|Black and Yellow Garden Spider - Argiope aurantia |
Family Araneidae - Orb weavers. Also commonly called black and yellow Argiope, writing spider, banana spider. Live adult male and female spiders photographed at Georgia and northern Illinois, USA.
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|This sexually mature female Argiope has reached her maximum size, about 15mm|
Female Argiope are the largest spiders most people in the U.S. ever encounter. The body can measure 40 mm (1-1/2"). With legs, 8 cm (3") diameter. These charming creatures can be found in gardens and weedy fields from early summer to mid-Autumn, but females become conspicuous in their large webs only toward the end of summer. They can be extremely numerous in small areas, making walking difficult without disturbing their webs.
Argiope females lay eggs in egg sacs attached to low foliage. These eggs hatch in early summer, and the spiderlings disperse. Spiders grow in increments - being encased as they are inside a hard and rigid exoskeleton, they cannot grow unless they shed the old armor in a process called molting, much the same as snakes and other animals.
The timing of the molt depends on many factors, including the ambient temperature, the amount of food and water available, even the length of the day. Only after the final molt does the spider become a sexually mature individual. 
Extreme closeup: spider's ventral abdomen and spinnerets
The much smaller Argiope males keep wide berth, remaining hidden in outlying portions of the web or in foliage nearby
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 them. 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 study at Behavioral Ecology magazine.
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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