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Orbiculariae
Orbiculariae is a possible clade of araneomorph spiders, uniting two groups that make orb webs. Phylogenetic analyses based on morphological characters have generally recovered this clade; analyses based on DNA have sometimes concluded that the group is not monophyletic. , the evidence is not sufficient to decide between these competing hypotheses. The issue relates to the origin of orb webs: whether they evolved early in the evolutionary history of entelegyne spiders, with many groups subsequently losing the ability to make orb webs, or whether they evolved later, with fewer groups having lost this ability.
==History==
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.)
Although cribellate and non-cribellate orb weavers had earlier been placed in the same taxon (from at least 1789), after the possession of a cribellum was prioritized over the form of the web, the two kinds of orb weaver were placed in separate taxa. Following John Blackwall in 1841 and Philipp Bertkau in 1878, for a long time the majority of araneologists accepted spiders with a cribellum as a coherent taxon, Cribellatae. Many also held that cribellate and ecribellate spiders had separately evolved orb webs from other kinds of web. By the early 1970s, it had become apparent that cribellate spiders were a paraphyletic group, the cribellum being an ancient feature of araneomorph spiders that had been lost in many descendants, so that grouping spiders together based on the retention of this feature did not produce a monophyletic taxon.
This still leaves open questions relating to the origin of the orb web. Did it evolve only once, with araneids later losing the cribellum (and hence cribellate silk), or did it evolve separately in uloborids and araneids? If it evolved only once, how early did this happen in the evolutionary history of entelegyne spiders? If late, then uloborids and araneids may form a monophyletic group, Orbiculariae. If very early, then uloborids and ananeids may not be closely related, and many more araneoids that do not spin webs would have lost this ability secondarily.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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