It is impossible to understand the morphology of the brain unless we examine the mode of origin of the nervous system in the early life of the embryo. The head of an embryo insect consists of six segments, i.e. the ocular, antennal, premandibular, mandibular, and the 1st and 2d maxillary segments, so named from the appendages they bear. Of these the first three in the larva and adult are preoral, and the last three are postoral. The antennal segment was probably either postoral in the progenitors of insects, or the antennæ were inserted on the side of the mouth, the latter finally moving back.[[42]]
The nervous system in the early embryonic condition, as shown by Wheeler (Fig. 245), at first consists of nineteen pairs of primitive ganglia, called neuromeres. Those of the head, which later in embryonic life fuse together to form the brain, are the first three, corresponding to the protocerebrum, deutocerebrum, and tritocerebrum of Viallanes. The first pair of primitive ganglia, and which is situated in front of the mouth, is divided into three lobes.
Fig. 245, A-D.—Diagrams of four consecutive stages in the development of the brain and nerve-chain of the embryo of Xiphidium: I, cephalic,—II, thoracic,—III, abdominal, region; st, stomodæum or primitive mouth; an, anus; e, optic plate; pc(og), 1st protocerebral lobe, or optic ganglion; pc2, pc3, 2d and 3d protocerebral lobes; dc, deutocerebrum; tc, tritocerebrum; 1–16, the 16 postoral ganglia; po. c, postoral commissure; fp, furcal pit; ac, anterior,—pc, posterior, ganglionic commissure; ag, anterior,—pg, posterior,—cg, central,—lg, lateral gangliomeres.—After Wheeler.
The first or outermost lobe, according to Wheeler, forms the optic ganglion of the larva and imago, while the second and third lobes. (pc2, pc3) ultimately form the bulk of the brain proper, or the protocerebral lobes. The second (primitively postoral) brain-segment or pair of ganglia gives origin to the antennæ, while the third brain, or premandibular (intercalary) segment, gives origin to a temporary embryonic pair of appendages found in Anurida and Campodea (the premandibular ganglia), and also to the nerves supplying the labrum. These three pairs of ganglia later on in embryonic life become preoral, the mouth moving backwards. The three pairs of primitive ganglia, behind, i.e. the mandibular and 1st and 2d maxillary ganglia, become fused together to form the subœsophageal ganglion, and which in larval and adult life is postoral.
If the tongue (ligula, or hypopharynx) represents a distinct pair of appendages, then there are seven segments in the head.
Fig. 246.—Section through head of a carabid, Anopthalmus telkampfii: br, brain; fg, frontal ganglion; soe, subœsophageal ganglion; co, commissure; n. l, nerve sending branches to the lingua (l); mn, maxillary nerve; mx, 1st maxilla; mm, maxillary muscle; mx′ 2d maxilla; mt, muscle of mentum; le, elevator muscle of the œsophagus; l′ of the clypeus, and a third beyond raising the labrum (lbr); eph, epipharynx; g. g′, salivary glands above; g2, lingual gland below the œsophagus (œ); m, mouth; pv, proventriculus; md, mandible.
The brain, then, supplies nerves to the compound and simple eyes, and to the antennæ, and gives origin to the sympathetic nerves; it is thus the seat of the senses, also of the insect’s mind, and coördinates the general movements of the body.
