Centipede bites can be extremely painful, they are not generally fatal to humans.

 a few scorpion species have a venomous sting that can be lethal to humans.Scorpion venom is a neurotoxin, a chemical that affects the nervous system, ultimately killing or paralyzing their prey.

The mechanical injury from a spider bite is not a serious concern for humans. However, it is generally the toxicity of spider venom that poses the most risk to human beings; severalspiders are known to have venom that can cause injury to humans in the amounts that aspider could inject when biting.

Not all crabs are safe to eat, however, and a few can carry lethal doses of toxins. ... So far, only a small number of species of this family are known to be toxic, but it is a good idea to avoid eating them regardless. These crabs are not truly poisonous and the crabs do not produce the toxins themselves.