A deadly male Sydney Funnel Web spider (Atrax robustus) trying to climb. The male's venom is 6 times more potent than the female's. These primitive spiders cannot actually climb smooth surfaces, but the inside corners of fish tanks normally have exposed silicone glue, which is rough enough for them to get a grip on, given the right circumstances in this case, he had nothing on the other side to steady his grip, so he didn't get very far off the ground.