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Spiders were machines built into the fake jewels on Tychus's vest. Once activated, the spiders sprouted legs and begin to wander the floor. They were programmed to go toward any heat source between 96 and 102 degrees Fahrenheit. Within them was a dose of lethal toxin, which it would inject at any movement more than normal breathing or facial expressions. They had a timed life, and after activation would self destruct after half an hour, leaving nothing to trace.
During the Covington Bank operation, Ash Thompson activated the spiders after an argument with Raynor and Tychus, killing the staff of the bank. This caused Raynor and Findlay walk out of the robbery, and led to a firefight that killed Thompson and his associates.