“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.”

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).

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).

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.

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.

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).

LITERATURE ON THE NERVOUS SYSTEM

a. General

Newport, George. On the nervous system of the Sphinx ligustri L., and on the changes which it undergoes during a part of the metamorphoses of the insect. (Phil. Trans. Roy. Soc., London, 1832, pp. 383–398; 1834, pp. 389–423, Pls.)

Helmholtz, H. L. F. De fabrica systematis nervosi evertebratorum. Diss. in aug. Berolini, 1842.

Blanchard, E. Recherches anatomiques et zoologiques sur le système nerveux des animaux sans vertèbres. Du système nerveux des insectes. (Annales des Sciences nat., Sér. 3, v, 1846, pp. 273–379, 8 Pls.)

—— Du système nerveux chez les invertèbres dans ses rapports avec la classification de ces animaux. Paris, 1849.

—— in Cuvier’s Règne animal. (Edition accompagnée de planches gravées. Insectes. Pl. 3, 3a, and 4.)

Leidy, Joseph. History and anatomy of the hemipterous genus Belostoma. (Memoirs Amer. Acad. Arts and Sc., N. S. iv, 1849, pp. 57–67, 1 Pl.)

Scheiber, S. H. Vergleichende Anatomie und Physiologie der Œstridenlarven. (Sitzungsb. k. Akad. wiss. Wien. Math.-Naturwiss. Cl., xli, 1860, pp. 439–496; xlv, 1862, pp. 7–68; 5 Taf.)

Tullberg, Tycho. Sveriges Podurider. (K. Svenska vet. Akad. Handl. x, 1872, pp. 1–70, 12 Taf.)

Berlese, A. Osservazione sulla anatomia descrittiva del Gryllus campestris L. (Atti della soc. Veneto-Trentina, 1880, vii, pp. 200–299.)

Baudelot, E. Contributions à la physiologie der système nerveux des insectes. (Revue d. sc. nat., i, pp. 269–280, 1872.)

Studer, Th. Ueber Nervenendigung bei Insekten. Kleine Beiträge zur Histologie der Insekten. (Mitt. Naturf. Ges., Bern, 1874, pp. 97–104, 1 Taf.)

Brandt, E. Recherches anatomiques et morphologiques sur le système nerveux des insectes Hyménoptères. (Compt. rendus de l’Acad. Sc., Paris, 1875.)

—— Ueber das Nervensystem der Apiden. (Sitzungsb. d. naturf. Ges., in Petersbourg, vii, 1876.)

—— Ueber das Nervensystem der Schmetterlingsraupen. (Verhandl. der Russ. Ent. Gesellsch., x, 1877. Also 16 other articles with plates, in Horæ Soc. Ent. Ross., 1878–1882.)

Mark, E. L. The nervous system of Phylloxera. (Psyche, ii, pp. 201–207, 1879.)

Riley, Charles Valentine. The nervous system and salivary glands of Phylloxera. (Psyche, ii, pp. 225, 226, 1879.)

Cholodkowsky, N. Zur Frage über den Baue und über die Innervation der Speicheldrüsen der Blattiden. (Horæ Soc. Ent. Ross., 1881, xvi, pp. 6–9, 2 Taf.)

Liénard, V. Constitution de l’anneau œsophagien. (Archives de Biologie, i, pp. 381–391, 1880, 1 Taf.)

Michaels, H. Nervensystem von Oryctes nasicornis im Larven-, Puppen-, und Käferzustande. (Zeits. f. wissens. Zool., xxxiv, 1880, pp. 641–702, 4 Taf.)

Rossi, A. Sul modo di terminare dei nervi nei muscoli dell’ organo sonoro della Cicala commune (Cicada plebeja). (Mem. accad. sc. Bologna, 1880, 4 Ser., i, pp. 661–665.)

Foettinger, A. Sur le termination des nerfs dans les muscles des insectes. (Archiv de Biologie, i, 1880.)

