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Family Araneidae - Orb Weavers

Orb Weaver - Araneus iviei - female Black & Yellow Argiope - Argiope aurantia - female trashline - Cyclosa turbinata Bolas spider - Mastophora cornigera - female Spiny - Micrathena gracilis Large red and yellow spider - Araneus marmoreus Immature Marbled Orbweaver? - Araniella displicata - female big spider - Argiope trifasciata
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Chelicerata (Chelicerates)
Class Arachnida (Arachnids)
Order Araneae (Spiders)
Infraorder Araneomorphae (True Spiders)
No Taxon (Entelegynes )
Family Araneidae (Orb Weavers)
Other Common Names
Garden Spiders, (or used to be referred to as Argiopidae(1), rather than Araneidae)
Explanation of Names
The common name comes from the web-spinning behavior of these spiders - they make the classic, round "orb" web that most people associate with spiders.
Numbers
There are approximately 3,500 species worldwide, with 180 occurring north of Mexico(1).

Acacesia - 1 species
Acanthepeira - 4 species
Aculepeira - 3 species
Allocyclosa - 1 species
Araneus - more than 50 species
Araniella - 2 species
Argiope - 5 species
Cercidia - 1 species
Colphepeira - 1 species (AL, AR, GA, LA, MI)


(list not finished)
Size
NOTE: When measuring the size of a spider, only the body length is measured (do not include the legs).
The smallest orb weavers are less than 1/4 inch (6 mm)
The larger orb weavers can grow over 1 inch (25 mm)
Identification
The Araneidae are ecribellate, entelegyne, three-clawed spiders, having eight eyes in two rows. The lateral eyes are usually adjacent and some distance from the medians; the four medians form a trapezoid. (2)

The Web:
Often the web is a very good indicator for this type of spider (hence the name of the spider: orb weaver). Note the structure of the webs below:


All orb weavers spin some sort of web consisting of concentric circles (smaller circles within larger circles) with "spokes" radially going from the center outwards toward the anchor points. Some genera of orb weavers (specifically Argiopes) also spin a thick zig-zag pattern through the center of their webs, called the stabilimentum; there are several theories as to what purpose the stabilimentum serves, some of which are covered on the guide page for Argiopes. Most orb weaver webs are vertical (perpendicular to the ground), but there are a handful that will spin a horizontal web (parallel to the ground). Some of the webs can be extremely large (over 3 feet in diameter).

The Spider:
The orb weaver spider family is one of the most varied (in size and appearance) of all the families of spiders.

Female: Most people recognize the female orb weaver spider shape: a large "golf ball"-like abdomen and a smaller head:
   


    The other common shape is a female Argiope (which is noticed more for its size, bright colors, and long, sprawling legs):
   

Male: Adult male orb weavers are smaller, and are not seen as often, as they generally do not spin webs, but wander in the search for potential mates. Note the differences in size:

    Note: Size is not reliable for determining the spider's gender, but males are smaller than their female counterparts.

    This image is of a male, and at the bottom of the photo is the abdomen of a female of the same species (Argiope aurantia):
   

    Some male orb weavers:
   

Identification of a Species:
For most orb weavers, the ability to classify a specimen to species level often requires a microscopic inspection of the genitalia. A genus level identification is considered a good ID for most of the orb weavers. However, there are a few species of orb weavers that can be identified just by appearance. Some of these are:
Almost all Argiope genus spiders.


Almost all Micrathena genus spiders.


Many of the Gastercantha genus spiders.


A handful of Araneus genus spiders.
A handful of Neoscona genus spiders.

Also see our collection of ventral images of araneids.
Range
Prolific throughout entire continental United States, Canada, Alaska, and Hawaii.
Habitat
Anywhere with abundant prey and structures to support the web - around light fixtures used at night, or tree branches, tall grasses, and bushes.
Season
Most noticeable in late summer in fall, when webs and adults reach their largest size.
Food
Food sources vary, but typically any small insects they catch in their webs. Some of the bigger orb weavers (Argiope genus) have been observed eating small frogs and humming birds (only if ensnared within the web).
Life Cycle
Most orb weavers appear in the spring, but are not noticed until summer to fall.

Male:
The adult males will wander in search of a mate, and as such, typically do not stay on a web for long, if they spin a web at all. Adult males are not observed as much as the females, since they are always on the move, looking for that "Mrs. Right". Typically after mating, the males will die.

