Chapter XIII: Part I: Morphology and Physiology (9)

A. G. Mayer believes that the pigments of the scales are derived from the hæmolymph or blood of the pupa, for the following reasons: (1) He is unable to find anything but blood within the scales during the time when the pigment is formed. (2) In Lepidoptera generally the first color to appear upon the pupal wings is a dull ochre-yellow, or drab, and this is also the color assumed by the blood when it is removed from the pupa and exposed to the air. (3) He has succeeded by artificial means in manufacturing several pigments from the blood which are similar in color to various markings upon the wing of the imago; chemical reagents have the same effect upon these manufactured pigments that they do upon the similarly colored pigments of the wings. “It should be here noted,” he says, “that in 1866 Landois pointed out the fact that the color of the dried blood of many caterpillars is similar to the ground color of the wings of the mature insect.”

=Ontogenetic and phylogenetic development of colors.=—The colors of the wings of Lepidoptera, as is well known, are acquired at the end of the pupal state. The order of development of the colors in the pupal wings has been observed by Schaeffer, Van Bemmelen, Urech, Haase, Dixey, Spuler, and A. G. Mayer. The immature wings are at first transparent and full of protoplasm. The transparent condition of the wings corresponds to the period before the scales are formed, and when they are full of protoplasm; they then become whitish as the scales develop; the latter are at first filled with protoplasm, and afterwards turn whitish, being little hollow sacks filled with air. After the protoplasm has completely withdrawn from the scales, the blood of the pupa enters them, and then the coloring-matter forms. (Mayer.) He adds that “about twenty-four hours after the appearance of the dull yellow suffusion the mature colors begin to show themselves. They arise, faint at first, in places near the centre of the wings, and are distinguished by the fact that they first appear upon areas between the nervures, never upon the nervures themselves. Indeed, the last place to acquire the mature coloration are the outer and costal edges of the wings, and the nervures.”

The faint color of the scales gradually increases in intensity. “For example, if a scale be destined to become black, it first becomes pale grayish brown, and this color gradually deepens into black.”

Urech states that in _Vanessa io_ first a white, and in _V. urticæ_ a pale reddish hue, are spread over the entire wings, and then successively arise other colors in the following order: yellow, yellow to brown, red, brown and black.

Spuler, however, claims that the differentiation of colors and markings do not follow one another, but arise simultaneously, and that his view is confirmed by Fischer. This may be the case with the highly specialized and diversely marked butterflies, but certainly taking the Lepidoptera as a whole the yellows and drabs must have been the primitive hues, the other colors being gradually added in the later more specialized forms.

It is noticeable that the most generalized moths, such as the species of Micropteryx, Tinea, Psychidæ, Hepialidæ (in general), etc., are dull brown or yellow-drab without bars, stripes, or spots of bright hues. These shades prevail in others of the more primitive Lepidoptera, such as many bombycine moths, and they even appear to a slight extent in certain caddis-flies. The authors mentioned, especially Mayer, whom we quote, claim that “dull ochre-yellows and drabs are, phylogenetically speaking, the oldest pigmental colors in the Lepidoptera; for these are the colors that are assumed by the hæmolymph upon mere exposure to the air. The more brilliant pigmental colors, such as bright yellow, reds, greens, etc., are derived by more complex chemical processes. We find that dull ochre-yellow and drabs are at the present day the prevalent colors among the less differentiated nocturnal moths. The diurnal forms of Lepidoptera have almost a monopoly of the brilliant colorations, but even in these diurnal forms one finds that dull yellow or drab colors are still quite common upon those parts of their wings that are hidden from view.”

The more primitive moths being more or less uniformly yellowish or drab, the next step was the formation of bars, stripes, finally spots, and eyed spots, these markings in the later forms appearing simultaneously in one and the same species of certain highly specialized moths and butterflies. All that has been said will prepare the reader for the consideration of the subject of insect coloration. The origin of such markings has been discussed by Weismann, Eimer, Haase, Dixey, Fischer, and others.

