Fairyflies have two to four larval stages, all apparently without functional spiracles or tracheae. The tracheae of insects are attached to the spiracles, excluding the head. The bubble covers at least some of the spiracles, permitting air to enter the tracheae. Marine chelicerates have gills, while the air-breathing forms generally have both book lungs and tracheae. Spiders have developed several different respiratory anatomies, based either on book lungs or on tracheae. Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis). They do not have book lungs, and breathe through tracheae. They breathe through tracheae, stigmata (small openings of the skin), intestines, and the skin itself. This network of transverse and longitudinal tracheae equalizes pressure throughout the system. Cockroaches, like all insects, breathe through a system of tubes called "tracheae". They have long legs, tracheae, and no median eyes. Each opens into an internal pouch, and connects to a system of tracheae. Spiders that have tracheae generally have higher metabolic rates and better water conservation.