Friday, January 23, 2009

覺 Sometimes I need more Legs and more Eyes.., Somewhere in the Mountain in Korea 2004

(*wikipedia for spider)
Most spiders have four pairs of eyes on the top-front area of the cephalothorax, arranged in patterns that vary from one family to another. The pair at the front are of the type called pigment-cup ocelli ("little eyes"), which in most arthropods are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However the main eyes at the front of spiders' heads are pigment-cup ocelli that are capable of forming images. The other eyes are thought to be derived from the compound eyes of the ancestral chelicerates, but no longer have the separate facets typical of compound eyes. Unlike the main eyes, in many spiders these secondary eyes detect light reflected from a reflective tapetum, and wolf spiders can be spotted by torch light reflected from the tapeta. On the other hand jumping spiders' secondary eyes have no tapeta. Jumping spiders' visual acuity exceeds by a factor of ten that of dragonflies, which have by far the best vision among insects; in fact the human eye is only about fives times sharper than a jumping spider's. They achieve this by a telephoto-like series of lenses, a four-layer retina and the ability to swivel their eyes and integrate images from different stages in the scan. The downside is that the scanning and integrating processes are relatively slow.
As with other arthropods, spider's cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly bristles called setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. Spider also have in the joints of their limbs slit sensors that detect vibration. In web-building spiders all these mechanical and chemical sensors are more important than the eyes, while the eyes are most important to spiders that hunt actively.
Like most arthropods, spiders lack balance and acceleration sensors and rely on their eyes to tell them which way is up. Arthropods' proprioceptors, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. On the other hand little is known about what other internal sensors spiders or other arthropods may have.

2 comments:

Janice Thomson said...

Well that is a very colorful fella - is he poisonous? Spider webs are one of nature's most beautiful items - I'm always amazed at the work that goes into making them. They glisten so beautifully in sunlight.

Peter.Chris said...

Most spiders in Korea have no poison. Those yellow/green striped ones are most popular spiders in the forest in Korea. Especially in the morning we see the beautiful dews dangling on the web..