LITERATURE

=Heer, O.= Einfluss des Alpenklimas auf die Farbe der Insecten.
(Froebel u. Heer, Mitth. aus dem Gebiete der theoret. Erdkunde,
1836, i, pp. 161–170.)

=Goureau.= Mémoire sur l’irisation des ailes des insectes. (Ann. Soc.
Ent. France, 2 sér., i, 1848, pp. 201–215.)

=Laboulbène, A., et M. Follin.= Note sur la matière pulvérulente qui
recouvre la surface du corps des Lixus et de quelques autres
insectes. (Ann. Soc. Ent. de France, 1848, vi, pp. 301–305, Fig.)

=Coquerel, Ch.= Note sur la prétendue poussière cryptogamique qui
recouvre le corps de certains insectes. (Ann. Soc. Ent. France,
1850, viii, pp. 13–15.)

=Brauer, F.= Beobachtungen in Bezug auf den Farbenwechsel bei
_Chrysopa vulgaris_. (Verhandl. k. k. zool.-botan. Gesellsch. Wien.,
1852, pp. 12–14.)

=Prittwitz, O. F. W. v.= Bemerkungen über die geographische
Farbenverteilung unter den Lepidopteren. (Stett. Ent. Zeit., 1855,
xvi, pp. 175–185.)

=Latham, A. G.= The causes of the metallic lustre of the scales on the
wings of certain moths. (Proc. Lit. and Phil. Soc. Manchester, iii,
1864, pp. 198–199. Quart. Journ. Micr. Sc., new ser., iv, 1864, pp.
48–49.)

=Sorby, H. C.= On the coloring matter of some Aphides. (Quart. Journ.
Micr. Sc., new ser., xi, 1871, pp. 352–361.)

=Leydig, Franz.= Bemerkungen über Farben der Hautdecke und Nerven der
Drüsen bei Insekten. (Archiv f. mikr. Anatomie, xii, 1876, pp.
536–550, 1 Taf.)

=Weismann, A.= Studien zur Descendenz-Theorie, ii, 1876.

=Hemmerling, Hermann.= Ueber die Hautfarbe der Insecten. (Bonn, 1878,
p. 27.)

=Buckton, C. B.= Monograph of the British Aphides. (London, 1879, ii,
p. 167.)

=Cameron, P.= Notes on the coloration and development of insects.
(Trans. Ent. Soc. London, 1880, pp. 69–79.)

=Hagen, Hermann A.= On the color and pattern of insects. (Proc. Amer.
Acad. Arts and Sc., 1882, pp. 234–267.)

=Poulton, Edward Bagnall.= The essential nature of the colouring of
phytophagous larvæ (and their pupæ), etc. (Proc. Roy. Soc. London,
xxxviii, pp. 269–315, 1884–1885.)

—— An inquiry into the cause and extent of a special colour-relation
between certain exposed lepidopterous pupæ and the surfaces which
immediately surround them. (Phil. Trans. Roy. Soc. London, clxxviii,
pp. 311–441, 1 Pl., 1887.)

=Krukenberg, C. Fr. W.= Grundzüge einer vergleichenden Physiologie der
Farbstoffe und der Farben. (Heidelberg, 1884, pp. 102.)

=McMunn, C. A.= Krukenberg’s chromatological speculation. (Nature,
xxxi, p. 217, 1885.)

=Müller, Fritz, and Dr. H. A. Hagen.= The color and pattern of
insects. (Kosmos, xiii, 1886, pp. 466–469.)

=Slater, J. W.= On the presence of tannin in insects and its influence
on their colors. (Trans. Ent. Soc. London, 1887, iii, Proceed., pp.
32–34.)

=Bemmelen, J. F. van.= Ueber die Entwicklung der Farben und Adern auf
den Schmetterlingsflügeln. (Tijdschrift der nederland. Dierkundige
Vereeniging, ser. 2, pp. 235–247, 1889.)

