Diagrams of inner views of maxillae of three types of S-T bees. a, Halictus quadricinctus (Fabricius); b, Oxaea flavescens Klug; c, Andrena erythrogaster (Ashmead). (The membranous labiomaxillary tube is stippled.) From Michener, McGinley, and Danforth, 1994. (source: Michener, 2007)