I shot a fox last night that was silly enough to stop at 20m, after having walked out from under the high seat I was in and had scented me.
The shot was heart and lung (.25-06 so more than enough gun ), but it still bolted so I ended up tracking it by torch light through all sorts of cover.
I have noticed it before when tracking blood at night, but it is amazing how quickly spiders and other bugs and critters come out to start feeding on the blood. A few times when I thought I had lost the trail I found it again by looking for spiders (in the main ones with small bodies and long legs) and seeing if they were feeding on blood.
I was tempted to put this in the "Tips" section, but wondered if anyone else looks for spiders other pointers to help them find blood or a trail?