Molecular procedures
To extract DNA from the gut of each spider, the opisthosoma (abdomen) was first cleaned using 70% ethanol and water then removed using a sterile scalpel blade and placed in Qiagen cell lysis solution. The opisthosoma was then ground using two 3 mm steel beads on a Genogrinder (Spex SamplePrep, Matuchen, NJ, USA) for 2 min at 1,200 hz. DNA from ground samples was extracted using the Qiagen Puregene kit (Qiagen, Hilden, Germany) according to the manufacturer’s protocol. DNA was then amplified using primer sets for 28s, 18s, and 16s (Primers listed in S1 of Supplemental Information). 28s has been used for fungal identification (Xu 2016; Zhao et al. 2011) and was therefore appropriate for detecting both arthropod and fungi. Amplification was performed using the Qiagen multiplex PCR kit (Qiagen, Hilden, Germany) in 10 ul reactions with 1ul of template DNA, and 1ul 10uM primer dilutions for 35 PCR cycles at 46°C annealing temperature. A second round of PCR was performed to add 8bp indexing tails on 5‘ end and Illumina (Illumina San Diego, CA, USA) TruSeq adapters. PCR products were visualized using a 1.5% agarose gel. Products were pooled in approximately equal amounts based on band strength. 1X AmpureBeads were used to clean the pooled products. The cleaned library was then sequenced on an Illumina Miseq using V3 chemistry. Negative controls were included on the sequencing run.