Copepods feed upon an array of microscopic organisms and some species prey upon creatures as large as themselves. Their first pair of trunk limbs, called the maxillipeds, plays a central role in the process of food collection. These structures are followed by four pairs of swimming legs and sometimes a fifth pair of greatly modified legs utilized for reproductive purposes. Through the use of the swimming legs and beating antennae, free-swimming copepods may move easily about their environments. When present in lakes or deep ponds, the organisms often travel from location to location in a periodic cycle of vertical migrations.