**4. The antennal lobes of the Sphinx Moth** *M. sexta*

The insect antenna consists of three segments, namely, the scape, pedicel, and flagellum. The entire length of the antenna has hairs or sensilla on its surface. On the first two segments, the sensilla houses mechanosensitive neurons. These project to mechanosensory centers in the deutocerebrum [62]. In the sphinx moth *M. sexta*, the long flagellum, divided into 85–90 annuli, is equipped with about 4x105 sensilla. These represent several modalities, such as mechanosensation, hygroreception, and olfaction [10, 63–65]. The sensory neurons in olfactory and possibly other antennal sensilla send their axons to the antennal lobes (**Figure 1**). The sensory neurons converge onto central interneurons. In *P. americana*, the convergence ratio between olfactory sensory neurons and projection neurons can be as high as 5000 to 1, and in rabbits, the ratio between sensory neurons and mitral cells is about 1000 to1 [40, 67]. The antennal lobe of *M. sexta* contains about 64 spheroidal glomeruli [68, 69]. In male *M. sexta*, a macroglomerular complex located near the entrance of the antennal nerve into the antennal lobes has been identified (**Figure 1**) [6].

*Neuronal Architecture and Functional Organization of Olfactory Glomeruli DOI: http://dx.doi.org/10.5772/intechopen.108728*

#### **Figure 1.**

*This figure shows how antennal sensory neurons project to glomeruli in the antennal lobes of Manduca sexta. Sensory neurons were anterogradely labeled with rhodamine dextran. The diagram and images show frontal views. (a) Olfactory receptor neurons in long antennal trichoid sensilla project to the three glomeruli of the macroglomerular complex (MGC: C-cumulus, T1-toroid-1, and T2-toroid-2). (b) a schematic diagram illustrates how receptor neurons project to the antennal lobe. Receptor neurons from a trichoid sensillum project to the three glomeruli of the macroglomerular complex. (c) if receptor neurons in long trichoid sensilla and other antennal sensilla were labeled, axonal projections would go to the macroglomerular complex and ordinary glomeruli in the antennal lobes. Optical sections were taken at different depths in anterior to posterior direction through the antennal lobes shown from left to right. C – Cumulus, do – Dorsal, la – Lateral, T1 – Toroid-1, T2 – Toroid-2. Scale bar: 100 μm. From [66].*

The first glomeruli in insects were described in the deutocerebrum of the bee by Kenyon [25, 70, 71]. In *M. sexta*, a closer anatomical analysis of glomeruli revealed a complex substructure of discrete domains and laminae within individual glomeruli [7, 72]. In bees, however, glomeruli have a relatively simple organization [73].

In contrast to the large differences in the number of glomeruli among different animal species, insect antennal systems present highly invariant glomerular organizations with regard to shape, size, location, and number within a species [74]. This has been shown for a variety of species including the fruit fly *Drosophila melanogaster* [75, 76], sphinx moth *M. sexta* [68], moth *Mamestra brassicae* [31], bee *Apis mellifera* [77], and cockroach *Blaberus craniifer* [78]. This invariance was also found to be true for the iulid *Cylindroiulus punctatus* (Diplopoda) [79] and in a vertebrate, the zebrafish (*Brachydanio rerio*) [80]. The number of glomeruli in all of these species is relatively small (18 for *C. punctatus* to 174 in worker bees). It is more difficult to verify numerical invariance in vertebrates with several thousand glomeruli [55]. The only identified vertebrate glomerulus is the modified glomerular complex for detection of the maternal suckling pheromone in rats [81].
