The Monophyletic Origin of the Orb Web

TWO VENERABLE groups of spiders spin orb webs: the cnbellate Ulobondae, weavers of calamistrated sticky silk, and the ecribellate Araneidae, weavers of true viscid silk (Figs. 12.2, 12.1). "Cnbellate" spiders possess a cnbellum and a calamistrum. The cnbellum is a flat, complex spinning plate from which the cnbellate silk issues; the calamistrum is a comb of bristles on the fourth metatarsi used to comb the cnbellate silk from the cnbellum. "Ecnbellate" spiders lack these structures. Other aspects of the morphology of the two families of spiders also differ considerably, yet both groups spin very similar orb webs. When juxtaposed to the morphological disparity between the spiders themselves, the similanty in the form of cheir webs and in the manner in which they are constructed has inspired a classic evolutionary controversy•the single versus the dual origin of the orb web. Behavioral evidence has defined one group, "orb weavers," including some but not all cnbellate spiders, but morphological evidence has defined another, "Cnbellatae," including some but not all orb weavers. The problem can be viewed as two contingent dilemmas. First, are all orb webs homologous as orb webs, and/or all cnbella homologous as cnbella? Second, if the answer to both of these questions is yes, then which homologuc is primitive and which derived, or do they both define the same group? There are several possible answers to these two dilemmas, and all have been suggested at one time or another in the history of the controversy. An additional problem contributing to the controversy over the orb web is secondary loss or complete modification of features in only some species of a monophyletic group. Occasionally, the species retaining the features are split off as a separate taxon defined by the retained primitive features. That taxon is not then synonymous with any real evolutionary lineage, and is