Binet. Contribution à l’étude der system nerveux sous intestinal des insectes. (Journ. l’anat. et phys., xxx, pp. 449–580, 1894.)

Paulowa. Zum Bau des Eingeweide Nervensystems der Insekten. (Zool. Anzeiger., xviii, Feb. 25, 1895, pp. 85–87.)

Also the writings of Lyonet, Cuvier, Rolando, Straus-Durckheim, Leydig, Newport, Graber, Viallanes, Grassi, Oudemans.

b. The brain

Dujardin, F. Mémoires sur le système nerveux des insectes. (Annales des Sciences nat, Sér. 3, 1850, xiv, pp. 195–206, Pl. 1, 1850.)

Rabl-Rückhard. Studien über Insectengehirne. (Archiv für Anatomie, Physiologie, etc., herausg. von Reichert u. R. du Bois-Raymond, 1876, p. 480, Taf. i.)

Dietl, M. J. Die Organization des Arthropodengehirns. (Zeitschr. wissens. Zool., xxvii, 1876, p. 488, Taf. xxxvi.-xxxviii.)

Flogel, T. H. L. Ueber den einheitlichen Bau des Gehirns in den verschiedenen Insectenordnungen. (Zeitschr. wissens. Zool., xxx, Suppl., 1878, p. 556, Taf. xxiii, xxiv.)

Newton, E. T. On a new method of constructing models of the brains of insects, etc. (Journ. Quekett Microscopical Club, pp. 150–158, 1879.)

—— On the brain of the cockroach, Blatta orientalis. (Quart. Journ. Microscopical Science, July, 1879, p. 340, Pl. xv, xvi.)

Packard, A. S. The brain of the locust. (Chapter xi, Second Report of the U. S. Entomological Commission, pp. 223–242, Pls. ix-xv, 1880.)

Cuccati, Giovanni. Sulla stuttura del ganglio sopraesofageo di alcuni ortotteri. (Acrydium lineola, Locusta viridissima, Locusta (species?), Gryllotalpa vulgaris, Bologna, 1887, 4º, pp. 1–27, Pl. i-iv.)

—— Intorno alla struttura del cervello della Sonomya erythrocephala, nota preventiva. Bologna, 1887.

—— Ueber die Organization des Gehirns des Sonomya erythrocephala. (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 240–269, 2 Taf.)

Viallanes, H. Études histologiques et organologiques sur les centres nerveux et les organes des sens des animaux articulés.

1. Mémoire. Le ganglion optique de la langouste (Palinurus vulgaris). (Annal. d. Sc. Nat. Zool., 1884, 6^e Sér., xvii, Art. 3, pp. 1–74, 5 Pls.)

2. Mémoire. Le ganglion optique de la Libellule (Æschna maculatissima). (Ibid., 1885, 6^e Sér., xviii, Art. 4, pp. 1–34, 3 Pls.)

3. Mémoire. Le ganglion optique de quelques larves de Diptères (Musca, Eristalis, Stratiomys). (Ibid., 1886, 6^e Sér., xix, Art. M. 4, pp. 34, 2 Pls.)

4. Mémoire. Le cerveau de la guêpe (Vespa crabro et vulgaris). (Ibid., 1887, 7^e Sér., ii, pp. 5–100, 6 Pls.)

5. Mémoire. 1. Le cerveau du criquet (Œdipoda cœrulescens et Caloptenus italicus). 2. Comparaison du cerveau des Crustacés et des Insectes. 3. Le cerveau et la morphologie du squelette céphalique. (Ibid., 1888, 7^e Sér., iv, pp. 1–120, 6 Pls.)

—— Sur la structure interne du ganglion optique de quelques larves de Diptères. (Bull. Soc. Phil., Paris, 1885, 7^e Sér., ix, pp. 75–78.)

—— La structure du cerveau des Hyménoptères. (Bull. Soc. Philomat., Paris, 1886, 7^e Sér., x, pp. 82, 83.)

