A simple device to collect, store and study samples of two-dimensional spider webs

<jats:p>Silk production and spinning are the most prominent characteristics of spiders (Araneae). The high diversity of web architectures correlates both with preying behaviors and with the ultra-structure of the silk threads that compose the webs, which in turn have a corresponding diversity of silk gland spigot complements. The details of web construction, thread structure, and gland spigot morphology have proven extremely informative for phylogenetic hypotheses (e.g., Eberhard 1982, 1988; Eberhard &amp; Pereira 1993; Coddington 1989; Griswold et al. 2005; Hormiga et al. 1995; Lopardo &amp; Ramírez 2007; Platnick et al. 1991; Agnarsson &amp; Blackledge 2009; Eberhard 2010), and a fascinating system to study spider diversification and evolutionary trends (e.g., Opell 1996; Opell &amp; Bond 2000; Blackledge et al. 2009, 2012; Dimitrov et al. 2012). </jats:p>

section_content: ruler. The stripes are then adhered onto the microscope slide with the paper-covered sticky side facing up. One tip of the slide has space for an adhesive label, the opposite tip has space for manipulation or a further label, and the longer frame stripes are separated 0. 5 mm from the edges of the slide to prevent the sampler from sticking to the containers. For very small webs (e. g., those of symphytognathoids) or for sampling a minute area of a web, a smaller frame can be simply adjusted by producing a smaller squared frame, or adding further transversal stripes inside the sampling rectangle. Collection and storage. At the moment of collection, the exposed side of the frame is peeled from the protective paper, and the sampler is gently pushed on the web, which immediately sticks on the adhesive surface. Once the contact with the adhesive is complete, the portion of the web outside the frame can be removed with the fingertips. While the softer webs made of thin threads (e. g., those of symphytognathoids or synotaxids) can be sampled directly, stronger webs (e. g., those of araneids or austrochilids) can slide through the sticky surface at the moment of taking the sample. For these, pressing the web with the fingertips against the sharp sides of the glass slide helps cutting the threads without distorting the structure. Once the sample is taken, a voucher code and the relevant data are written in the sampler label, a similar label is added to the vouchered specimen, and more detailed notes are taken in the field book if necessary. The silk samplers are conveniently carried (e. g., in the pockets of a collecting vest) using small five-slide containers with a lateral lid (Fig. 2 ; http://www. emsdiasum. com item #71550). Because the silk samplers are thicker than a regular glass slide, only two of them will fit in each of these boxes. Once the samples are taken and properly labeled, they can be stored in regular slide boxes, leaving one empty slot between samplers (Fig. 4 ). We have not detected any alteration in 15 year-old samples of webs of austrochilid spiders. Viscid silk however may alter its structure by evaporation in storage, and in the vacuum chamber of a standard SEM, although that effect may not be very drastic (Fig. 14 ). 

Observation. While there is a wealth of information on web architecture, construction routines and morphology of spinnerets, the knowledge of the detailed composition of cables, junctions and web surfaces is still fragmentary (see Eberhard 2010). We provide here some images to illustrate that silk samples can be easily studied and provide informative data to fill in such gaps. Silk samples mounted as described above can be directly examined on the compound or stereo microscope (Figs. 5 6 7 ), or sputter-coated for SEM study. For SEM observation, we use a flexible copper stripe of the same width of a glass slide, but slightly longer, so that the tips are bent about 5 mm over each extremity to hold the silk sampler. We glued two aluminum stubs on the back of the copper stripe, fitting the sample holder of the SEM (Fig. 3 ). The axial lines and reserve warp of cribellate bands are inside the cribellate mass, and thus can be better observed on a compound or stereo microscope (Figs. 5-7; see also Griswold et al. 2005: figs. 120-125). SEM observations can be used to reveal structural details, such as the surface of cribellate fibrils (Figs. 9, 10) and the composition of cables and junctions.

section_name: Acknowledgments

section_content: Silk samplers and field work in Chile were supported by a Sigma-Xi Grant-in-Aid of Research to MJR. Field work in South Africa was supported by a Fessenden Research Fellowship from the American Museum of Natural History to MJR, in Australia by German Research Foundation ( DFG Mi 1255/5-1 ) and ANPCyT. The Corporación Nacional Forestal (Chile), KwaZulu-Natal Nature Conservation Service (South Africa ), and Department of Sustainability and Environment (Australia, Victoria) issued permits for collection of specimens. Cristian Grismado, Luis Compagnucci, Matjaž Kuntner and Peter Michalik helped with the collection of silk samples and the organization of field trips. Gustavo Hormiga and an anonymous reviewer provided useful suggestions for the manuscript. This work was supported by grants PICT 1393 and 1007 from ANPCyT to MJR.
section_name: Acknowledgments