=I.= =The optic or procerebral segment= is composed of a median portion, _i.e._ two fused procerebral lobes (median protocerebrum), and of two lateral masses, the optic ganglia (_protocerebrum_), and comprises the following regions fused together and forming the median procerebral mass (Viallanes):— 1. Procerebral lobes. 2. Optic ganglia. 3. Layer of postretinal fibres. 4. Ganglionic plate. (_Periopticon_ of Hickson.) 5. External chiasma. 6. External medullary mass. (_Epiopticon_ of Hickson.) 7. Internal chiasma. 8. Internal medullary mass. (_Opticon_ of Hickson.) 9. Optic ganglia and nerves. 10. Pedunculated or stalked body. (Mushroom body of Dujardin.) 11. Bridge of the procerebral lobes. 12. Central body. [Illustration: FIG. 249.—Diagram of an insect’s brain: _cc_, central body; _cg_, ganglionic cells; _che_, external, _chi_, internal chiasma; _cœ_, œsophageal commissure; _cp_, mushroom body; _ctc_, tritocerebral commissure; _fpr_, postretinal fibres; _goc_, ocellar ganglion; _goc_^1, œsophageal ganglion, the dotted ring the œsophagus; _gv^1_, _gc^2_, _gv^3_, 1st, 2d, 3d, unpaired visceral ganglion; _gvl_, lateral visceral ganglion; _ld_, dorsal lobe of the deutocerebrum; _lg_, ganglionic plate; _lo_, olfactory lobe; _lpc_, protocerebral lobe; _me_, external, _mi_, internal medullary mass; _na_, olfactory or antennal nerve; _nl_, nerve to labrum; _no_, ocular nerve; _nt_, tegumentary nerve; _œ_, œsophagus; _plp_, bridge of the protocerebral lobes; _rvd_, visceral root arising from the deutocerebrum; _rvt_, visceral root arising from the tritocerebrum; _tr_, tritocerebrum; _to_, optic nerve or tract.—After Viallanes. ] =Optic ganglia.=—Each of the two optic ganglia is formed of a series of three ganglionic masses situated between the compound eyes and the median procerebral mass, _i.e._ the ganglionic plate (Fig. 249, _lg_), the external medullary mass (_me_), and the internal medullary mass (_mi_). The postretinal fibres (_fpr_) arising from the facets or single eyes of the compound eye (ommatidia) pass into the ganglionic plate (_lg_), which is united within by the chiasmatic fibres (_che_, external chiasma) of the external medullary mass (_me_). The last is attached to the internal medullary mass (_mi_) by fibres (_chi_), some of which are chiasmatic, and others direct. Finally, the internal medullary mass connects with the median part of the protocerebrum by direct fibres forming the optic nerve or tract (_to_). =Procerebral lobes.=—The median procerebral lobes are fused together on the median line, forming a single central mass. From each side or lobe arises the mushroom or stalked body. In the middle of the mass is the central body, and directly in front is the procerebral bridge (_plp_). The latter is a band uniting the two halves of the brain.