 A web spider 's web is an extension of the web spider 's senses , informing the spider of vibrations that signal the arrival of prey and predators . If the intruder is another web spider , these vibrations vary widely depending on the new web spider 's species , sex and experience . A Portia can pluck another spider 's web with a virtually unlimited range of signals , either to lure the prey out into the open or calming the prey by monotonously repeating the same signal while the Portia walks slowly close enough to bite it . : 340 – 341 Such tactics enable Portias to take web spiders from 10 % to 200 % of a Portia ′ s size , and Portias hunt in all types of webs . : 491 In contrast , other cursorial spiders generally have difficulty moving on webs , and web @-@ building spiders find it difficult to move in webs unlike those they build : sticky webs adhere to cursorial spiders and to web @-@ builders of non @-@ sticky webs ; builders of <unk> webs have difficulty with non @-@ <unk> webs , and vice versa . : 424 Where the web is sparse , a Portia will use " rotary probing " , in which it moves a free leg around until it meets a thread . : 433 – 434 When hunting in another spider 's web , a Portia ′ s slow , choppy movement and the flaps on its legs make it resemble leaf detritus caught in the web and blown in a breeze . : 514 P. labiata and some other Portias use breezes and other disturbances as " smokescreens " in which these predators can approach web spiders more quickly , and revert to a more cautious approach when the disturbance disappears . : 313 A few web spiders run far away when they sense the un @-@ rhythmical gait of a Portia entering the web – a reaction Wilcox and Jackson call " Portia panic " . : 418 