—— La structure du cerveau des Orthoptères. (Bull. Soc. Philomat., Paris, 1886, 7^e Sér., xi, pp. 119–126.)

—— Sur la morphologie comparée du cerveau des Insectes et des Crustacés. (Compt. rend. Acad. Sc. Paris, 1887, civ, pp. 444–447.)

Kenyon, F. C. The meaning and structure of the so-called “mushroom bodies” of the hexapod brain. (Amer. Naturalist, xxx, 1896, pp. 643–650, 1 fig.)

—— The brain of the bee. (Journ. Comp. Neurology, vi, fasc. 3, 1896, pp. 133–210.)

—— The optic lobes of the bee’s brain in the light of recent neurological methods. (Amer. Nat., xxxi, 1897, pp. 369–376, 1 Pl.)

With the embryological works of Graber, Heider, Korscheldt, Patten, Wheeler, etc.

c. Histology of the nervous System

Helmholtz. De fabrica systematis nervosi evertebratorum. Diss. Berolini, 1842.

Remak. Ueber d. Inhalt d. Nervenprimitivröhren. (Archiv f. Anat. u. Phys., 1843.)

Leydig. Lehrbuch der Histologie der Menschen und der Thiere. 1857.

—— Vom Bau des thierischen Körpers. i. 1864.

—— Tafeln zur vergleichenden Anatomie. i. Tübingen, 1864.

—— Zelle und Gewebe, neue Beiträge zur Histologie des Tier-Körpers. Bonn, 1885, pp. 219, 6 Taf.

Walter. Mikroscopische Studien über das Centralnervensystem wirbelloser Thiere. 1863.

Dietl, M. J. Die Gewebselemente des Centralnervensystems bei wirbellosen Thieren. (Aus den Berichten des naturw.-medic. Vereins in Innsbruck.) Innsbruck, 1878.

Berger. Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Arbeiten des zool. Instituts zu Wien, Heft 2, p. 173, 1878.)

—— Nachtrag zu den Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. (Ibid., Heft 3.)

Viallanes, H. Recherches sur l’histologie des insectes, etc. Paris, 1882. (Annales des Sciences nat., pp. 1–348, Pls. 1–18.)

—— Sur la structure de la substance ponctuée des insectes. Paris, 1885.

Haller, B. Ueber die sogenannte Leydig’sche Punktsubstantz im Centralnervensystem. (Morp. Jahrb., xi, 1886.)

Nansen, F. The structure and combination of the histological elements of the central nervous system. (Bergen’s Museum Aarsberetning for 1886. Bergen, 1887.)

Also the writings of Benedicenti, Holmgren.

a. The eyes and insect vision

Of the eyes of insects there are two kinds, the simple and the compound. Of the former there are usually three, arranged in a triangle near the top of the head, between the compound eyes (Fig. 259, B). The compound or facetted eyes, which are usually round and prominent, differ much in size and in the number of facets.

The number of facets varies from 12 in Lepisma,—though in a Brazilian beetle (Lathridius) there are only seven unequal facets,—to 50 in the ant, and up to 4000 in the house-fly, 12,000 in Acherontia atropos, 17,000 in Papilio, 20,000 in the dragon-fly (Æschna), 25,000 in a beetle (Mordella), while in Sphinx convolvuli, the number reaches 27,000. The size of the facets seems to bear some relation to that of the insect, but even in the smallest species none have been observed less than 1
2000 of an inch in diameter. Day-flying Lepidoptera have smaller facets than moths (Lubbock).

The simple, or single-lensed eye (ocellus).—Morphologically the simple eye is a modified portion of the ectoderm, the pigment enclosing the retinal cells arising from specialized hypodermal cells, and covered by a specialized transparent portion of the cuticula, forming the corneal lens. The apparatus is supplied with a nerve, the fibres of which end in a rod or solid nerve-ending, as in other sensory organs.

