this jumping spider s main ocelli (center pair) acute. outer pair secondary eyes , there other pairs of secondary eyes on sides , top of head.
spiders have 4 pairs of eyes on top-front area of cephalothorax, arranged in patterns vary 1 family another. principal pair @ front of type called pigment-cup ocelli ( little eyes ), in arthropods capable of detecting direction light coming, using shadow cast walls of cup. however, in spiders these eyes capable of forming images. other pairs, called secondary eyes, thought derived compound eyes of ancestral chelicerates, no longer have separate facets typical of compound eyes. unlike principal eyes, in many spiders these secondary eyes detect light reflected reflective tapetum lucidum, , wolf spiders can spotted torch light reflected tapeta. on other hand, jumping spiders secondary eyes have no tapeta.
other differences between principal , secondary eyes latter have rhabdomeres point away incoming light, in vertebrates, while arrangement opposite in former. principal eyes ones eye muscles, allowing them move retina. having no muscles, secondary eyes immobile.
some jumping spiders visual acuity exceeds factor of ten of dragonflies, have far best vision among insects; in fact human eye 5 times sharper jumping spider s. achieve telephoto-like series of lenses, four-layer retina , ability swivel eyes , integrate images different stages in scan. downside scanning , integrating processes relatively slow.
there spiders reduced number of eyes, of these six-eyes numerous , missing pair of eyes on anterior median line, others species have four-eyes , two. cave dwelling species have no eyes, or possess vestigial eyes incapable of sight.
other senses
as other arthropods, spiders cuticles block out information outside world, except penetrated many sensors or connections sensors nervous system. in fact, spiders , other arthropods have modified cuticles elaborate arrays of sensors. various touch sensors, bristles called setae, respond different levels of force, strong contact weak air currents. chemical sensors provide equivalents of taste , smell, means of setae. pedipalps carry large number of such setae sensitive contact chemicals , air-borne smells, such female pheromones. spiders have in joints of limbs slit sensillae detect forces , vibrations. in web-building spiders, these mechanical , chemical sensors more important eyes, while eyes important spiders hunt actively.
like arthropods, spiders lack balance , acceleration sensors , rely on eyes tell them way up. arthropods proprioceptors, sensors report force exerted muscles , degree of bending in body , joints, understood. on other hand, little known other internal sensors spiders or other arthropods may have.
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