Female:
The most commonly noticed orb weaver is female, since she sits on her web, feeding and eventually waiting for the males to find her. Toward the end of fall, the females will lay their last clutch of eggs, and then die at the first frost. The eggs can survive the winter (even withstanding freezing) due to the simplistic nature/chemistry of the eggs. The warmth of the spring will cause the eggs to mature, releasing a new generation of orb weavers.

Juveniles:
A single egg sac can contain upwards of several hundred eggs. Juvenile orb weavers will spin "perfect" orb webs, but as they mature, the webs will become more distinctive and adapted for that particular species. (1)
Remarks
Behavior:
Orb weavers are very docile, non-aggressive spiders that will flee at the first sign of a threat (typically they will run or drop off the web). They are not dangerous to people & pets, and are actually quite beneficial because they will catch and eat a lot of pest-type insects.

Eating:
Put a medium-sized insect in the web of a large orbweaving spider in the garden. You will see the spider bite the prey, wrap it in silk, wait for it to die, then begin to eat. As a first step in eating, the spider will literally vomit digestive fluid over the prey. Then the prey is chewed with the "jaws" (chelicerae), and the fluid is sucked back into the mouth together with some liquefied "meat" from the prey. The spider repeats this process as often as necessary to digest, and ingest, all but the inedible hard parts. What is discarded afterwards is a small ball of residue. (3)

Being Bitten:
Being bitten by an orb weaver is very uncommon, and typically the individual was "asking" to be bitten. Orb weavers will only bite if they feel threatened and trapped without a chance for escape (e.g. - like trying to pick them up). A bite is often compared to a bee sting, and for most people, is nothing serious. However, it is recommended to observe them in their environments (e.g. - on their web) and not to pick them up.

Daytime/Nighttime Activities:
Orb weavers are typically nocturnal. During the day, the spider will prefer to either sit motionless in the web or move off the web. If the spider moves off the web (but does not abandon it), she will be nearby in some cover (rolled up leaves, or on a branch) with a trap line nearby. If prey becomes ensnared in the web, the trap line will vibrate, indicating a possible meal. The spider will investigate; if it is "meal worthy", she will bite it to immobilize it, and wrap it with silk to either eat later, or to continue to subdue the meal while eating. If the trapped insect is not meal worthy, she will ignore it or eject it from the web.

At night, the orb weaver will become more active, working to repair any damage on the web, and sitting in the middle of the web. For some species, once morning starts to arrive, the spider will tear down the web and eat most of the silk (reabsorption of moisture plus consuming any dew that might have settled on the web). They will rebuild their web at dusk/night.

Famous Orb Weavers:
Some orb weavers are influential enough to make history. One such spider was the inspiration of E. B. White's "Charlotte's Web". Charlotte, who spun her webs in Maine, was an Araneus cavaticus, sometimes called a "Barn Spider". (NOTE: Not all barn spiders are A. cavaticus; there are several species of spiders that are commonly called barn spiders.)

Several species of Neoscona and Araneus that are considered "barn spiders" can only be identified by examination of the carapace groove. Neoscona have a longitudinal groove on the carapace (parallel with the long axis of the body), whereas Araneus have angular (transverse) grooves. However, an apparent problem is that in Araneus the groove may appear as little more than a dimple, making it tough to tell. Here is a diagram detailing the carapace grooves of the two genera.
See Also
Long-Jawed Orb Weavers (Tetragnathidae): Info, Images
Hackled Orb Weavers (Uloboridae): Info, Images
Print References
Almost all general books on spiders have information about orb weavers. One good, inexpensive book for the most common orb weavers (and other common spiders and arachnids) is Spiders and Their Kin(1). (Please see the "Works Cited" section below for more information about this reference.)

(Click here for a list of recommended resources on spiders and other arachnids.)
Internet References
www.emporia.edu - Checklist of Kansas Orbweaving Spiders (PDF) - Nice booklet with images.
"Spiders seduced into yielding secrets of web." (An interesting New York Times article from 1985 about the work of Dr. Peter N. Witt with orbweavers and their webs)

More to be added soon...
Works Cited
1.Spiders and Their Kin: A Golden Guide from St. Martin's Press
Herbert W. Levi, Lorna R. Levi, Nicholas Strekalovsky. 2001. St. Martin's Press.
2.Keys to the Genera of Araneid orbweavers of the Americas
H.W. Levi. 2002. The Journal of Arachnology 30:527–562.
3.The Spider Myths Site