Orb weaver’s: Natural Artists

With the weather beginning to cool off and leaves starting to change into striking colors of red and orange, many of you will likely take to the outdoors for an evening stroll. It is within this time of year that many orb weaver spiders reach full maturity and their brilliant webs can be seen on full display.

The orb weaver spider group is comprised of many species, thus making it difficult to distinguish them from other spider groups. In fact, the orb weaver spider family, Araneidae, is one of the most variable in size and appearance of all the spider families. Like all other spiders, orb weavers have a cephalothorax (a fused head and thorax), abdomen, 8 legs and fang-like mouthparts called chelicera. Many orb weavers are brightly colored, have hairy or spiny legs and a relatively large abdomen that overlaps the back edge of the cephalothorax. Abdomens vary between species and some orb weaver spiders have spiny, smooth, or irregularly shaped abdomens. Most nocturnal orb weavers are usually brown or gray in color, while diurnal species exhibit bright colors of yellow or orange along with black markings. However, the most observable appearance of orb weavers is that of the large characteristic webs they produce. In general, orb weavers construct organized, circular grid webs that are similar in shape to webs depicted in Halloween decorations. More specifically, orb weaver webs are made of radial strands of silk that resemble the spokes of a wheel with the spokes connected by numerous concentric circular silk strands. While most spiders have two claws on each foot, orb weavers possess an additional claw which helps them spin their complex webs. The web of the garden orb weaver spider is very large and can measure up to three feet in diameter and is engineered to capture flying insects.

Orb weavers are typically nocturnal spiders and many species will build or repair their webs at night. Some orb weaver spiders tear down and even consume much of the web’s silk as the morning begins to dawn. Silk is one of the trademarks of spiders and it is a protein produced in liquid form by special abdominal glands. The silk is extruded through a battery of spigots mounted on 3 pairs of spinnerets on the abdomen, a process that transforms the liquid into dry fibers. Spiders, as a group, can manipulate the thickness, strength, and elasticity of their silk to produce up to seven different types, each intended for a different purpose.

Small insects such as flies, moths, beetles, wasps and mosquitoes are examples of insects that make up the spider’s diet. Orb weavers tend to inhabit locations where there is abundant prey and structures that can support their web. Typical habitats include areas around night-lights, tree branches, tall grass, weeds, fences, walls and bushes. Since orb weavers are not hunters or wanderers, they will sit and wait for prey to get trapped. Should the spider move off the web, it will remain nearby and hidden in a protected site such as some rolled up leaves or on the branch of a plant. However, the spider remains aware of prey that becomes ensnared in the web by a trap line of silk that will alert the spider if something enters the web. If a prey insect is trapped in the web, the trap line vibrates notifying the spider to rush to the web. At this point the spider will bite and paralyze the prey, and then wrap it in silk for later consumption. Orb weavers are most often noticed by homeowners in the late summer and fall since the adult spiders have attained their largest size and have constructed large webs.

One species that is frequently encountered by homeowners is the yellow garden spider (Argiope aurantia). This orb weaver is fairly large, with females ranging in size from 3/4 to 1” in body length, which is three times larger than the males. The top side of the abdomen in females is black with symmetrical patches of bright yellow and the legs are reddish brown at the base and black towards the tips. The yellow garden spider spins webs in sunny areas where plants are present on which they can anchor their web, which is frequently in gardens around the home. One characteristic of the yellow garden spider web is that it includes a ribbon of silk laid down in a zigzag pattern that is called a stabilimentum. The true purpose of the stabilimentum is open for debate. It was originally thought to contribute to the structural integrity of the web. Another theory suggests that it is clearly perceived by birds and mammals, preventing them from crashing through and destroying the web. Some researchers have recently proposed that it may help camouflage the spider as it rests in the hub of its web. Others have suggested the reflective qualities of the stabilimentum, especially in the ultraviolet range, may attract prey into the web.

Despite their large size and fearsome appearance, orb weavers are not considered to be medically important. Orb weavers rarely bite and only do so when threatened and unable to escape. If bitten by an orb weaver, the bite and injected venom is comparable to that of a bee sting, with no long-term implications unless the bite victim happens to be hyper-allergic to the venom.

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