THE NERVOUS SYSTEM
a. The nervous system as a whole
Fig. 239.—Central nervous system of Machilis maritima: au, eye; lo, optic tract; g, brain; an, antennal nerve; oe, œsophagus passing between the œsophageal commissures; usg, infraœsophageal ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last (Sabc) consisting of three fused ganglia; s, sympathetic nervous system of the ventral cord.—After Oudemans, from Lang.
The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal.
A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve; by means of these nerves the ganglia are connected with other ganglia, and with the sensory cells and muscle-fibres. The ganglia may be simple, and arranged in pairs, corresponding to each segment of the body, or they may be compound, the result of the fusion of several pairs of ganglia, which in the early stages of the embryo are separate. Thus the brain of insects is a compound ganglion, or ganglionic mass.
The nerves are of two kinds: 1. Sensory, which transmit sensations from the peripheral sense-cells to the ganglion, or brain; 2. Motor, which send stimuli from the brain or any other ganglion to the muscles.
Of ganglion cells, some are tactile, and others give rise to nerves of special sense, being distributed to the eyes, or to the organs of hearing, smell, taste, or touch.
Fig. 240.—Nervous system of Melanoplus spretus: sp, supraœsophageal ganglion, sending off the large optic nerve (op) to the eyes, and an ocellar nerve to each ocellus (the dotted line oc stops short of the left ocellus); if, infraœsophageal ganglion; 1, 2, 3, thoracic ganglia; 1–5, five abdominal ganglia (the fifth the largest, and sending branches to the ovipositor, etc.) The sympathetic nerve and ganglia are represented by the two main nerves which arise from the medio-cephalic (as) resting on and above the œsophagus, and two ganglia (ps) on the under side of the crop. From each of these ganglia, two nerves are sent under the crop, and a larger nerve on each side to as far as the stomachal cæca, ending the figure at the dotted line 2, near the second thoracic ganglion. u, a round, shining body, connected by a nerve with the medio-cephalic ganglion, its nature unknown.
Fig. 241.—Section through the head of Machilis, showing the brain (br), and subœsophageal ganglion (soe. g); cl, clypeus; lbr, labrum; oc, ocellus.
While the supraœsophageal ganglion, or “brain,” of the insect is much more complex than any other ganglion, consisting more exclusively both of sensory as well as motor ganglia and their nerves, it should be borne in mind that the subœsophageal ganglion also receives nerves of special sense, situated on the palpi and on the tongue, as in the bee and other insects; hence this ganglion is probably complex, consisting of sensory and motor cells. The third thoracic ganglion is also, without doubt, a complex one, as in the locusts the auditory nerves pass into it from the ears, which are situated at the base of the abdomen, while in the green grasshoppers, such as the katydids and their allies, whose ears are situated in their fore legs, the first thoracic ganglion is a complex one. In the cockroach and in Leptis (Chrysopila), a common fly, the caudal appendages bear what are probably olfactory organs, and as these parts are undoubtedly supplied from the last abdominal ganglion, this is probably composed of sensory and motor ganglia; so that we have in the ganglionated cord of insects a series of brains, as it were, running from head to tail, and thus in a still stronger sense than in vertebrates the entire nervous system, and not the brain alone, is the organ of the mind of insects.
The simplest, most primitive form of the nervous system of insects is seen in that of the Thysanura. That of Campodea has not yet been fully examined, but in that of the more complicated genus, Machilis (Fig. 239), we see that there is a pair of ganglia to nearly each segment, while the brain (Fig. 241) is composed of three lobes, viz. the optic, the cerebral (Fig. 239, g), behind which is the antennal lobe, from which the antennal nerve takes its origin. Behind the opening for the throat (oe) is situated the first ganglion of the ventral cord, the subœsophageal ganglion, which gives rise to the nerves supplying the jaws and other mouth-parts.
Fig. 242, A-D.—The nervous systems of 4 genera of Diptera, to demonstrate their various degrees of fusion of ganglia: A, non-concentrated more primitive nervous system of Chironomus plumosus, with 3 thoracic and 6 abdominal ganglionic masses. B, nervous system of Empis stercorea, with 2 thoracic and 5 abdominal ganglionic masses. C, nervous system of Tabanus bovinus, with 1 thoracic ganglionic mass, and the abdominal ganglia closely approximated. D, highly modified nervous system of Sarcophaga carnaria, in which all the ganglia of the ventral cord behind the subœsophageal ganglion are fused into a single ganglionic mass.—After Brandt, from Lang.
