What Kind Of Spider Looks Like A Crab?

What Kind Of Spider Looks Like A Crab?

One of the more colorful spiders in Florida is the spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus) 1767. Although not as large as some of the other common orb weavers (e.g., Argiope, Levi 1968; Neoscona, Edwards 1984), the combination of color, shape, and web characteristics make Gasteracantha cancriformis one of the most conspicuous of spiders. The colloquial name for this spider in parts of Florida is crab spider, although it is not related to any of the families of spiders commonly called crab spiders, e.g., Thomisidoe.

Figure 1. The spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus), in its web. Photograph by Andrei Sourakov, Florida Museum of Natural History.

Systematics

Because of the variations in color and shape of the abdominal «spines» throughout its range, Gasteracantha cancriformis has been described by numerous early scientists under a plethora of names (Levi 1978). Although Kaston (1978) continued the use of the name Gasteracantha elipsoides (Walckenaer) 1841, resurrected by Chamberlin and Ivie (1944), Levi (1978) examined this species and found it to be a synonym of Gasteracantha cancriformis.

Distribution

This species belongs to a pantropical genus which contains many species in the Old World. With the possible exception of the West Indian Gasteracantha tetracantha (L.) (which may be only a geographic race), Gasteracantha cancriformis is the only species of its genus to occur in the New World, ranging from the southern United States to northern Argentina (Levi 1978).

Identification

This species can be easily distinguished from all other spiders in Florida. Females may be 5 to nearly 9 mm in length, but 10 to 13 mm wide. They have six pointed abdominal projections frequently referred to as «spines.» The carapace, legs, and venter are black, with some white spots on the underside of the abdomen. The dorsum of the abdomen is, typically for Florida specimens, white with black spots and red spines. Specimens from other areas may have the abdominal dorsum yellow instead of white, may have black spines instead of red, or may be almost entirely black dorsally and ventrally. Males are much smaller than females, 2 to 3 mm long, and slightly longer than wide. Color is similar to the female, except the abdomen is gray with white spots. The large abdominal spines are lacking, although there are four or five posterior small humps (Levi 1978, Muma 1971).

Figure 2. Female spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus). Photograph by University of Florida.

Biology (Back to Top)

Muma (1971) discussed the life cycle and web construction of Gasteracantha cancriformis in Florida. Although males have been found in every month except December and January (Levi 1978), they are most common in October and November. Females, which are found as adults throughout the year, are most common from October through January. Mixed-mesophytic woodlands and citrus groves are where they are most frequently found. Males hang by single threads from the females’ webs prior to mating, described by Muma (1971).

Ovate egg sacs, 20 to 25 mm long by 10 to 15 mm wide, are deposited on the undersides of leaves adjacent to the female’s web from October through January. The egg mass consists of 101 to 256 eggs, with a mean of 169 (based on 15 egg masses). After the eggs are laid on a white silken sheet, they are first covered with a loose, tangled mass of fine white or yellowish silk, then several strands of dark green silk are laid along the longitudinal axis of the egg mass, followed by a net-like canopy of coarse green and yellow threads. Eggs are frequently attacked by specialized predators, primarily Phalacrotophora epeirae (Brues) (Diptera: Phoridae), and occasionally Arachnophago ferruginea Gahan (Hymenoptera: Eupelmidae) (Muma and Stone 1971). Eggs take 11 to 13 days to hatch, then spend two to three days in a pink and white deutova stage before molting to the first instar.

Figure 3. Egg sac of the spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus). Photograph by Lyle J. Buss, University of Florida.

After another five to seven days, the spiderlings acquire dark coloration. Spiderlings dispersed within a week later in disturbed laboratory colonies, but remained in the eggsacs an additional two to five weeks in the field. Spiderlings make tiny, inconspicious orb webs or hang from single strands. In the late summer and early fall, significant increases occur in both body and web size. The larger webs have 10 to 30 radii. The central disk where the spider rests is separated from the sticky (viscid) spirals by an open area 4 to 8 cm wide. There may be as many as 30 loops of the viscid spiral, spaced at 2 to 4 mm intervals. The catching area of the web may be 30 to 60 cm in diameter. Conspicuous tufts of silk occur on the web, primarily on the foundation lines. The function of these tufts is unknown, but one hypothesis suggests that the tufts make the webs more conspicuous to birds (Eisner and Nowicki 1983), preventing the birds from flying into and destroying the webs. The webs may be less than 1 m to more than 6 m above ground. The spiders prey on whiteflies, flies, moths, and beetles that are caught in the webs.

See also:  What Is The Scientific Name Of A Spider?

Figure 4. The spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus), in its web. Photograph by Andrei Sourakov, Florida Museum of Natural History.

Survey and Detection

Citrus workers frequently encounter this species, and it may occur on trees and shrubs around houses and nurseries. Specimens may be easily collected in small vials, and are best preserved, as are all spiders, in 70 to 80% ethyl or isopropyl alcohol.

