Olfactory systems from phylogenetically diverse animals

have similar organizational features. Afferents of olfactory receptor neurons terminate in glomeruli in primary olfactory centers in the brain of vertebrates (Pinching and Powell, 1971; Mori and Yoshihara, 1995; Hildebrand and Shepherd, 1997; Mori et al., 1999) and many invertebrates; for example, molluscs (Chase and Tolloczko, 1993), insects (Maynard, 1966; Tolbert and Hildebrand, 1981; Boeckh et al., 1984; Laissue al., 1999) and crustaceans (Mellon and Munger, 1990; Sandeman et al., 1992; Schmidt and Ache, 1996b). Odor quality is represented in glomerular neuropils as spatial odotopic patterns (Cinelli et al., 1995; Mombaerts et al., 1996; Friedrich and Korsching, 1998; Galizia et al., 1999; Sachse et al., 1999; Wachowiak et al., 2000; Belluscio and Katz, 2001). The fact that olfactory glomerular neuropils may have evolved independently among several arthropod groups (Strausfeld, 1998) and in other phyla suggests that a glomerular organization to the primary olfactory neuropils might be a superior adaptation to minimize neuropilar volume and to capture the diverse and complex nature of chemical signals (Hildebrand and Shepherd, 1997). The olfactory neurons of some invertebrates, such as snails (Chase and Tolloczko, 1993) and decapod crustaceans (Schmidt and Ache, 1996a), project into non-glomerular neuropils, suggesting that non-glomerular neuropilar organizations can also efficiently process chemosensory information. The glomerular and non-glomerular pathways operate in parallel in some species. For example, in lobsters and other decapods, 851 The Journal of Experimental Biology 205, 851–867 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3890