=Hopkins, F. G.= Uric acid derivatives functioning as pigments in
butterflies. (Proc. Chem. Soc. London, 1889, p. 117; also Nature,
xl, p. 335.)

—— Pigment in yellow butterflies. (Nature, xlv, p. 197, 1891.)

—— The pigments of the Pieridæ. (Proc. Roy. Soc. London, lvii, No.
340, pp. 5, 6, 1894. Phil. Trans. Roy. Soc. London, clxxxvi, pp.
661–682, 1896.)

=Coste, F. H. P.= Contributions to the chemistry of insect colors.
(The Entomologist, xxiii, 1890; xxiv, 1891, pp. 9–15, etc. Nature,
xlv., pp. 513–517, 541–542, 605.)

=Urech, F.= Beobachtungen über die verschiedenen Schuppenfarben und
die zeitliche Succession ihres Auftretens. (Zool. Anzeiger, xiv, pp.
466–473, 1891; Ibid., August 1, 1892.)

—— Beiträge zur Kenntniss der Farbe von Insektenschuppen. (Zeits. f.
Wissens. Zool., lvii, pp. 306–384, 1893.)

=Griffiths, A. B.= Recherches sur les couleurs de quelques insectes.
(C. R. Acad. Sc. Paris, cxv, pp. 958, 959.)

=Mayer, Alfred Goldsborough.= On the color and color-patterns of moths
and butterflies. (Proc. Bost. Soc. Nat. Hist., xxvii., March, 1897,
pp. 243–330, 10 Pls. See also p. 201 under Mayer.)

Also the writings of Bates, Beddard, Belt, Butler, Darwin, Dimmock,
Dixey, Eimer, Haase, Higgins, Müller, Poulton, Seitz, Wallace,
Weismann.

2. INTERNAL ANATOMY

THE MUSCULAR SYSTEM

In its general arrangement the muscular system of insects corresponds to the segmented structure of the body. Of the muscles belonging to a single segment, some extend from the front edge of one segment to that of the next behind it, and others to the hinder edge; there are also sets of dorsal and ventral muscles passing in an oblique or vertical course (Figs. 16–18). As Lang observes, “the greater part of the muscles of the body can be traced back to a paired system of dorsal and ventral intersegmental longitudinal muscles.” The muscular system is simplest in larval insects, such as caterpillars, where the musculature is serially repeated in each segment.

In the larva of Cossus Lyonet found on one side of the body 217 dorsal, 154 lateral, 369 ventral, and in the thoracic legs 63, or 803 muscles in all. “Adding to this number the 12 small muscles of the second segment, and 8 others of the third, which he did not describe, there would be for all the muscles on one side of the caterpillar 823. This would make for the entire body 1646, without counting a small single muscle which occurs in the subdivision of the last segment,” and also those of the internal organs as well as those of the head, so that the total number probably amounts to about 2000, not 3000, as usually stated in the books. Lubbock admits that Lyonet was right in his mode of estimating the number. In the larva of _Pygærci bucephala_ he found that “the large muscles scarcely vary at all,” though certain smaller ones are very variable. Lubbock observed that certain of the longitudinal muscles in the caterpillar of Diloba split up into numerous, not less than ten, separate fascicles. “This separation of the fibres composing a muscle into separate fascicles is carried on to a much greater extent in the larvæ of Coleoptera. Of course in the imago the number of thoracic muscles is greatly increased, or at least in Dyticus and the wood-feeding Lamellicorns, which alone I have examined. In these two groups each of the larger muscles is represented by at least twenty separate fascicles, which makes it far more difficult to distinguish the arrangement of the muscles.”

The muscles are whitish or colorless and transparent, those in the thorax being yellowish or pale brown; and of a soft, almost gelatinous consistence. In form they are simply flat and thin, straight, band-like, or in rare cases pyramidal, barrel or feather shaped. They act variously as rotators, elevators, depressors, retractors, protractors, flexors, and extensors.