In the Collembola, which are retrograde Thysanura, there are from one (Smynthurus), to three or four ventral ganglia.
In the winged insects, where the ganglia are more or less fused, the fusion taking place in the head and at the end of the abdomen; there are in the more simple and generalized forms, such as Ephemera, the grasshopper, locusts (Fig. 240), etc., thirteen ganglia besides the two pairs of compound ganglia in the head, three pairs of thoracic ganglia, and usually from five to eight pairs of ganglia in the abdomen.
Fig. 243.—Nervous system of the May beetle, Lachnosterna fusca: w1, nerve to 1st,—w2, nerve to 2d, pair of wings; ig, infraœsophageal ganglion.
Fig. 244.—The same of the stag-beetle, Lucanus dama, where there are 3 thoracic, and 3 separate abdominal ganglia.
In certain winged insects the process of fusion or degeneration is carried to such an extreme that there are either no abdominal ganglia (Fig. 242, D), or their vestiges are situated in the thorax and partially fused with the thoracic ones, as in the May beetle, in which the prothoracic pair of ganglia is separate, while the two other thoracic ganglia are fused with the abdominal, the latter being situated in the thorax; this fusion is carried to a further extent than in any other Coleoptera yet examined. In many Diptera and Hemiptera the abdominal ganglia are either absent or the vestiges are fused with the thoracic ganglia.
Rhizotrogus, which is allied to our May beetle, as also Hydrometra and the Stylopidæ are said to lack the subœsophageal ganglion (Brandt).
In numerous Coleoptera (Acilius, Gyrinus, Necrophorus, Melolontha, Bostrichus, Rhynchænus); in many Diptera (Culex, Tipula, Asilus, Xylophaga, and Phora); and in the higher Hymenoptera (Crabronidæ, Vespidæ, and Apidæ), as well as in many Lepidoptera (Vanessa, Argynnis, and Pontia), two of the thoracic ganglia are fused together, while all three are partially fused into a single mass in many brachycerous Diptera (Conops, Syrphus, Pangonia, and the Muscidæ); in certain Hemiptera (Pentatoma, Nepa, and Acanthia); also in a beetle (Serica brunnea). Sometimes the subœsophageal ganglion is fused with the first thoracic, as in Acanthia, Nepa, and Notonecta. The greatest amount of variation is seen in the number of abdominal ganglia, all being fused into a single one or from one to eight. The fusion is usually greatest where the abdomen is shortened, due to the partial atrophy and modification of the terminal segments which bear the ovipositor, where present, and the genital armature.
There is only one pair of abdominal ganglia in Gyrinus and in certain flies (Conops, Trypeta, Ortalis, and Phora); two in Rhynchænus, a weevil, and in the flies, Syrphus and Volucella; three in Crabro and Eucera; four in Sargus, Stratiomys and in butterflies, five in the beetle, Silpha, and in the fly, Sciara, and the moth, Hepialus.
The nervous system in the larvæ of the metabolous orders is not concentrated, though in that of the neuropterous Myrmeleo it has undergone fusion from adaptation to the short compressed form of this insect.
b. The brain
The brain of insects appears to be nearly, if not quite, as complex as that of the lower vertebrates. As in the latter, the pair of supraœsophageal ganglia, or brain, is the principal seat of the senses, the chief organ of the insect’s mind.
It is composed of a larger number of pairs of primitive ganglia than any of the succeeding nerve-centres, and is, structurally, entirely different from and far more complicated than the other ganglia of the nervous system. It possesses a central body in each hemisphere, a “mushroom body,” optic lobes and optic ganglia and olfactory lobe, with their connecting and commissural nerve-fibres, and a number of other parts not found in the other ganglia.
In the succeeding ganglia the lobes are in general motor; the fibres composing the œsophageal commissures, and which arise from the œsophageal commissural lobes, extend not only to the subœsophageal ganglion, but pass along through the succeeding ganglia to the last pair of abdominal nerve-centres.[[40]] Since, then, there is a direct continuity in the fibres forming the two main longitudinal commissures of the nervous cord, and which originate in the brain, it seems to follow that the movements of the body are in large part directed or coördinated by the brain.[[41]] Still, however, a second brain, so to speak, is found in the third thoracic ganglion of the locust, which receives the auditory nerves from the ears situated in the base of the abdomen; or in the first thoracic ganglion of the green grasshoppers (katydids, etc.), whose ears are situated in their fore legs; while even the last pair of abdominal ganglia in the cockroach and mole cricket, is, so to speak, a secondary brain, since it distributes sensory nerves to the caudal stylets, which are provided with organs probably olfactory in nature.
