SYNOPSIS. Sensory events presumably guide the orb-weaver in the sequence of placing radii. However, at a given stage of construction, several sectors of the web may be equivalent in the sense that they possess features capable of eliciting radius-building. Observation of the natural progress of construction cannot reveal these characteristics. Some delineation of the problem may be obtained by compelling the spider to identify equivalent sectors. This procedure was followed for Araneus diademntus by selectively destroying threads, in effect repeatedly forcing the spider into the same web-array. The “candidate” sectors—those regions which are placed and replaced by the spider—are sufficiently described by the size of their central angles in early stages of radius-building, but not in later stages. Angles of relatively large size are left open in the upper half of the web. Sensory guidance in construction of radii may depend upon response to a complex array of forces in the web.