These are the muscles of the prothorax, and its organs of locomotion. The reader is referred for a further account of the muscles of the hinder thoracic and of the abdominal segments to Straus-Durckheim’s original work.

Minute structure of the muscles.—The muscular fibres of insects are striated (Figs. 235–238), even those of the alimentary canal; the only notable exception being the alary muscles of the pericardial septum, while Lowne states that certain of the thoracic muscles of the blow-fly are not striated (Miall and Denny).

In describing the minute structure of the muscles of ants, wasps, and bees, C. Janet states that each consists of a group of fibres diverging from a tendon, which is an integumentary invagination (Fig. 236). Each fibre may be regarded as a multinucleate cell; the sarcolemma represents the cell-membrane. It forms a resistant and extremely elastic tube. The longitudinal (Fig. 236, E) and radiating filaments or reticulum (spongioplasm of Gehuchten) lie in a nutritive filling substance (the hyaloplasm of Gehuchten). The radiating filaments are formed of an exceedingly elastic substance, and serve to sustain the longitudinal filaments, to transmit the nervous stimulus to them, and to bring them back into position after contraction. Janet’s account agrees on the whole with that of Gehuchten.

The muscles of flight are said to be penetrated by fine tracheal branches, probably to supply a greater amount of oxygen, as the most energetic movements of the insect are made in moving the wings during flight; while the other muscles of the body are only surrounded by the air-tubes. (Sharp.)

Without entering into tedious details, the reader is referred to figures or references to the more important systems of muscles, such as those of the legs and other appendages, of the wings, of respiration, etc., to the sections treating of those organs or functions; also to Figs. 16, 17, 18, 22, 48, 74, 81, 83, 84, 115, 116, 172, 173, 174, etc.

Muscular power of insects.—The most detailed and careful experiments are those of Plateau. His experiments prove that even the weakest insects pull at least five times their own weight; many of them, however, get the better of a burden twelve to twenty fold as heavy as themselves, while a strong man or a draught horse, for example, is not even able to pull a burden which is equal to the weight of his body. Plateau came to the following results as to the relation of the weight of the body to the load drawn (1 and 2 are to be compared with each other, 1 being the larger, and 2 the smaller insect; it will be seen that the smaller insect is the stronger).

As regards the pushing power, the relation of the load to the size of the body in different large beetles, gave the following figures:—

The leaping force of locusts was found by Straus-Dürckheim to be in Œdipoda grossa as 1.6, in Œ. parallela as 3.3 of their weight.

A humble bee (Bombus terrestris) can carry while flying a load 0.63 of its own weight, and a honey bee 0.78; here, as usual, the smaller insect is the stronger.^[39]

a. General

Lyonet, P. Traité anatomique de la chenille. La Haye, 1762.

Cornalia, E. Monographia del Bombyce del gelso. (Mem. R. Instituto Lombardo Sc. Lett. ed Arte, 1856.)

Basch, S. Skelett und Muskeln des Kopfes von Termes. (Zeitschr. f. wissens. Zool., xv, 1865, pp. 55–75, 1 Taf.)

Lubbock, John. Arrangement of the cutaneous muscles of the larva of Pygæra bucephala. London, 1858. 2 Pls.

—— On some points in the anatomy of ants. (Month. Micr. Journ., xviii, pp. 121–142, 1877, 4 Pls.)

—— On the anatomy of ants. (Trans. Linn. Soc., Ser. 2; Zool., ii, 1879, pp. 141–154, 2 Pls.)

Poletajeff, N. Du développement des muscles d’ailes chez les Odonates. (Horæ Soc. Ent. Ross., xvi, 1879, pp. 10–37, 5 Pls.)

—— Die Flugmuskeln der Lepidopteren und Libelluliden. (Zool. Anzeiger, 1880, pp. 212, 213.)

—— Ueber die Flugmuskeln der Rhopaloceren. (Arbeiten d. Russ. Ent. Ges., 1881, xiii, p. 9, 1 Taf., in Russian.)

Lendenfeld, R. von. Der Flug der Libellen. (Sitzb. k. Akad. Wissens., 1 Abth. Wien, 1881, lxxxiii, pp. 289–376, 7 Taf.)

Luks, Constantine. Ueber die Brustmuskulature der Insekten. (Jena. Zeitschr. f. Naturwissen., xvi, N. Folge IX, 1883, pp. 520–552, 2 Taf.)

Carlet, G. Sur les muscles de l’abdomen de l’abeille. (Comptes rend., 1884, xcviii, pp. 758, 759.)

Janet, Charles. Sur les muscles des fourmis, des guêpes et des abeilles. (Comptes rend., cxxi, p. 610, 1 Fig., 1895.)

Also the writings of Straus-Durckheim, Newport, Graber, Burgess, Leydig, Dahl, Ockler, Dogiel, Dimmock, Kraepelin, Becher, Langer, Kolbe.

b. Histology

Aubert, H. Ueber die eigenthümliche Struktur der Thoraxmuskeln der Insekten. (Zeitschr. f. wissens. Zool., iv, 1853, pp. 388–399, 1 Taf.)

Verson, E. Zur Insertionsweise der Muskeln. (Sitzsb. Akad. d. wiss. math. naturw. Cl. Wien., lvii, 1 Abth., pp. 63–66, 1868.)

Künckel d’Herculais. Sur le développement des fibres musculaires striées chez les insectes. (Compt. rend. de l’Acad. Sc. Paris, lxxv, 1872.)

Grunmach, Emil. Ueber die Structur der quergestreiften Muskelfaser bei den Insekten. Berlin, 1872. pp. 47.

Fredericq, L. Note sur la contraction des muscles striés de l’Hydrophile. (Bull. Acad. Roy. Belgique, xli, p. 583, 2 Pls.)

Gehuchten, A. van. Étude sur la structure intime de la cellule musculaire striée. (La Cellule, ii, pp. 289, 293–453, 1886, 6 Pls.)

Janet, Charles. Études sur les fourmis, les guêpes et les abeilles. 12^e note. (Structure des membranes articulaires des tendons et des muscles, Limoges, 1895, pp. 25, 11 Figs.)

Also the writings of Burmeister, Chabrier, Leydig, Meckel, Lebert, Wagner, Wagener, Amici, Krause, Heppner, Retzius, Rollet, G. Elias Müller, F. Merkel, Hensen, Kölliker, Dogiel, Dönitz, Hagen, Vosseler, Bütschli u. Schewiakoff, Lowne, Ciaccio, Biedermann, Cohnbeim, Brücke, Haycraft, Melland, Bowman.

Plateau, Félix. Sur la force musculaire des insectes. (Bull. Acad. Roy. Belgique, 2 Sér. xx, 1865, pp. 732–757; xxii, 1866, pp. 283–308.)

—— Recherches sur la force absolue des muscles des invertébrés. 1884.

Radan, R. La force musculaire des insectes. (Revue de deux mondes, 2 Sér., lxiv, 1866, pp. 770–777.)

Bibiakoff, Paul von. Zur Muskelkraft der Insekten. (Natur, xvii, 1868, p. 399.)

Delbœuf. Nains et géants, Étude comparative de la force des petits et des grands animaux. Bruxelles. (Also in Kosmos, xiii, 1883, pp. 58–62.)

Camerano. Mem. Acc. Torino (2), xliii, 1893, p. 229.

Also Newport, Art. Insecta, p. 76. Kirby and Spence, Burmeister, Graber, Kolbe, pp. 375, 376.

a. The nervous system as a whole

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.

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.

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.

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.

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.

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.

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):—

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).

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.

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.