All "Nepenthes" pitchers share several basic characteristics. Traps consist of the main pitcher cup, which is covered by an operculum or lid that prevents rainwater from entering the pitcher and displacing or diluting its contents. A reflexed ring of hardened tissue, known as the peristome, surrounds the entrance to the pitcher (only the aerial pitchers of "N. inermis" lack a peristome). A pair of fringed wings run down the front of lower traps and these presumably serve to guide terrestrial insects into the pitchers' mouth. Accordingly, the wings are greatly reduced or completely lacking in aerial pitchers, for which flying insects constitute the majority of prey items.