There are several types of spiders that are commonly called «Crab Spiders.» The most commonly encountered crab spiders found in Kentucky are in the family Thomisidae. These are often called «thomisid» crab spiders, or simply «thomsids.» Thomisids tend to have a flat shape compared to other spiders, and their front two pairs of legs are typically very long compared to the back two pairs. Thomisid crab spiders hold their front legs out and up, similar to the way a crab holds its claws. Many thomisid crab spiders have bright, «neon» colors (yellow, green, and orange). Others have gray and brown color patterns.

Crab spiders are considered beneficial to humans. Although they eat a few beneficial insects (like bees), they also kill flies, mosquitoes, moths, and other insect pests. Some species are large enough to bite people, but no crab spiders are known to be of medical significance.

FLOWER SPIDERS
FAMILY: Thomisidae | GENERA: Misumenops, Misumenoides, Misumena, others
Flower Spiders may be the most commonly encountered crab spiders in Kentucky. Flower spiders have bright colors which allow them to blend in with the flowers on which they wait for prey. Some flower spiders are able to change color to become camouflaged on different kinds of flowers, although the color change may take a few days. Pictured below is a flower spider from the Misumenops genus and three views of Misumenoides formosipes. These and related crab spiders are commonly found on Kentucky wildflowers and flowering plants (like milkweed and Queen Anne’s Lace). Thanks to Dr. Gary Dodson from Ball State University for help with flower spider identifications, along with biological information about these genera.

Xysticus and Coriarachne spp.
FAMILY: Thomisidae | GENUS: Xysticus and Coriarchne
Crab spiders in the Xysticus and Coriarachne genera, like the ones pictured below, resemble flower spiders except that they often have more subdued colors (like brown, black, and gray). This might be because crab spiders in these genera often hunt on the ground or on bark (instead of in flowers) where dark colors are best for camouflage.

Tmarus spp.
FAMILY: Thomisidae | GENUS: Tmarus
Crab spiders in the genus Tmarus are infrequently encountered in Kentucky and they don’t look very much like the other crab spiders in Thomisidae. Little is known about their biology. The ones pictured below were less than 1/2″ long.

RUNNING CRAB SPIDERS
FAMILY: Philodromidae
Running Crab Spiders belong to the family Philodromidae. They are not encountered as often as crab spiders in the family Thomisidae, but they are common nevertheless. Pictured below are running crab spiders from the genus Tibellus and Philodromus.

Although crab spiders are fairly common, finding one might take some patience because they are often well camouflaged, especially the ones that live on the ground. Crab spiders make great photography subjects, especially the brightly colored flower spiders. Flower spiders are very common in garden flowers and in wildflowers. If you want to find a flower spider, try to think like a flower spider: where would you sit if you wanted flies and bees to come to you?

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Scientists think that the venom of certain crab spiders is more potent than that of most other spiders: this allows crab spiders to quickly paralyze the large and tough bees that often visit flowers. However, crab spider venom is not known to be especially dangerous to humans.

The University of Nebraska Entomology website has some great pictures of crab spiders at: http://entomology.unl.edu/images/spiders/spiders3.htm

Do you know any myths, legends, or folklore about crab spiders? If so, let us know.

Crab Spiders

Have you been bitten by a crab spider or is one lurking around your living area? First off, don’t panic; crab spiders’ venom are not poisonous to you (unless you are a bee or unless you happen to be allergic). And if you don’t know if you are dealing with a crab spider (Thomisidae), check to see if they look like a crab in that their front legs are held up and out, their 4 – 10mm bodies are a bit flatter than other spiders, and, like a crab, they are walking sideways. Also check to see if they look a bit neon in color (yellow, green or orange) or sometimes white or brown. You probably should also stop looking for cobwebs; these crabby walkers tend to wait on foliage and on the grounds of gardens for prey.

Although you may not want to hear this, crab spiders are considered helpful for humans as they kill those pesky flies, mosquitoes, moths and other insects (like we do!), however not many people want crab spiders habituating with them. What these crab spiders really love, however, are flowers (hence their bright colors). Like a chameleon, the crab spider can change her color to camouflage itself into the flower she is waiting on to ambush an unsuspecting insect and to hide from other predators.

The crab spider will stay put in its hunting spot for days or even weeks, sometimes changing color with it’s surroundings. When it sees a living meal it can catch, it will drop a silk line to hunt, similar to a fisherman. Again, the crab spider can bite and will bite a human, but its venom is not harmful or poisonous.

If a crab spider is found in your house, it’s probably a mistake on its part for navigation. The crab spider most likely wants to be on a leaf, under a piece of bark, on a flower or in the crevasse of a tree trunk in your garden. If you’ve noticed a brown-speckled, flat, crab-legged spider hanging around and need to test it to see what kind it is, gently poke it with a long stick and watch to see if it widens those long front legs and moves sideways. If it does, that will be a crab spider.

