The Deutocerebrum

The second preoral neuromere is called the deutocerebrum. It consists of sensory and motor neuropils and is constructed along the ground pattern typical of postoral ganglia.

Ventrally its neuropils comprise two paired sensory centers: the antennal lobe and, caudal to it, the vertical lobe. Three recent reviews by Vosshall, Hildebrand, and Hansson provide useful summaries of how olfactory receptors wire into the antennal lobes, how the lobes themselves are structured, and how these structures relate to the rest of the brain. Antennal lobe neuropils consist of discrete islets, called glomeruli. Most of these receive mainly olfactory receptor endings from the antenna's funiculus, although certain glomeruli receive inputs from the maxillary palps. Studies on Drosophila and the moth Manduca sexta, which are reviewed, have demonstrated that as a rule glomeruli have unique identities and positions in the antennal lobe.

The vertical lobes receive mainly mechanosensory terminals from the scapus and pedicellus. Some glomeruli situated caudally in the antennal lobe, bordering the dorsal lobe, receive inputs from antennal thermoreceptors, receptors responding to water vapor, and some mechanoreceptors. Motor neurons controlling antennal musculature originate lateral to and dorsal to these sensory regions. However, there is little information about the arrangements of interneurons and relay neurons supplying antennal lobe motor neurons.

Glomerular antennal lobes appear to be typical of neopteran insects. The antennae of primary apterygotes and palaeopterans supply columnar and layered neuropils of the vertical lobes, as do mechanosensory axons from the first two antennal segments (scapus and pedicellus) in neopterans.

Cellular arrangements in the antennal lobes are reminiscent of arrangements in malacostracan crustaceans and in vertebrates. Whether these similarities are a consequence of convergent evolution is debatable. However, in both phyla olfactory receptor neurons tuned to a specific odor molecule converge to the same address (glomerulus) in the antennal lobe. These addresses are then represented as a coarser map in the neuropils of the lateral protocerebrum by the axons of projection neurons originating in glomeruli. Antennal lobes also integrate olfactory information by virtue of complex connections provided by local inhibitory local interneurons. Sexual dimorphism also occurs in the antennal lobes, particularly in the Lepidoptera, in which receptors encoding components of the female pheromone blend send their axons to discrete glomeruli of the male-specific macroglomerular complex.

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