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.
Fig. 247.—Median longitudinal section through the head of Blatta orientalis. The nervous system of the head is drawn entire. hyp, hypopharynx; os. oral cavity; lbr, upper lip; gf, frontal ganglion; g, brain; na, root of the antennal nerve; no, root of the optic nerve; ga, anterior,—gp, posterior ganglion of the paired visceral nervous system; œ, œsophagus; c, œsophageal commissure; usg, infraœsophageal ganglia; cc, longitudinal commissure between this and the first thoracic ganglion; sg, common duct of the salivary glands; lb, labium (2d maxillæ); nr, recurrent nerve; d, nerve uniting the frontal ganglion with the œsophageal commissure; e, nerve from this commissure to the labrum; f, nerve from the infraœsophageal ganglion to the mandible, —g, to the 1st maxillæ, —h, to the lower lip (2d maxillæ).—After Hofer, from Lang.
Fig. 248.—1, front view of the brain of Melanoplus femur-rubrum: opt. gang, optic ganglion; oc, ocelli and nerves leading to them from the two hemispheres, each ocellar nerve arising from the region containing the calices; m. oc, median ocellar nerve; opt. l, optic lobe sending off the optic nerve to the optic ganglion; ant. l, antennal or olfactory lobe; ant. n, antennal nerve; f. g, frontal ganglion of sympathetic nerve; lbr. n, nerve to labrum; x, cross-nerve or commissure between the two hemispheres; œ. c, œsophageal commissure to subœsophageal ganglion. 2, side view of the brain and subœsophageal ganglion (lettering of brain as in 1): s. g, stomatogastric or sympathetic nerve; a. s. g, anterior, and p. s. g, posterior, sympathetic ganglia; g2, subœsophageal ganglion; md, nerve to mandible; mx, maxillary nerve; ln, labial nerve; nl, unknown nerve,—perhaps salivary. 3, interior view of the right half of the head, showing the brain in its natural position: an, antenna; cl, clypeus; lbr, labrum; m, mouth-cavity; md, mandible; t, tongue; œ, œsophagus; c, crop; en, right half of the endocranium or X-shaped bone, through the anterior angle of which the œsophagus passes, while the great mandibular muscles play in the lateral angles. The moon-shaped edge is that made by the knife passing through the centre of the X. 4, view of brain from above (letters as before). 5, subœsophageal ganglion from above: t. c, commissure to the succeeding thoracic ganglion (other letters as before). Fig. 3 is enlarged 8 times; all the rest 25 times.—Drawn from original dissections, by Mr. Edward Burgess, for the Second Report of the U. S. Entomological Commission.
The pair of subœsophageal ganglia distributes nerves to the mandibles, to the 1st and 2d maxillæ, and to the salivary glands (Fig. 248).
Its general shape and relations to the walls and to the outer organs of the head is seen in Figs. 247, 248. In all the winged insects (Pterygota) its plane is situated more or less at right angles to the horizontal plane of the ventral cord. On the dorsal and anterior sides are situated the ocular lobes, and below these the antennal lobes.
Viallanes first, independently of embryonic data, divided the brain of adult insects into three regions or segments; i.e. the “protocerebron,” “deutocerebron” and “tritocerebron,” which he afterwards found to correspond with the three primitive elements (neuromeres) of the brain and with the segments of the head of the embryo.
The brain of the locusts (Melanoplus and Œdipoda) being best known will serve as the basis of the following description, taken mainly from Viallanes, with minor changes in the name of the three segments, and other modifications.
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.
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.
The shape and relation of the mushroom bodies are represented in Figs. 252 and 253. The bodies are connected by commissural fibres, and are connected with the optic ganglion of the same side, and with the central body; while they are connected with the antennal lobes by the optico-olfactory chiasma.
Fig. 251.—Sagittal section through the brain of the locust: l. oc. n, lateral ocellus nerve; a. t, anterior tubercle of the mushroom body; i. t, internal tubercle of the mushroom body; c. l, cerebral lobes; l. l, lateral lobe of the middle protocerebrum; com, commissural cord; c. mol, central mass of the olfactory lobe; ac. an. l, fibres uniting the median lobe of the middle protocerebrum with dorsal lobes of the deutocerebrum; gc. trit. l, ganglionated cortex of the tritocerebral lobe; c. an. l, cortex of antennal (olfactory) lobe; lab. fr, labrofrontal nerve; oe. com, œsophageal commissure; tr. com, transverse commissure of œsophageal ring; other letters as in Fig. 250.—After Viallanes.