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How Did I Get Spinybacked Orb Weaver Spiders?

Like many other spider species, these pests enter homes through door gaps and torn screens. Thick brush and tree limbs that touch windows or siding also help them move into houses. The small size of spinybacked orb weaver spiders keeps them hidden from view, especially in low light.

How Serious Are Spinybacked Orb Weaver Spiders?

Spinybacked orb weaver spiders are mostly harmless. Their large webs often startle and annoy people, but the pests pose no serious health risks. Infestations can become large, as their egg sacs contain over 200 offspring. Because these spiders are predacious on insects that often cause damage to crops and garden plants, they are actually very beneficial spiders.

How Can You Get Rid of Them?

Preventing a spinybacked orb weaver spider infestation begins with making sure the population its food source is kept to a minimum. Sealing holes, cracks and gaps in the home’s doors, windows and foundation may also help prevent entrance into the home’s living space.

Managing lighting to minimize attracting insects is a good practice to reduce prey insects that will attract spineybacked orb weaver spiders.

In addition, removing ground litter that serves as harborage for spiders is also helpful.

Your pest management professional can then use his or her inspection findings to prepare a comprehensive pest management plan that will effectively and efficiently deal with the specific pest problem.

See also:  What Does A Wolf Spider Bite Look Like?

Signs Of An Infestation

The appearance of the spiders, their webs and egg sacs are evidence of an infestation. Spinybacked orb weavers are considered a nuisance when they build large webs in places that are bothersome to homeowners.

Behavior, Diet & Habitat

Typical habitats for the spineybacked orb weaver spider are the edges of woodlands, shrubs and gardens. They also frequent plant nurseries and are commonly seen in citrus groves.

Where can you find their webs?

They frequently spin their webs in shrubs, trees, and the corners of windows and doors and on porches and patios. This spider normally does not enter homes, but can be unintentionally brought inside with potted plants and other outside items.

Spinyback Spider Web What do they eat?

The spineybacked orb weaver’s diet consists of small insects that are captured in its web. Insects such as beetles, moths, mosquitoes and whiteflies are likely prey and after biting and paralyzing their prey, they consume the liquefied insides of their “catch”, much like most other spiders. An interesting characteristic of its web is the presence of silk tufts the female spider builds into the web. Possibly, the purpose of these tufts of silk is to make the webs more conspicuous to birds, thus reducing the likelihood of them flying into and destroying the spider’s web.

Do they bite?

The spineybacked orb weaver can bite, but they are not aggressive spiders. They do not bite people unless picked up or otherwise provoked and are not known to cause serious symptoms if they do bite someone.

Life Cycle Reproduction

The Spineybacked orb weavers’ typical lifespan only lasts for about one year. In most of their distribution area, females can be found at any time of the year, but mostly from October to January, while males are most often found during October and November.

The female produces an egg sac in the summer or fall that contains from 100-260 eggs. The female spider attaches eggs under a leaf, wraps them in webbing and dies. Eggs take about eleven to thirteen days to hatch.

Distribution

Spineybacked orb weaver spiders are found across the world, with U.S distributions ranging from southern California to Florida.

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Facts, Identification, and Control

  • Characteristics: Crab spiders are named for their crablike appearance and movements.
  • Body: Crab spiders have two large, strong front legs that are used to grasp prey. They scuttle sideways with their hind legs, although some species do move like other spiders.

How Did I Get Crab Spiders?

These spiders don’t build webs, but they don’t go out to hunt either. Instead, they use camouflage to hide and wait for prey to come to them. This means they seek places where food is common. Gardens and landscaped areas often attract crab spiders because the pests can find insect prey in abundance. They get their name because of their appearance, which is crab-like and their ability to walk sideways like a crab.

How Serious Are Crab Spiders?

Because they eat pests like flies and mosquitoes, crab spiders are generally beneficial. They are venomous, but most crab spiders have mouthparts too small to pierce human skin. Even the giant crab spider, which is large enough to successfully bite people, typically causes only mild pain and no lasting side effects.

Signs of Infestation

Crab spiders excel at not being seen. When detected, it is the spider itself that would be the sign. Crab spiders typically do not infest indoors and would prefer to remain outside.

Behavior, Diet, & Habits

Instead of spinning webs to catch prey, crab spiders use camouflage. Some resemble bird droppings, while others look like fruits, leaves, grass, or flowers. Others are capable of changing colors entirely. When prey approaches, these spiders attack and administer poisonous bites. Crab spider venom is potent enough to render large insects immobile.

Reproduction

Crab spiders produce eggs within a few weeks after mating. These eggs are deposited into two silken egg sacs, which are joined at the center. However, their eggs are not housed within a web. Females commonly remain near egg sacs in order to protect their young from predators. Hatching time depends on environmental conditions. After spiderlings emerge, they resemble adults. These small spiders undergo a series of molts before becoming mature and fertile.

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