FIG. 231.—Diagram of the muscles and nerves of the ventral surface of
the segments in the larva of _Sphinx ligustri_: _A_, _A_, recti
muscles; 1, 2, ventral recti muscles (1, recti majores; 2, recti
minores); 3, ridge giving origin to recti muscles of one segment,
and insertion to the same of the adjoining segment; 4, ridge for
attachment of muscle; 5, retractor ventriculi, connecting the
mid-intestine with the outer integument of the body. _B_, 6, first
oblique,—7, second oblique,—9, 10, third oblique, muscles; 11,
fourth oblique,—12. third rectus,—13, fifth oblique,—14,
triangularis, muscle; 15, transversus medius; 16, transverse ridge;
17, transversi abdominales; 18, abdominales anteriores; 19, 20,
abdominales laterales, some (20) longer than others; 21, obliquus
posterior; 22, postero-laterales obliqui; 23, transversus lateralis;
24, second transversus lateralis; 25, retractor spiraculi, or
constrictor of the spiracles, attached by a long tendon (26); 27,
retractor valvulæ.

Nerves: _a_, ganglion,—_c_, transverse nerves, of which _p_ is the
first, _q_ the second, _r_ the third,
and _s_ the fourth branch; _t_, the main trunk, which crosses the
great longitudinal trachea, receives a
filament from the transverse nerve (_n_), and divides into two
branches (_t_);—some of these branches
form a small plexus (_u_); the nerve _t_ divides in two divisions (_p_
and _v_). The second division ends
in _w_ and _x_; the branch _q_ divides into _y_ and _z_. For other
explanations, see Newport, art. _Insecta_.—After
Newport.
]

FIG. 232.—Musculature of the European cockchafer, _Melolontha
vulgaris_: _a_, _a_, levatores capitis; _b_, depressores capitis;
_c_, rotatores capitis; _d_, depressors externi; _e_, retractor or
flexor of the jugular plate; _f_, oblique extensor of the jugular
plate; _g_, the other retractor of the jugular plate; _h_, retractor
prothoracis superior; _i_, inferior retractor, the proper depressor
of the prothorax; _k_, elevator prothoracis; _l_, one of the
rotatores prothoracis; _m_, _n_, _o_, flexors of the coxa; _x_,
great depressor muscle of the wing; _y_, _y_, elevators and
protractors attached to the metaphragma and base of the postfurca;
_z_, second flexor of hind leg; _a_, _a_, extensors of hind leg;
_c_, _c_, dorsal recti of abdomen. _Q_, ejaculatory duct; _R_,
penis; _S_, its prepuce. _M_, rectum.—After Straus-Durckheim, from
Newport.
]

Our knowledge of the muscular system of insects is still very
imperfect. To work it out thoroughly one should begin first with
that of Scolopendrella, then some generalized synapterous form, as
Japyx or Lepisma, then passing to that of a caterpillar, and ending
with some of the more highly specialized forms, such as a beetle,
etc. Thus far our knowledge is confined to that of the caterpillars
(Lyonet, Newport, and Lubbock) and the beetle (Straus-Durckheim) and
ants (Forel, Lubbock, and Janet).

=Musculature of a caterpillar.=—Newport’s account of that of the
larva of _Sphinx ligustri_ is the most useful (Fig. 231). The
muscles here present, he says, great uniformity of size and
distribution in every segment, the motions of each of these
divisions of the body being almost precisely similar, especially in
the 4th to 9th trunk segments. In these segments the first layer
seen on removing the fat and viscera are the flat straight recti
muscles. They are the most powerful of all the trunk muscles, and
are those which are most concerned in shortening the body, in
effecting the duplicature of the external teguments during the
changes of the insect, and which during the larval state mainly
assist in locomotion. There are four sets, two dorsal and two
ventral (Fig. 231, _A_, _A_). Without entering into farther details,
the reader is referred to the works of Newport and to Fig. 231.

