Cross Orb Weaver Spider - Araneus diadematus
Family Araneidae - Orb-Weavers
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Live adult female orbweaver spider photographed at Wheaton, Illinois, USA Size: 18mm
Cross Orb Weaver Spider
This large female orb weaver was found in a wooden picnic shelter at Wheaton, Illinois. I was able to pull a picnic table over to where she was hiding in the rafters, under the eaves of the structure, and photograph while standing with one leg on the picnic table and one foot on the fence rail 4 feet away. It was tricky, to say the least. Plus I had to avoid damaging her orb web which was about 1 foot away from her hiding place. I'll admit I prodded her to get the side shots. Although the air temp. was 80 degrees, she was rather lethargic.

I visited her twice more over the space of 4 weeks. Last I saw her, Oct. 10th, she was still sitting in the same place, but no longer maintained an orb web. I assume her business was done, although I never was able to locate any of her mates and I never saw any egg cases.

Cross Orbweaver Spider
Spider Anatomy
There are over 3,000 species of orb weavers in 170 genera worldwide. They form the third largest family of spiders, behind only Salticidae and Linyphiidae [1]. They are of course known for their large (sometimes very large) circular webs, which the females build amongst weedy fields, in trees, and around many man-made wooden structures such as bridges and barns.

Male orb weavers are considerably smaller and less conspicuous than their sometimes gaudily decorated love interests, and they usually build smaller ancillary webs nearby - they do not as a rule approach the females for any reason other than doing the nasty.

Like all spiders, orb weavers have eight legs. Like most spiders, they have eight eyes, fang-like mouthparts called chelicerae and the little "feeler-thingies" known as pedipalps (or just palps for short) in front of their face. (See below)

Orb Weaver Spider Eyes
Prosoma, eyes and pedipalps detail
Pedipalps are the second pair of appendages (the first being the chelicerae, not visible here) of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustaceans and insects, although more recent studies suggest they are probably homologous with the crustacean second antennae.

Palps are appendages with six segments: the coxae, a single trochanter, the femur, a short patella, the tibia, and the tarsus. In spiders, the coxae frequently have extensions called maxillae or gnathobases, which function as mouth parts. In sexually mature male spiders, the final segment of the pedipalp, the tarsus, develops into a complicated structure that is used to transfer sperm to the female seminal receptacles during mating. The details of this structure vary considerably between different groups of spiders and are useful for identifying species [2].

Cross Orbweaver Spider
Spider is suspended on and has hold of  tensioned communications silk with her claws and spinnerets.
Orbweaver spiders often add stabilimenta, or heavy zigzagging portions, to their webs. Stabilimenta are conspicuous lines or spirals of silk, included by many diurnal spiders at the center of their otherwise cryptic webs. It has been shown spider webs using stabilimenta catch, on average, 34% fewer insects than those without. However, webs with the easily-visible markings are damaged far less frequently by birds flying through the web. It is an evolutionary tradeoff the spider can influence every time it builds a new web. The inclusion of stabilimenta is influenced by many factors, including prey density and web location.  Read the scientific study at Behavioral Ecology magazine.

In any event, stabilimenta or no, a large orb weaver female planted firmly head-down in her web amongst tall weeds and grasses remains maddeningly invisible to man and beast. The cryptic markings of the fat abdomen and striped legs function as camouflage much like the tigers' stripes do in the jungle - the geometric elements serve to break up the outline of the spiders' body and confuse the eye of the beholder into not recognizing the image. It is this principle upon which warships' hull camouflage was painted during the World Wars, with stark diagonal lines and shapes intended to keep an enemy from discerning the outline and identifying the size of the ship and the extent of its armaments [3].

References
  1. Wikipedia, the free encyclopedia, "Orb Weaver Spider"
  2. Wikipedia, "Pedipalp"
  3. Thomas Eisner, Maria Eisner, and Melody Siegler, "Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures" (Belknap Press, 2005)
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Class Arachnida / Order Araneae: Spiders are the largest group of arachnids.  They are easily recognized by their eight legs, and there are few creatures great or small that elicit such irrational fear in mankind. The vast majority of spiders are completely harmless and offer beneficial services, chief of which is keeping the burgeoning insect population in check. I am continually amazed at the resourcefulness of these supremely successful predators.
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