers are found in open areas – gardens, grasslands, roadsides and woodland edges – in the US east of the Rockies and into Canada.
These are small spiders https://bugguide.net/node/view/712593/bgimage, with females measuring ¼” and less, and males much smaller. Like other orbweavers, they come in a range of colors, with some with more lines, and some with more spots – https://bugguide.net/node/view/823822/bgimage, https://bugguide.net/node/view/1059263/bgimage. Good pictures here: https://www.marylandbiodiversity.com/view/8531 and here’s a nice “face-to-face” https://bugguide.net/node/view/1423274/bgimage.
The webs they build in sheltered areas in grass or brush may be horizontal or slightly angled and are sizable webs (about 12” across) for such a small spider. They have a “bull’s-eye” stabilimentum that is sometimes open and sometimes more solid https://bugguide.net/node/view/1236100/bgimage and that is often occupied by its resident during the day.
The “Arachnids of North Carolina” website tells us that in October, it “builds web at dawn orienting its web perpendicular to the rising sun, to warm up in its web quicker.” And, adds the usaspiders.com website, “Such orientations to sunrise would maximize the surface area of the body exposed to insolation and allow the spiders to warm quickly during the coolest part of the day. A quick warmup in the morning may be advantageous to prey capture, particularly during the cooler months of the year.”
Female Lined orbweavers conceal their egg sacs by folding a leaf around them and webbing it shut. Although the eggs within the sac hatch in fall, the tiny spiderlings stay inside the sac through winter (absorbing yolk material in their abdomen) and emerge in summer.
The tiny Lined orbweaver, of course, is up against competition from other spiders as it tries to make a living. In a study published about 15 years ago, Richardson and Hanks looked at the division of potential prey among four species of orb-weaving spiders living in close proximity in a grassland.
As suspected, it was not a zero-sum game – the spiders survived by occupying different niches within their habitat. The researchers noted the spiders’ sizes, the web size and height, the density of the webs’ “mesh,” and the kind of plant that the web was attached to. They found that spider size (and therefore the size of prey they were able to subdue) allowed a variety of species to coexist.
Kate Redmond, The BugLady
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