Insects have a system of internal tubes (called tracheae) that lead from the outside to internal regions of the body by what are called spiracles, where gases are exchanged. Some insects rely on muscles to pump the air in and out of the tracheae, while in other insects, the process is a passive exchange of gases. In addition, some insects, such as spiders, have “lungs” in addition to tracheae—hollow, leaflike structures through which the blood flows. These lungs hang in an open space that is connected to a tube, with the other side of the tube in open contact with the air.