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; goc1, œsophageal ganglion, the dotted ring the œsophagus; gv1, gc2, gv3, 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.
The procerebral lobes also give origin to the nerves to the ocelli (no).
Fig. 250.—Transverse section through the brain of the locust (Œdipoda and Caloptenus): c′, lower part of the wall of c, calyx;—st, stalk of the same; bpcl, bridge of the protocerebral lobes; mo, nerve of median ocellus; ch, transverse fascia of the optico-olfactory chiasma; fcb, fibrous region of the central body; lcb, tubercle of the central body; fch, descending fascia of the optico-olfactory chiasma; choo, superior fascia of the optico-olfactory chiasma; pt, protocerebral lobes; ld, dorsal lobe of the deutocerebrum; lt, tritocerebral lobe; gcld, gc, ganglion cells.—After Viallanes.
The mushroom or stalked bodies.—These remarkable organs were first discovered by Dujardin, who compared them to mushrooms, and observed that they were more highly developed in ants, wasps, and bees than in the lower insects, and thus inferred that the higher intelligence of these insects was in direct relation to the development of these bodies. We will call them the mushroom bodies.
These two bodies consist of a rounded lobular mass (the trabecula) of the procerebral lobe, from which arises a double stalk (Fig. 253), the larger called the cauliculus, the smaller the peduncle (or pedicel); these support the cap or calyx. The calices of the bee were compared by Dujardin to a pair of disks on each side of the brain as seen from above, “each disk being folded together and bent downwards before and behind, its border being thickened, and the inner portion radiated.” In the locust there are but two divisions of the calyx; in the cockroach, ants, wasps, and bees, four.