No abstract

Spider silk possesses unique mechanical properties like large extensibility, high tensile strength, super-contractility, etc. Understanding these mechanical responses require characterization of the rheological properties of silk beyond the simple force-extension relations which are widely reported.Here we study the linear and non-linear viscoelastic properties of dragline silk obtained from social spiders Stegodyphus sarasinorum using a Micro-Extension Rheometer that we have developed.Unlike continuous extension data, our technique allows for the probing of the viscoelastic response by applying small perturbations about sequentially increasing steady-state strain values.In addition, we extend our analysis to obtain the characteristic stress relaxation times and the frequency responses of the viscous and elastic moduli. Using these methods, we show that in a small strain regime (0-4%) dragline silk of social spiders shows strain-softening response followed by strain-stiffening response at higher strains (> 4%). The stress relaxation time, on the other hand, increases monotonically with increasing strain for the entire range. We also show that silk stiffens while ageing within the typical lifetime of a web. Our results demand the inclusion of the kinetics of domain unfolding and refolding in the existing models to account for the relaxation time behaviour.

Understanding the evolution of eusociality in insects has been a long-standing and unsolved challenge in evolutionary biology. For decades, it has been suggested that haplodiploidy plays an important role in the origin of eusociality. However, some researchers have also suggested that eusociality is unrelated to haplodiploidy. Surprisingly, there have been no large-scale phylogenetic tests of this hypothesis (to our knowledge). Here, we test whether haplodiploidy might help explain the origins of eusociality across 874 hexapod families, using three different phylogenetic comparative methods. Two of the methods used support the idea that the evolution of eusociality is significantly associated with haplodiploidy, providing possibly the first phylogenetic support for this decades-old hypothesis across insects. However, some patterns were clearly discordant with this hypothesis, and one phylogenetic test was non-significant. Support for this hypothesis came largely from the repeated origins of eusociality within the haplodiploid hymenopterans (and within thrips). Experimental manipulations of the data show that the non-significant results are primarily explained by the origins of eusociality without haplodiploidy in some groups (i.e., aphids, termites). Overall, our results offer mixed phylogenetic support for the long-standing hypothesis that haplodiploidy helps drive the evolution of eusociality.