The stalked bodies are enveloped by the cortical layers of ganglion-cells, those filling the hollow of the calyx having little or no protoplasm around the nucleus.
Structure of the mushroom bodies.—By staining the brain of the honey bee with bichromate of silver, Kenyon has worked out the structure of the mushroom bodies, with their cells. The cup-shaped bodies or calyces are composed of fibrillar substance (punktsubstanz). Each of these cups, he says, is “filled to overflowing with cells having large nuclei and very little cytoplasm.” From the under surface of each of these cups there descends into the general fibrillar substance of the brain “a column of fibrillar substance, which unites with its fellow of the same side to send a large branch obliquely downward to the median line of the brain, and an equally large or larger branch straight forwards to the anterior cerebral surface.”
The cells of the mushroom bodies, observes Kenyon, “stand out in sharp contrast to all other nerve cells known, though they recall to some extent the cells of Purkinje in the higher mammals. Each of the cells contained within the fibrillar cup sends a nerve-process into the latter, where it breaks up into a profusely arborescent system of branchlets, which often appear with fine, short, lateral processes, such as are characteristic of the dendrites of some mammalian nerve-cells.” Just before entering the fibrillar substance, a fine branch is given off that travels along the inner surface of the cup along with others of the same nature, forming a small bundle to the stalk of the mushroom body, down which it continues until it reaches the origin of the anterior and the inner roots above mentioned. “Here it branches, one branch continuing straight on to the end of the anterior root, while the other passes to the end of the inner root. Throughout its whole course the fibre and its two branches are very fine. Nearly the whole stalk and nearly the whole of each root is made up of these straight, parallel fibres coming from the cells within the cup of the mushroom bodies. What other fibres there are enter these bodies from the side, and branch between the straight fibres very much as the dendrites of the cells of Purkinje branch among the parallel fine fibres from the cells of the granular layer in the mammalian cerebellum. These fibres are of the nature of association fibres.”
Viallanes showed that from the olfactory or antennal lobes, as well as from the optic ganglia, there are tracts of fibres which finally enter the cups of the mushroom bodies, and Kenyon has confirmed this observation. Kenyon has also, by the Golgi method, detected another tract, before unknown, “passing down the hinder side of the brain, from the cups to the region above the œsophagus, where it bends forward and comes in contact with fibres from the ventral cord, which exists, although Binet was unable to discover any growth of fibres connecting the cord with the brain.
“The fibres entering the cups from the antennal lobe, the optic ganglia, and the ventral region, spread out and branch among the arborescent endings of the mushroom-body cells. The fibres branching among the parallel fibres of the roots and the stalk lead off to lower parts of the brain, connecting with efferent or motor-fibres, or with secondary association fibres, that in their turn make such connections. This portion of the circuit has not been perfectly made out, though there seems to be sufficient data to warrant the assumption just made.
Fig. 252.—Section 17, showing the central body (centr. b) and mushroom body, optic and antennal lobes (a. l), and procerebral lobes (pc. l); o. cal, outer division of the calyx; op. n, optic nerve; trab, trabeculum; tc. n, transverse nerve.
“Such fibres existing as described, there is then a complete circuit for sensory stimuli from the various parts of the body to the cells of the mushroom bodies. The dendritic or arborescent branches of these cells take them up and pass them on out along the parallel fibres or neurites in the roots of the mushroom bodies as motor or other efferent impulses.
“This, however, is not all. For there are numerous fibres evident in my preparations, the full courses of which I have not been thus far able to determine, but which are so situated as to warrant the inference that they may act as association fibres between the afferent fibres from the antennæ, optic ganglia, and ventral system, and the efferent fibres. There is then a possibility of a stimulus entering the brain and passing out as a motor impulse without going into the circuit of the fibres of the mushroom bodies; or, in other words, a possibility of what may be compared to reflex action in higher animals.”
Fig. 253.—Enlarged view of the trabeculum (the dotted lines tcn and obt. n pass through it) and its nerves, of the mushroom body,—its calices and stalk, and the origin of the optic nerve × 225 diameters: atn, ascending trabecular nerve; obt. n, oblique trabecular nerve; tcn, transverse nerve; lat. n, lateral nerve; cent. n, central nerve.