=Musculature of a beetle.=—The best general account of the
musculature of a perfect insect is that of Straus-Durckheim in his
famous work on the Melolontha. We will copy the summary of Newport,
who adopted the nomenclature applied to these parts by Burmeister:—

“The muscles that connect the head with the thorax are contained
within the prothorax (Fig. 232, 2), and are of three kinds,
extensors, flexors, and retractors. The extensors, _levatores
capitis_ (_a_, _a_), consist of two pairs, one of which arises from
the middle line of the pronotum, and diverging laterally from its
fellow of the opposite side, passes directly forwards, and is
inserted by a narrow tendon into the anterior superior margin of the
occipital foramen. The other arises further back from the
prophragma. It is a long, narrow muscle that passes directly
forwards through the prothorax, and is inserted by a tendon near the
superior median line of the foramen; so that, while this muscle and
its fellow of the opposite side elevate the head almost in a
straight line, the one first described, when acting alone or singly,
draws the head a little on one side; but when the whole of these
muscles act in unison, they simply elevate the head upon the
prothorax. The depressors or flexors, _depressores capitis_ (_b_),
are exceedingly short muscles, which arise from the jugular plate,
or, when that part does not exist, from the border of the
prosternum, and are attached to the inferior margin of the occipital
foramen. They simply flex the head on the prothorax. The lateral
flexors, _depressores externi_ (_d_), are two little muscles that
arise from the same point as the preceding, and are attached to the
lateral inferior margin of the occipital foramen. The rotatory
muscles, _rotatores capitis_ (_c_), are two flat muscles like the
elevators, which arise, one at the side of the antefurca and the
other from the posterior jugular plate, and passing upwards and
outwards are attached to the lateral margin of the occipital
foramen. The _retractor_ or flexor of the jugular plate is a small
muscle (_e_) that arises from the margin of the antefurca, and
passing directly forwards is inserted by a small tendon into the
middle of the jugular piece. The _oblique extensor_ of the jugular
plate is a long, slender muscle (_f_) that arises from the external
margin of the pronotum, and passing obliquely downwards and forwards
traverses the prothorax and is inserted by a narrow tendon to the
jugular plate immediately before the retractor. The other retractor
(_g_) arises from the anterior superior boundary of the pronotum,
and passing downwards is inserted into the jugular plate between the
larger levator and _flexor capitis_.

“The muscles proper to the prothorax consist of four pairs, by which
it is united to the succeeding segments. The first of these, the
superior retractor, _retractor prothoracis superior_ (_h_), arises
by a broad, fleshy head from the anterior external margin of the
pronotum, and passing directly backwards is inserted by a tendon
into the prophragma, a little on one side of the median line. The
next muscle of importance, the inferior retractor (_i_), arises from
the anterior border of the medifurca, and is united to the posterior
of the antefurca, thus forming with that muscle part of the great
recti of the larva. This muscle must be considered as the proper
depressor of the prothorax. The _elevator prothoracis_ (_k_) is
narrow, pyramidal, and arises fleshy from the lateral surface of the
prophragma. It passes downwards and is attached by a narrow tendon
to the superior portion of the antefurca. The _rotatores
prothoracis_ are the largest of all the muscles of this segment.
They arise, one on each side (_l_), by a narrow head from the
posterior part of the pronotum, and passing beneath the prophragma
are considerably enlarged and attached to the tegument between the
two segments, and also to the anterior portion of the mesothorax.
The remaining muscle proper to the prothorax is the closer of the
spiracle, an exceedingly small muscle not shown in the drawing.

