Tensile metrics of black widow attachment disc adhesion in wet and dry conditions

Spiders use piriform silk attachment discs to adhere threads during web construction and to secure safety lines. Water could degrade attachment disc adhesion either by interfering with placement of the discs or later reducing adhesion during loading. We tested the effect of water on the adhesion of attachment discs for the spider Latrodectus hesperus, which spins webs in mostly dry environments. We compared adhesion for discs spun on wet versus dry glass that were subsequently loaded in either wet or dry conditions. Attachment discs placed on wet glass showed similar adhesion to discs placed on dry glass. However, water significantly decreased both peak force of adhesion and work of adhesion when loading occurred under wet conditions, regardless of initial placement conditions. Furthermore, failure mode shifted from rupture of draglines in dry loading conditions to adhesive failure of discs in wet loading conditions. Our results show the importance of considering both the conditions in which biological structures are produced and those in which the structures perform as potentially independent factors for performance. Our results also suggest that adhesion in wet conditions can challenge some spiders, potentially leading to specialization of attachment discs for riparian or aquatic species.

Methods
Attachment discs are observed prior to testing under a compound microscope (Leica DMLB 2 Clinical Microscope, Leica microsystems) at 10x magnification, and images are taken using an Olympus Q-Color5 imaging system (Olympus Confocal) for area and morphology analysis. The sample is then placed in a Nano Bionix tensile tester (MTS System Corp., Oak Ridge, TN, USA) with the upper grip holding the slide and the lower grip holding the cardboard cutout where the dragline is secured. The cardboard “C” is cut near its midpoint, separating the dragline end from the attachment disc end, and the sample adjusted in the X-Y plane to ensure that the dragline will pull the attachment disc perpendicular to the slide surface, to control for the effect of differing pulling angles (Sahni et al., 2012). The dragline was then pulled at 0.1mm/sec until complete failure is observed (Blackledge et al., 2005)(Swanson et al., 2006). The force, time, and displacement are recorded, and work is calculated. The sample is then placed back in the compound microscope where images are taken of the attachment disc again to determine the failure type (cohesive, adhesive, or dragline).

Tests are conducted under two different conditions (dry and wet) for a total of four treatments (placement condition - loading condition): Dry-Dry, Wet-Dry, Dry-Wet, and Wet-Wet. Dry loading condition tests are conducted at room humidity (40-60%). For wet-placed dry-loading conditions (Wet-Dry), excess surface water is removed carefully using Kimtech wipes after the disc is placed. The samples are then allowed to fully dry in a desiccator for 5 minutes before testing. For the wet loading condition tests (Dry-Wet and Wet-Wet), samples are continuously sprayed with DI water from the atomizer until mounted and tested 5 minutes later. Water droplets visibly coated the attachment discs during wet loading. For the Dry-Wet condition, the spider places an attachment disc on a dry slide which is then immediately (within ~5-10 seconds) wetted using the DI water atomizer for 5 min before testing. For the Wet-Wet condition, the slide is wet while the attachment disc is placed, and the disc remains saturated with water from then on, including during loading. All samples are allowed to dry after tensile testing, before microscope imaging and failure type analysis.