The mushroom bodies have not yet been found to be present in the Synaptera, but occur in the larvæ, at least of those of most metamorphic insects (Lepidoptera and Hymenoptera), though not yet found in the larvæ of Diptera. The writer has found these bodies in the nymphs of the locust (Melanoplus spretus), but not in the embryo just before hatching. They occur in the third larval or nymph stage of this insect. It is evident that by the end of the first larval stage the brain attains the development seen in the third larval state of the two-banded species (C. bivittatus).
Fig. 254.—Section through the brain of Caloptenus bivittatus in the third larval stage, showing the two hemispheres or sides of the brain, and the ocelli and ocellar nerves, which are seen to arise from the top of the hemispheres directly over the calices (compare Fig. 251): o. cal, outer division of calyx of left mushroom body.
The result of our studies on the brain of the embryo locust was that from the embryonic cerebral lobes are eventually developed the central body and the two mushroom bodies. Fig. 254 shows the early condition of the mushroom bodies and their undoubted origin from the cerebral ganglia. Hence these bodies appear to be differentiations of the cerebral ganglia or lobes, having no connection with the optic or antennal lobes.
The central body (Fig. 252, centr. b).—This is the only single or unpaired organ in the brain. Dietl characterizes it as a median commissural system. Viallanes describes it as formed entirely of a very fine and close fibrillar web, like a thick hemispherical skull-cap, situated on the median line and united with the cerebral lobes. “It is like a central post towards which converge fibres passing from all points of the brain; being bound to the cerebral lobes, to the stalked bodies, to the optic ganglia, and to the olfactory lobes by distinct fibrous bundles.”
The antennal or olfactory lobes (Deutocerebrum).—This portion of the brain consists of two hemispherical lobes, highly differentiated for special sensorial perceptions, and connected by a slightly differentiated medullary mass, the dorsal lobe (Figs. 248, 249 lo), from which arise the motor fibres and those of general sensibility. The antennal lobes are in part attached to the optic ganglia, and partly to the stalked body on the same side, by the optic olfactory chiasma (Fig. 250 fch, choo), a system of fibres partially intercrossed on the median line.
The œsophageal lobes (Tritocerebrum) (Figs. 249, 250).—From this region the labrum and viscera are innervated, the nerves to the latter being called the visceral, sympathetic, or stomatogastric system. As Viallanes remarks, though plainly situated in front of the mouth, they are in fact post-œsophageal centres. The two lobes are situated far apart, and are connected by a bundle of fibres passing behind the œsophagus, called the transverse commissure of the œsophageal ring (Lienard). The œsophageal ganglia, besides giving rise to the labral nerves, also give origin to the root of the frontal ganglion.
c. Histological elements of the brain
The brain and other ganglia are composed of two kinds of tissue.
1. The outer slightly darker, usually pale grayish white portion consists of cortical or ganglion-cells differing in size. This portion is stained red by carmine, the cells composing it readily taking the stain.
The large ganglion cells (represented in Figs. 252 and 253) are oval, and send off usually a single nerve-fibre; they have a thin fibrous cell-wall, and the contents are finely granular. The nucleus is very large, often one-half the diameter of the entire cell, and is composed of large round refractive granules, usually concealing the nucleolus.
2. The medullary or inner part of the brain consists of matter which remains white or unstained after the preparation has remained thoroughly exposed to the action of the carmine. It consists of minute granules and interlacing fibres. The latter often forms a fine irregular network inclosing masses of finely granulated nerve matter.
This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des thierischen Körpers, p. 89, thus refers to it:—
“In the brain and ventral ganglia of the leech, of insects, and in the brain of the gastropods (Schnecken) I observe that the stalks (stiele) of the ganglion-cells in nowise immediately arise as nerve-fibres, but are planted in a molecular mass or punktsubstanz, situated in the centre of the ganglion, and merged with this substance. It follows, from what I have seen, that there is no doubt that the origin of the nerve-fibres first takes place from this central punktsubstanz.”
“This relation is the rule. But there also occur in the nerve-centres of the invertebrates single, definitely situated ganglion-cells, whose continuations become nerve-fibres without the intervention of a superadded punktsubstanz.” We may, with Kenyon, call it the fibrillar substance.