“The other muscles of this segment are those of the legs, which are
of considerable size. There are three distinct flexors of the coxa
(_m_, _n_, _o_). The first of these arises from the superior lateral
border of the pronotum, the second from the superior posterior
border, the third from the sides of the prothorax, and the fourth a
little nearer posteriorly, and the whole of them are attached by
narrow tendons to the sides of the coxa. But there is only one
extensor muscle to this part. In like manner, the extensor of the
trochanter is formed of three portions (Fig. 233, _a_, _b_, _c_);
but there is only one flexor (_d_), and one abductor (_e_). In the
femur, there is one extensor (_f_),—a long penniform muscle that
occupies the superior part of the thigh, and is attached by a tendon
to the anterior-posterior margin of the joint formed by the end of
the tibia. There is also but one flexor (_g_) in the femur, which,
like the preceding muscle, is penniform, and occupies the inferior
portion of the femur, and its tendon is attached to the inferior
border of the tibia. In the tibia itself there is also one flexor
and one extensor. The _flexor_ (_i_) occupies the superior portion
of the limb, and ends in a long tendon (_l_) that passes directly
through the joints of the tarsus, on their inferior surface, and is
attached to the inferior margin of the claw (_g_). The _extensor_
(_h_) occupies the inferior portion of the tibia, and is shorter
than the preceding muscle, like which it ends in a long tendon that
is attached to the upper margin of the claw. Besides these muscles,
which are common to the joints of the tarsus, there are two others
belonging to the claw, situated in the last joint. The first of
these, the _extensor_ (_m_), is short, and occupies the superior
portion of the last phalanx of the tarsus, and the other, the
_flexor_ (_n_), is a much longer penniform muscle, which occupies
nearly the whole of the upper and under surface of the posterior
part of the phalanx, and is attached, like the long flexor of the
tarsus, to the inferior part of the claw.”

FIG. 233.—Muscles of the fore leg of _Melolontha vulgaris_: _a_,
_b_, _c_, three divisions of the extensor of the trochanter; _d_,
flexor,—_e_, abductor, of the trochanter; _f_, extensor of the
femur; _g_, flexor of the femur; _h_, extensor of the tibia; _i_,
flexor of the tibia; _l_, tendon attached to the lower edge of the
claw (_g_); _m_, extensor,—_n_, flexor, of the claw.—After
Straus-Durckheim, from Newport.
]

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).

FIG. 234.-Section through the prothorax of _Diapheromera femoratum_:
_prov_, proventriculus; _tr_, trachea; _n. c_, nervous cord; _s.
gl_, salivary gland; _hyp_, hypodermis; _ur. t_, urinary tube; _ht_,
heart; _m_, _m″_, _m‴_, muscles for lowering and raising the tergum;
_m′_, another muscle, its use unknown.
]

FIG. 235.—Striated muscular fibre of Hydrophilus: _A_ and _B_, two
fibrillæ in a state of extension; _a_, thick disk; _b_, thin disk;
_c_, intermediate space. _C_, _D_, portion of the same fibrillæ seen
by moving the objective farther away and using a small diaphragm;
_n_, thick; _c_, thin disk. × 2000 diam.—After Ranvier, from
Perrier. _E_ after Gehuchten, from Lang.
]

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.

FIG. 236.—Preparations from the adductor muscle of the mandible of
_Vespa crabro_, worker, fixed by heat and alcohol several hours
after leaving its cell. _A_ to _E_ × 425; _F_ × 212: _A_, terminal
cupule of the tendon of a fibre. _B_, _C_, union of the fibres with
their tendon. _D_, branch of the tendon of a muscle sending out
tendons of some of the fibres; this branch is accompanied with
numerous nervous ramifications (_N_). _E_, fragment of a nerve which
furnishes the ramifications of Fig. _D_. _F_, fragment of the tendon
of the adductor muscle of the mandible; at the left are seen the
terminal cupules of the fibres (_td, c_); on the right, on the body
of the tendons, some sessile cupules, each of which forms the
attachment of a fibre; _td, b_, tendons of the fibres.—After Janet.
]

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).

