Brain and Optic Lobes

This chapter discusses different segments of brain and optical lobe. Brain includes neuropils of the subesophageal ganglion, which is composed of the fused ganglia from three postoral segmental neuromeres. These are located ventrally with respect to the digestive tract, as are ganglia of the thorax and abdomen. In most hemimetabolous insects, and in many aleopterans, the subesophageal ganglion is connected by paired circumesophageal commissures to the supraesophageal ganglion. In many crown taxa, the subesophageal and supraesophageal ganglia are fused, as is the case in honey bees or the fruit fly Drosophila melanogaster, which is the taxon used to summarize the major divisions of the brain. A consequence of fusion is that tracts of axons that would otherwise form the circumesophageal commissures are embedded within a contiguous neuropil. Further, the optic lobes of palaeopteran and neopteran insects consist of three retinotopic neuropils. These are the lamina, medulla, and lobula complex. In certain orders of insects, the lobula complex is divided into two separate neuropils: a lenticular lobula that is mainly composed of columnar neurons and a tectum-like lobula plate that is hallmarked by wide-field tangential neurons. However, in insects with an undivided lobula, deeper layers comprise tangential neurons that probably have the same functions as tangential neurons in the lobula plate.