Leydig subsequently (p. 91) further describes this fibrillar substance, stating that the granules composing it form a reticulated mass of fibrillæ, or, in other words, a tangled web of very fine fibres:—
“We at present consider that by the passage of the continuation of the ganglion-cells into the punktsubstanz this continuation becomes lost in the fine threads, and on the other side of the punktsubstanz the similar fibrillar substance forms the origin of the axis-cylinders arranged parallel to one another; so it is quite certain that the single axis-cylinder derives its fibrillar substance as a mixture from the most diverse ganglion-cells.”
d. The visceral (sympathetic or stomatogastric) system
This system in insects is composed (1) of a series of three unpaired ganglia (Fig. 249, gv1, gv2, gv3), situated over the dorso-median line of the œsophagus, and connected by a median nervous cord or recurrent nerve (nr, vagus of Newport). The first of these ganglia is the frontal ganglion, which is connected with the œsophageal ganglia by a pair of roots (rvt), which have an origin primitively common with that of the labral nerves (Fig. 248, fg and lbr).
Fig. 255.—Anterior portion of the paired and unpaired visceral nervous system of Blatta orientalis seen from above. The outlines of the brain (g) and the roots of the antennal nerve (na), which cover a portion of the sympathetic nervous system, are given by dotted lines. Lettering as in Fig. 247. nsd, nerve to salivary gland. The nervus recurrens (nr) enters an unpaired stomach ganglion farther back.—After Hofer, from Lang.
2. Of two pairs of lateral ganglia (Fig. 255, ga, gp) situated two on each side of the œsophagus. They are connected both with the antennal lobes by a nerve (rvd), and to the chain of unpaired ganglia by a special connective. The first pair of these ganglia sends nerves to the heart and aorta; the second pair to the tracheæ of the head.
The unpaired median or recurrent nerve (nr) extends back from under the brain along the upper side of the œsophagus, and (in Blatta), behind the origin of the nerves to the salivary glands, enters an unpaired ganglion, called the stomachic ganglion (ganglion ventriculare), situated in front of the proventriculus. The number of these stomachic ganglia varies in different orders of insects.
In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller, Brandt, ex Kolbe) that the nerve to each salivary gland arises from three different centres: the anterior end situated under the œsophagus is innervated by the paired visceral nerves from the hinder paired ganglia; the remaining part by nerves arising from each side of the recurrent nerve; and thirdly by a pair of nerves arising from the subœsophageal ganglion which accompanies the common salivary duct, and ends in branches which partly innervate the salivary glands and in part their muscles.
Hofer considers that the function of this complex system of paired and unpaired ganglia, with their nerves, is a double one, viz. serving both as a centre for the peristaltic action of the œsophagus, and as innervating the salivary glands.
Besides these a second portion of the visceral system arises from the thoracic and abdominal ventral cord. It may be seen in the simplest condition yet known in the nervous system of Machilis (Fig. 239 s). It consists of a fine, slender nerve, which extends along the surface of the ventral chain of ganglia, and sending off a pair of branches (accessory transverse nerves) in front of each ganglion. These accessory nerves receive nerve-twigs from the upper cord of the ventral chain, dilating near their origins into a minute elongated ganglion, and then passing partly outwards to the branches of the tracheæ and the muscles of the spiracles, uniting in the middle line of each segment of the body behind the head, i.e. of those segments containing a pair of ganglia.
e. The supraspinal cord
In the adult Lepidoptera has been detected, continuous with and on the upper side of the abdominal portions of the ventral cord, a longitudinal cord of connective tissue forming a white or yellowish band, and which seems to be an outgrowth of the dorsal portion of the neurilemma of the ventral cord. Muscles pass from it to the neighboring ventral portions of the integument. Its use is unknown, and attention was first called to it by Treviranus, who called it “an unknown ventral vessel” (Bauchgefäss). Afterwards it was re-discovered by Newport, who described it as “a distinct vascular canal.” But Burger has proved by cross-sections that it is not tubular, but a comparatively solid cord composed, however, of loose connective tissue. Newport found it in the larva of Sphinx ligustri, but Cattie states that it is not present in that of Acherontia atropos. It has not yet been observed in insects of other orders, but its homologue exists in the scorpion and in the centipede, and it may prove to correspond with the far more complete arterial coat which, with the exception of the brain, envelops the nervous system of Limulus.
f. Modifications of the brain in different orders of insects
There are different grades of cerebral development in insects, and Viallanes claimed that it was no exaggeration to say that the brain of the locust (Melanoplus) differs as much from that of the wasp as that of the frog differs from that of man. He insists that the physiological conditions which determine the anatomical modifications of the brain are correlated with 1, the food; 2, the perfection of the senses; and 3, with the perfection of the psychic faculties. For example, in those which feed on solid food and whose œsophagus is large (Orthoptera and Coleoptera), the connectives are elongated, the subœsophageal commissure free in all its extent, and the tritocerebrum is situated quite far from the preceding segment of the brain.
