Red Velvet mites, on forearm hair to show size. Water mites are arachnids, cousins to spiders, thus they have 8 legs. Their abdomen is unsegemented and broadly joind to the cephalothorax. They can grow up to 6 mm and are common inhabitants of ponds and streams. Not known as a vector for disease in humans, they are parasitic, as they pierce their prey and such out the juices. They are so small they absorb oxygen through their "skin. Trachae tubes carry oxygen around their body once it has been absorbed.