The Evolution of Dragline Initiation in Spiders: Multiple Transitions from Multi- to Single-Gland Usage

Jonas O Wolff

doi:10.3390/d12010004

Abstract

Despite the recognition of spider silk as a biological super-material and its dominant role in various aspects of a spider’s life, knowledge on silk use and silk properties is incomplete. This is a major impediment for the general understanding of spider ecology, spider silk evolution and biomaterial prospecting. In particular, the biological role of different types of silk glands is largely unexplored. Here, I report the results from a comparative study of spinneret usage during silk anchor and dragline spinning. I found that the use of both anterior lateral spinnerets (ALS) and posterior median spinnerets (PMS) is the plesiomorphic state of silk anchor and dragline spinning in the Araneomorphae, with transitions to ALS-only use in the Araneoidea and some smaller lineages scattered across the spider tree of life. Opposing the reduction to using a single spinneret pair, few taxa have switched to using all ALS, PMS and the posterior lateral spinnerets (PLS) for silk anchor and dragline formation. Silk fibres from the used spinnerets (major ampullate, minor ampullate and aciniform silk) were generally bundled in draglines after the completion of silk anchor spinning. Araneoid spiders were highly distinct from most other spiders in their draglines, being composed of major ampullate silk only. This indicates that major ampullate silk properties reported from comparative measurements of draglines should be handled with care. These observations call for a closer investigation of the function of different silk glands in spiders.

Highlights

Spider silk has been praised as a biological super-material combining extreme strength and extensibility at a low density, biodegradability and biocompatibility [1]
The gland’s ducts lead to nozzle-like openings, the spigots, that are scattered across usually three pairs of moveable spinnerets: the anterior lateral spinnerets (ALS), the posterior median spinnerets (PMS) and the posterior lateral spinnerets (PLS)
The combination of different silk materials into composite silk products is a topic that should not be neglected in the biomechanical study of spider webs, if we want to achieve a better understanding of silk and web function

Summary

Introduction

Spider silk has been praised as a biological super-material combining extreme strength and extensibility at a low density, biodegradability and biocompatibility [1]. The increased interest in spider silk has led to a boost in spider research; most silk studies focus on the pure material properties, macro-molecular synthesis and assembly in a few model species, with the biology and variation of spider silk being largely neglected. This dismisses the potential of broader comparative studies both for bioprospecting and a comprehensive understanding of the ecology and evolutionary history of spiders. Spiders possess a number of different types of silk glands that produce fibres with different physical properties and different biological functions [5,6]. Some spiders possess an anterior spinning plate, the cribellum

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