On the other hand, in insects which feed on fluid food (Hymenoptera, Lepidoptera, Diptera, Hemiptera), the œsophagus is slender and the nervous centres which surround them are very much condensed; the connectives are short, and the tritocerebrum is closely fused, partly to a portion of the antennal lobes (deutocerebrum) and partly to the mandibular ganglion.
As regards the perfection of the senses, where, as in dragon-flies, the eyes are very large, the optic ganglia are correspondingly so, and in the same insects the antennæ being very small, the antennal lobes are almost rudimentary. The ants exhibit inverse conditions; in their brain the antennal lobes are well developed, while the optic ganglia are reduced, and where, as in Typhlopone, the eyes are wanting, they are completely atrophied.
Fig. 256.—Head of Anophthalmus tellkampfii, showing the brain,—the optic ganglia, nerves, and eyes totally atrophied.
Fig. 257.—Head of another Carabid, with the brain and eyes normal: op, optic ganglion; pcl, brain.
In certain cave insects where the eyes are wanting, the optic ganglia are also absent. In the eyeless cave species of Anophthalmus the optic ganglia and nerves are entirely atrophied, as they are in Adelops, which, however, has vestiges of the facets (ommatidia). Fig. 257 represents the brain of Chlænius pennsylvanicus, a Carabid beetle, with its eyes and optic ganglia (op) which may be compared with Anopthalmus, in which these parts are totally atrophied.
Dujardin claimed that the degree of complication of the stalked body of the Hymenoptera was in direct relation with their mental powers. This has been proved by Forel, who has shown that in the honey bee and ants the mushroom bodies are much more developed in the workers than in the males or females and Viallanes adds that these bodies are almost rudimentary in the dragon-flies, whose eyes are so large; while on the contrary in the blind ants (Typhlopone), these bodies are as perfect and voluminous as in the ants with eyes.
Fig. 258.—Diagrammatic outlines of sections of the upper part of the brain of a cockroach. Only one side of the brain is here represented. The numbers indicate the position in the series of 34 sections into which this brain was cut. mb, mushroom bodies, with their cellular covering (c) and their stems (st); a, anterior nervous mass; m, median nervous mass.—After Newton.
Within the limits of the same order the stalked bodies are most perfect in the most intelligent forms. Thus in the Orthoptera, says Viallanes, the Blattæ, Forficulæ, and the crickets, the mushroom bodies are more perfect than in the locusts, which have simpler herbivorous habits. This perfection of the mushroom bodies is seen not only in the increase in size, but also in the complication of its structures. Thus in the groups with lower instincts (Tabanus, Æschna) the stalk does not end in a calyx projecting from the surface of the brain, but its end, simply truncated, is indicated externally only by an accumulation of the ganglionic nuclei which cover it.[[43]]
In types which Viallanes regards as more advanced, i.e. Œdipoda and Melanoplus, the end of the stalk projects and is folded into a calyx.
The brain of the cockroach (Periplaneta, Fig. 258) is a step higher than that of the locusts, each calyx being divided into two adjacent calices, although the cockroaches are an older and more generalized type than locusts.
The stalked bodies of cockroaches are thus complex, like those of the higher Hymenoptera, the calices in Xylocopa, Bombus, and Apis being double and so large as to cover almost the entire surface of the brain.
Finally, in what Viallanes regards as the most perfect type (Vespa), the sides of the calices are folded and become sinuous, so as to increase the surface, thus assuming an appearance which, he claims, strongly recalls that of the convolutions of the brain of the mammals.
Cheshire also calls attention to a progression in the size of these appendages, as well as in mental powers as we rise from the cockchafer (Melolontha vulgaris) to the cricket, up to the ichneumon, then to the carpenter bee, and finally to the social hive bee, “where the pedunculated bodies form the ⅕ part of the volume of the cerebral mass, and the 1
870 of the volume of the entire creature, while in the cockchafer they are less than 1
2300 the part. The size of the brain is also a gauge of intelligence. In the worker bee the brain is 1
174 of the body; in the red ant, 1
296; in the Melolontha, 1
3500; in the Dyticus beetle, 1
4400.” (Bees and bee-keeping, p. 54.)
g. Functions of the nerve-centres and nerves
As we have seen, the central seat of the functions of the nervous system is not the brain alone (supraœsophageal ganglion), but each ganglion is more or less the seat of vital movements, those of the abdomen being each a distinct motor and respiratory centre. The two halves of a ganglion are independent of each other.
