Ontogenetic Growth Patterns of Leg Segments in Three Theridiid Spiders

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Abstract
In adult spiders, leg lengths vary within and between species and is associated with functions in web construction, sensory activities, and prey capture. However, little is known about how leg proportions change during ontogeny. Here we investigate whether leg length relative to body size is maintained throughout development in three cobweb spiders ( Latrodectus geometricus , L. mactans , and Steatoda grossa ). Because spiderlings of these species build webs and capture prey from the first instar, we predicted proportional growth across instars. We also evaluate which model (linear, quadratic, or Gompertz) best describes femur and tibia growth, and estimated growth rates (first derivative) and accelerations (second derivative) for each sex and species. Legs were proportionally shorter in early than in later instars in L. geometricus and S. grossa , whereas proportions remained similar across development in L. mactans . Femur and tibia growth was best described by a quadratic model in both sexes of L. geometricus and L. mactans , and in males of S. grossa . In contrast, females of S. grossa followed a Gompertz trajectory; with high early growth rates that declined toward maturity. Despite these differences in trajectories, leg segments showed similar magnitudes of acceleration within species. Across the three theridiid species, females showed two distinct ontogenetic patterns–steady increases in growth rate (quadratic) or rapid early growth followed by deceleration (Gompertz)–whereas males consistently followed quadratic trajectories. Conserved acceleration among leg segments may reflect developmental canalization that preserves functional proportionality during growth.

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