The famous spider from Charlotte's Web is a barn orb-weaver spider, Araneus cavaticus. Orb weaving spiders produce the familiar flat, ornate, circular webs usually associated with spiders. Orbweavers come in many shapes and sizes, but the brightly colored garden orbweavers, Argiope spp., are the largest and best known.
Orbweavers are generally harmless and can be a nuisance when they build large webs in places inconvenient for humans. Despite their formidable appearance, orb weaver spiders are not considered dangerous.
One of the bright-hued spiders is the spiny orb weaver, Gasteracantha cancriformis. Although not as large as some of the other orb weavers, its combined color, shape, and distinctive web makes G. cancriformis a very recognizable spider.
Though there is a group of spiders classed in the Thomisidae family which are given the common name of crab spiders, biologists placed the Spiny Orb Weaver in the Family Arneidae (known by the common designation of Orb Weavers). Nevertheless, most gardeners in the local area know this spider as a "crab spider" due to its flattened body having spines sticking out from the abdomen.
The spines on the abdomen may have an anti-predator function but spiny orb weavers are small so it may be hard for a predator to see, much less attack, them. Other common names are crab-like orbweaver, spinybacked orbweaver and spiny orbweaver spider.
The species is found in many parts of the world and is seen along the southeast coast of the United States and in California. The spiny orb weaver spider lives on the edges of woodlands and shrubby gardens. They frequently live in nurseries as well as citrus groves.
The mating behavior of G. cancriformis seems somewhat formal. The male tentatively seeks out a female by visiting her web and uses a 4-tap rhythmical-pattern drumming on the silk to get her attention.
He then cautiously approaches her. The male hangs by single threads from the female’s web prior to mating, becoming strapped down with silk from the female. Once contact is made, the spiders vibrate the web and copulate. Mating may take 35 minutes or more and occur repeatedly. After mating, the male remains on the female's web.
The female produces an egg sac with 100 to 260 eggs while facing down near the center of her web. In order to protect and feed the young in their egg and larval stages, the female constructs an egg case. She deposits the sac on the underside a leaf for over-wintering (from October through January).
The egg case is constructed first from an oval egg sheet made of finely woven threads, which are firmly attached to the lower surface of a leaf. After the eggs are laid on this “silken sheet,” they are covered with a loose, spongy, tangled mass of yellow and white threads. Next, several strands of dark green silk are laid along the egg case.
The final cover is a net-like canopy made as the female moves along the mass, with several dozen coarse, rigid, dark- green and yellow threads. All parental care by spiny orb weavers occurs before the young hatch because after the female lays her egg mass, she dies.
Eggs take eleven to thirteen days to hatch. The spiderlings then take a few days to learn how to move correctly. If they are undisturbed, they will not disperse from the case for two to five weeks, when they reach maturity and acquire a dark coloration.
Once out of the case, spiderlings make tiny, inconspicuous orb webs or hang from single strands. By late summer and early fall, there are significant increases in both body and web size. Unfortunately, the spiny orb weaver does not live very long. Females die after producing an egg mass, and males die six days after implanting sperm in the female.
The spiny orb weaving spiders look like plant seeds or thorns hanging in their webs and are easily distinguished from other spiders. The male is smaller (1/16"-1/8") than the female (3/8") and seldom noticed.
Spiny orb weavers have a broad, hard abdomen that can be white, orange, or yellow with red markings. There are six pointy “spines” protruding from the edges. The carapace, legs, and venter are black.
The spiny orb weaver spins flat, round shaped webs in shrubs, trees, and in the corners of windows and similar outdoor areas of buildings. A new web is constructed each night to make sure that the structure is secure. Typically, adult females construct webs because male species hang from a single thread close by the nest of a female.
The web itself is constructed from a basic foundation, which consists of a single vertical strand. The foundation is connected with a second primary line or by a primary radius. After making this basic framework, the spider begins to construct a strong exterior radius, and continues to spin secondary non-viscid radii.
The larger webs have ten to thirty radii. There is a central disk where the spider rests. This is separated from the sticky (viscid) spirals by an open area with a catching area in the web. Conspicuous tufts of silk also occur on the web, primarily on the foundation lines.
The difference between foundation silk and tufted silk is visibly distinct. The true function of these tufts is unknown, but some studies suggest that the tufts serve as little flags to warn birds and prevent them from flying into and destroying the web. The spiny orb weaver’s web may be quite close to the ground. Females live solitarily on individual webs and up to three males may dangle on silk threads nearby.
The spiny orb weaver’s web captures flying, and sometimes crawling, pests such as beetles, moths, mosquitoes, whiteflies, and other small fly species. A female builds her web at an angle, where she rests on the central disk, face down, awaiting her prey. When a small insect flies into the web, she moves quickly to the quarry, determining its exact location, and size, and immobilizes it.
If the prey is smaller than the spider, she will carry it back to the central disk and eat it. If her victim is larger than she is, she will wrap the numbed creature on either side and either climb back up the web or swing down a drag line before climbing up to her resting area.
Sometimes several insects are caught at the same time. The spiny orb weaver must find and paralyze them all. If it is not necessary to relocate them elsewhere on her web, the spider may just feed on them where they are, then come back to them as she pleases. She feeds upon the liquefied insides of her meal, and the drained carcasses are then discarded from the web.
Spiny orb weavers are one of the many beneficial spiders we have, since it preys upon small pests that are present in crops and suburban areas. They help to control overpopulation of these insects. Spiny orb weavers are not dangerous and would easily be overlooked if not for their unique coloration.
Spiny orb weavers do not invade the indoors unless carried in while residing in a potted plant. Spiny orb weavers are not dangerous—they are beneficial animals. They should not be killed, if at all possible.
Beneficials in the Garden & Landscape is an Earth-KindTM program coordinated through Extension Horticulture at Texas A&M University. Earth-Kind uses research-proven techniques to provide maximum gardening and landscape enjoyment while preserving and protecting our environment.
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