170 SPIDERS 



the abdomen. By some extraordinary instinct the vital organs are 

 not eaten until last so that the spider does not die until the insect is 

 ready to emerge from the now empty skin of its host. The Acro- 

 ceridae (Diptera) are a family of flies which have evolved similar 

 parasitic habits and members of several other fly families as well as 

 the Pimplinae and Cryptinae (Hymenoptera) are parasitic on the 

 eggs of spiders. Apparently cryptic coloration and other protective 

 devices are of little avail against these creatures, but even so, 

 Ichneumons and other parasitoids are probably a far less important 

 factor in controlling the numbers of spiders than they are in con- 

 trolling insect population. 



With so many terrible foes it is not surprising to find that spiders 

 have evolved all kinds of protective devices, including prickly 

 spines, unpleasant flavour and scent: some even eject unpleasant 

 fluids that deter predators. Scytodes thoracica squirts gum from its 

 fangs for defensive purposes as well as when it is hunting. Many 

 hunting spiders build silken protective cells in which they rest 

 when not in search of prey. Scotophaeus hlackwalli has evolved the 

 habit of retreating with abdomen raised, trailing behind it a ribbon 

 of silk that serves as a protection against attack from the rear. 

 Many spiders achieve concealment by means of cryptic coloration, 

 closely resembling their natural background, whether this is a 

 coloured flower, a leaf, hchen, sand or bark. The crab spiders pro- 

 vide numerous fine examples of such camouflage. Sometimes the 

 outline of the spider is camouflaged by means of a 'dazzle- 

 pattern', as in the case of Salticus scenicus whose irregular patches 

 of contrasted colours tend to draw the attention of the observer 

 from the shape that bears them. Egg cases too are frequently con- 

 cealed with twigs and pieces of leaf. Poisonous and powerful 

 species may show conspicuous colorations, while all kinds of 

 poisonous and distasteful insects such as ants, wasps, bugs, beetles 

 and even scorpions are eflFectively mimicked by various spiders 

 (Bristowe, 1941).* 



For example, amongst the Argiopidae some species of the genus 

 Cyclosa resemble small snails which, on account of the hardness of 

 their shells, would be eaten by few birds and would certainly 

 be neglected both by Pompilidae and Ichneumonidae, the princi- 

 pal enemies of spiders. One such species has been recorded from 