FIG. 237.—_Vespa crabro_, worker, fixed by heat and alcohol some hours
after leaving its cell. _A_ × 425; _B_ to _D_ × 850 times: _A_,
muscular fibre of the motor muscles of the mandibles treated, for
ten minutes, by 1 per cent potassium to bring out the reticulum; the
nodes of union of the rayed filaments with the longitudinal
filaments are indicated by distinct granulations (_l.d_), and these
longitudinal filaments present accessory thickenings (_d.a_); T,
trachea; N, junction of a nervous filament with the muscular fibres.
_B_, fibre of the same muscle, not treated with potassium, stained
by hæmatoxylin; _C_, transverse section of a disk at the level of a
layer of rayed filaments; _Sarc_, sarcolemma. _D_, transverse
section of a disk at the level of the rods; _nuc_, nucleus.—After
Janet.
]

1. _Carabus auratus_ 17.4.
2. _Nebria brevicollis_ 25.3.
1. _Cetonia aurata_ 15.
2. _Trichius fasciatus_ 41.3.
1. _Melolontha vulgaris_ 14.3.
2. _Anomala frischii_ 24.3.
1. _Oryctes nasicornis_ 4.7.
2. _Geotrupes stercorarius_ 9.8.
3. _Onthophagus nuchicornis_ 14.4.
1. _Necrophorus vespillo_ 15.1.
2. _Silpha livida_ 24.4.
1. _Ocypus morio_ 17.
2. _Quedeus fulgidus_ 29.6.
1. _Donacia nymphææ_, 42.7.
2. _Crioceris merdigera_ 39.2.
1. _Bombus terrestris_ 16.1.
2. _Bombus rupestris_ 14.5.
3. _Apis mellifica_ 20.2.

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

_Oryctes nasicornis_ 3.2.
_Geotrupes stercorarius_ 28.4.
_Onthophagus nuchicornis_ 92.9.

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.

FIG. 238.—_Vespa crabro_, fixed and stained as in the subjects of the
other figures. _I_, _N_, _P_ × 1700; _H_, _J_, _M_ × 850; the others
× 425 times: _A-C_, motor muscles of the antennal scape. _D-P_,
motor muscles of the 3d coxa. _A_, _B_, the two ends, in very
different states of contraction, of the same fibre; on one side the
transverse striæ are near together, on the other very far apart.
_C_, a crushed and split fibre showing a fibrous appearance, owing
to the rupture of the radiated filaments, and the separation of the
longitudinal filaments. _D_, muscular disk seen in section, with two
rows of nuclei. _E_, a muscular fibre with three rows of nuclei.
_F_, a nucleus, accompanied with coagulated protoplasm, oozing from
a previous break of the muscular fibre. _G_, nerve-terminations very
near each other on the same muscular fibre. _H_, longitudinal
filaments, evenly covered with the coagulated substance, and
forming, throughout the mass of the fibre, continuous filaments.
_I_, filaments widely separated. _J_, longitudinal filaments showing
the beginning of one of the transverse breaks which isolate some of
the disks. _K_, oblique view of a disk obtained by such a break, and
of a fibre in circular section, with an axial row of nuclei; this
piece comprises three stages of radiated filaments. _L_, muscular
fibre with a row of nuclei; at the lower part, the nuclei have
issued from a longitudinal fissure in the fibre, and have remained
attached in a chain. _M_, edge of fibre in which there is quite a
large, clear space between the sarcolemma and the rods. _N_, passage
of the trachea, with the spiral thread, into three capillaries with
a smooth cuticula. _O_, elliptical disk from a fibre, with two rows
of nuclei, and showing a layer of radiated filaments. _P_, fragment
(highly magnified) of the edge of a disk seen in section.—After
Janet.
]

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]

LITERATURE ON THE MUSCLES

_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.

_c_. Muscular power of insects

=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.

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_:
_w^1_, nerve to 1st,—_w^2_, 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; _pc^2_, _pc^3_, 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. (_pc^2_, _pc^3_) 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; _g^2_, 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; _g^2_, 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;
_goc_^1, œsophageal ganglion, the dotted ring the œsophagus; _gv^1_,
_gc^2_, _gv^3_, 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.