The Significance of Spider Size to the Diversification of Spider-Web Architectures and Spider Reproductive Modes

Abstract

The ability of spiders to withstand long periods of food deprivation and to respond rapidly through reproduction when food is available may be the most important physiological factors influencing araneoid evolution. For spiders, foraging effectiveness influences size at sexual maturation, rate of maturation, and reproductive output. I discuss the correlation between size changes, the diversification of web architecture (foraging tactics), and reproductive mode in araneoid spiders. I explore some of the constraints and potential for evolutionary diversification of web architectures and web silks. The data collected up to now suggest that scaling webs and web fibers is the simplest (but not the only) means of evolutionary differentiation among the kinetic properties of webs and, hence, among the classes of prey intercepted by orb webs. Although orb webs are built by spiders of all sizes, webs of other-than-orb design are built, in general, only by small spiders. Changes in web design allow derived araneoids to make use of new habitats not available to their ancestors. Spider reproductive output is a function of the intake of prey biomass. The evolution of alternative foraging tactics among spiders is correlated with the evolution of alternative reproductive modes. The present data suggest that small spiders reach reproductive maturity after fewer developmental stages than do large spiders, producing several egg sacs per spider. Large araneoids, in contrast, take longer to mature; after maturation, they fatten before they reproduce. Larger spiders tend to produce few egg sacs at the end of their lifetime, but each sac contains many eggs. Available data on the time to sexual maturity suggest that the evolutionary diversification of modes of spider foraging and reproduction have evolved through progenesis and, hence, through the reorganization of developmental pathways. The goal of this work is to begin to identify the ecological and developmental factors that have contributed to the evolution of the diverse web architectures and reproductive tactics of the araneoid spiders. This work does not prove or disprove the mechanisms or patterns through which web-weaving spiders have evolved, but it provides a new conceptual framework in which to study the evolution of web spinners.