According to Faivre, the brain is the seat of the will and of the power of coördinating the movements of the body, while the infraœsophageal ganglion is the seat of the motive power and also of the will.
The physiological experiments of Binet, which are in the line of those of Faivre, but more thorough, demonstrate that an insect may live for months without a brain, if the subœsophageal ganglion is left intact, just as a vertebrate may exist without its cerebrum. As Kenyon says: “Faivre long ago showed that the subœsophageal ganglion is the seat of the power of coördination of the muscular movements of the body. Binet has shown that the brain is the seat of the power directing these movements. ‘A debrained hexapod will eat when food is placed beneath its palpi, but it cannot go to its food even though the latter be but a very small space removed from its course or position. Whether the insect would be able to do so if the mushroom bodies only were destroyed, and the antennal lobes, optic lobes, and the rest of the brain were left intact, is a question that yet remains to be answered’” (Kenyon).
In insects which are beheaded, however readily they respond to stimulation of the nerves, they are almost completely wanting in will power. Yet insects which have been decapitated can still walk and fly. Hymenoptera will live one or two days after decapitation, beetles from one to three days, and moths (Agrotis) will show signs of life five days after the loss of their head.
That the loss of will power is gradual was proved by decapitating Polistes pallipes. A day after the operation she was standing on her legs and opening and closing her wings; 41 hours after the operation she was still alive, moving her legs, and thrusting out her sting when irritated. Ichneumon otiosus, after the removal of its head, remained very lively, and cleaned its wings and legs, the power of coördination in its wings and legs remaining. A horse-fly, a day after decapitation, was lively and flew about in a natural manner.[[44]]
When the abdomen is cut off, respiration in that region is not at first interrupted. The seat of respiratory movements was referred by Faivre to the hinder thoracic ganglion, but Plateau says that this view must be entirely abandoned, remarking: “All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and in insects a respiratory centre, for the somite to which it belongs” (Miall and Denny’s The Cockroach, p. 164).
The last pair of abdominal ganglia serve as the nervous centre of the nerves sent to the genital organs.
The recurrent or stomatogastric nerve, which, through the medium of the frontal ganglion, regulates digestion, has only a slight degree of sensibility; the insect remains quiet even when a powerful allurement is presented to the digestive tract (Kolbe).
Faivre states that the destruction of the frontal ganglion, or a section of the commissures connecting it with the brain, puts an end to swallowing movements; on the other hand, stimulation results in energetic movements of this nature.
Yersin, by cutting through the commissure in different places, and thus isolating the ganglia of the nervous cord of Gryllus campestris, arrived at the following results:—
1. The section of a nerve near its origin rendered the organ supplied by this nerve incapable of performing its functions.
2. If the connectives between two ganglia, i.e. the second and third thoracic ganglia, are cut through, the fore as well as hinder parts of the body retain their power of motion and sensation; but a stimulus applied to the anterior part of the body does not pass to the hinder portion.
3. Insects with an incomplete metamorphosis after section of the connectives are not in every case unable to moult and to farther develop.
4. If only one of the two connectives be cut through, the appendages of the side cut through which take their origin between the place injured and the hinder end of the body, often lose sensation and freedom of motion, or the power of coördination of movements becomes irregular. Sometimes this is shown by an unsteadiness in the gait, so that the insect walks around in a circle; after a while these irregularities cease, and the movements of the limbs on the injured side are only slightly restrained. By a section of both connectives in any one place the power of coördination of movements is not injured.
5. The section of the connectives appear to have no influence on nutrition, but affects reproduction, the attempt at fertilization on the part of the male producing no result, and the impregnated female laying no eggs.
6. Injury to the brain, or to the subœsophageal, or one of the thoracic ganglia, is followed by a momentary enfeeblement of the ganglion affected. Afterwards there results a convulsive trembling, which either pervades the whole body or only the appendages innervated by the injured ganglion.
7. As a result of an injury to the brain there is such a lack of steadiness in the movements that the insect walks or flies in a circle; for instance, a fly or dragon-fly thus injured in flying describes a circle or spiral. Steiner, in making this experiment, observed that the insect circled on its uninjured side. The brain is thus a motor centre.
8. By injuring a thoracic ganglion, one or all the organs which receive nerves from the ganglion are momentarily weakened. Afterwards the functions become restored. Sometimes, however, the insect walks in a circle. Faivre observed that after the destruction of the metathoracic ganglion of Dyticus marginalis the hind wings and hind legs were partially paralyzed (Kolbe, ex Yersin).