whether spiders that weave true orb webs form a coherent group, and so should be classified together, is a question that has a long history. two groups of spiders that spin orb webs are the uloboridae and the araneidae. although their webs have a very similar overall architecture, the sticky catching threads are created in different ways. uloborid spiders have a cribellum – a flat plate from which a particular kind of silk emerges – and a calamistrum – a structure formed of bristles on the metatarsus of the fourth leg, used to "comb" the cribellate silk into extremely fine strands that are adhesive without having any "glue" present. araneid spiders have silk-producing organs that add fine droplets of a glue-like substance to silk of normal thickness to create viscid silk. (spiders that do not spin orb webs can also be divided into those that produce cribellate silk and those that produce viscid silk.)