As seen in the ocellus of Dyticus (Fig. 260), under the corneal lens the hypodermis forms a sort of pit, and the cells are modified to form the vitreous body (vitrella) and retina. Each retinal cell (re) is connected with a fibre from the optic nerve, contains pigment, and ends in a rod directed outwards towards the lens. The cells at the end of the pit or depression are, next to the lens, without pigment, and, growing in between the retina and the lens, fill it up, and thus form a sort of vitreous body.

The ocellus appears to be a direct heirloom from the eyes of worms, while the many-facetted compound eye of the crustaceans and of insects is peculiar to these classes. The compound eye of the myriopod Scutigera differs structurally in many respects from the compound eye of insects, and that of Limulus still more so.

It should be observed that in the young nymph of Ephemera, as well as in the semipupa of Bombus, each of the three ocelli are situated on separate sclerites. In Bombus the anterior ocellus has a double shape, being broad, transversely ovate, and not round like the two others, as if resulting from the fusion of what were originally two distinct ocelli.

The ocelli are not infrequently wanting, as in adult Dermaptera, in the Locustidæ, and in certain Hemiptera (Hydrocora). In Lepidoptera there are but two ocelli; in geometrid moths they are often atrophied, and they are absent in butterflies (except Pamphila).

The compound or facetted eye (ommateum).—The facetted arthropod eye is wonderfully complex and most delicately organized, being far more so than that of vertebrates or molluscs. The simplest or most primitive facetted eye appears to be that of Lepisma. As stated by Watase, the compound eye of arthropods is morphologically “a collection of ectodermic pits whose outer open ends face towards the sources of light, and whose inner ends are connected with the central nervous system by the optic nerve fibres.”

The facetted eye is composed of numerous simple eyes called ommatidia, each of which is complicated in structure. The elements which make up an ommatidium are the following: (1) The facet or cornea, which is a specialized portion of the cuticula; and (2), the crystalline lens or cone; (3), the nerve-ending or retinula, which is formed out of the retinula cells and the rhabdom or rod lying in its axis; and (4) of the pigment enclosing the lens and rod; the last three elements are derived from the hypodermis. The single eyes are separated from each other by pigment cells.

The facet or cornea.—This is biconvex, clear, transparent, usually hexagonal in outline, and refracts the light. The corneal lenses are cast in moulting.

The corneal lenses are circular in most cases where they are very convex, as in Lathridius and Batocera. The hexagonal ones are very irregular. When they are very convex the eye has a granular appearance, but when not greater than the convexity of the eye itself, the eye appears perfectly smooth (Bolbocerus, etc.). The facets in the lower part of the eye of Dineutes are a trifle larger than in the upper part (about nine to ten). In many insects the reverse is the case, the upper facets being larger than the lower, a notable instance being Anax. The intervening lines between the facets are often beset with hairs, sometimes very long and dense, as in the drone bee and Trichophthalmus; and the modifications of the hairs into scales which takes place on the body occurs on the eyes also, the scales on the eyes of some beetles of the family Colydiidæ being very large, arranged in lines over the eyes like tombstones (Trachypholis).^[45]

The crystalline lens or cone.—Behind or within the facets is a layer composed of the cones, behind which are the layers of retinulæ and rhabdoms, and which correspond to the layer of rods and cones, but not the retina as a whole, of vertebrate animals.

The crystalline lens is, when present, usually more or less conical, and consists of four or more hypodermis-cells.

The cones are of various shapes and sizes in insects of different groups, or are entirely wanting, and Grenacher has divided the eyes of insects into eucone, pseudocone, and acone. As the pseudocone seems, however, to be rather a modification of the eucone eye, the following division may be made:—

1. Eucone eyes, comprising those with a well-developed cone. They occur in Lepisma, Blatta (Fig. 262), and other Orthoptera, in Neuroptera, in Cicadidæ, in those Coleoptera with five tarsal joints, in the dipterous genus Corethra, and in the Lepidoptera and Hymenoptera (Fig. 263).