Evolution beyond the orb web: the web of the araneid spider Pasilobus sp., its structure, operation and construction

The araneid spider Pasilobus sp., common in the Morobe District, New Guinea, builds its web at night close to bushes and small trees. The more-or-less horizontal web has a triangular frame that is divided into halves by a midline thread running from the apical angle to bisect the base. From the midline thread hang 4–11 pairs of widely spaced spanning threads; these are the only adhesive elements in the web. The spanning threads are viscid for only part of their length and are strongly attached to the web only at their junction with the midline thread. The outer end of each spanning thread forms an easily ruptured, low-shear joint with the lateral frame thread of the web. When a flying insect strikes a spanning thread, the low-shear joint breaks and the thread drops below the web, leaving the insect tethered to the midline. The insect may continue to fly, on the tether, or may spin down to motionlessness. The spider rushes to the midline thread end of the tether, hauls up the spanning thread and then bites the insect. Experimental investigations of the low-shear joints and the adhesiveness and elasticity of the spanning thread are described and the results analysed. The web-building behaviour of Palilobus differs in several ways from that of most araneids and is described and compared with that of Gasteracantha and other species. The possible evolutionary origins of the Pasilobus web are outlined.