From Complete Orb to Semi-Orb Webs: Developmental Transitions in the Web of Nephilengys Cruentata (Araneae: Tetragnathidae)

Abstract

Marked developmental changes in web pattern provide an opportunity to assess the degree of interdependency between web building routines. The first webs of the tropical orb web spider Nephilengys cruentata are small, closely knit, complete orb webs, built in a great variety of sites. Older spiders built 'incomplete' orbs, which lack upper components (frame, radii, spirals), and in which a silken, cylindrical retreat, placed near a ceiling or a corner, opens into the hub. We studied the transition from the complete orb stage to the semi-orb stage, recording several web parameters and the stages of retreat formation. Two types of ontogenetic changes were identified: global increases in size (allometric growth) and gradual suppression of upper components (vertical asymmetry). The difference between upper and lower parts was evidenced in the curves which describe development: whereas growth in the lower part of the web usually followed linear trends, trends in the upper parts were quadratic, which means decreasing growth rates. Turning points (the temporal threshholds at which upper web parameters passed from increasing to decreasing trends) were not simultaneous: they clustered into two transitional periods. During the first transitional period, number of radii, number of viscid and dry spirals, percentage of orbs with an empty sector and web inclination had turning points; during the second transitional period, horizontal diameter, the length of radii and the percent occurrence of tent-like retreats. Our results (changes in upper and lower parts of the web governed by different ontogenetic rules; asynchrony of changes in the upper part; independence of retreat emergence from web modifications to which it is normally associated) reinforce a modular conception of the control of web building. This conception has implications for the understanding of the evolution of web patterns: aspects or parts of the building program could be independently subjected to selection, leading to specialized webs. The developmental uncoupling between algorithms opens, at the evolutionary level, the possibility of structural novelties in web building.