Coste, and more especially Urech, have s_A Text-book of Entomology Including the Anatomy, Physiology, Embryology and Metamorphoses of Insects for Use in Agricultural and Technical Schools and Colleges as Well as by the Working Entomologist

may be dissolved out of the scales by means of chemical reagents, giving

colored solutions, and leaving the scales white or colorless. They have

also shown that some of these pigments may be changed in color by the

action of reagents, and then restored to their original color by other

reagents. They have proved that reds, yellows, browns, and blacks are

always due to pigments, and in a few cases greens, blues, violets,

purples, and whites, and not, as is usually the case, to structural

conditions, such as striæ on the scales (Mayer). They confined

themselves solely to the chemical side of the problem, not considering

the structure of the scales themselves.

Urech has also discovered a beautiful smaragd-green coloring-matter in

the wings (not in the scales) of the pupa of _Pieris brassicæ_. It is

not chlorophyll, and Urech suggests that it may be either the germinal

substance of the pigments of the scales or its bearer. It is not the

pigment of the blood.

Urech has also demonstrated that in many Lepidoptera the color of the

urine which is voided upon emergence from the chrysalis is similar to

the principal color of the scales.

Hopkins has worked on the pigments within the scales of butterflies.

The yellow pigment in _Gonopteryx rhamni_ is a derivation of uric

acid, and he calls it lepidotic acid. Its aqueous solution is

strongly acid to litmus, and must be bad-tasting to birds.

Hopkins has dissolved the red pigment from the border of the hind

wing of _Delias eucharis_, an Indian butterfly, in pure water,

finding as the result a yellow solution; but if the solution be

evaporated to dryness, the solid residue of pigment is red once

more. He has obtained from this pigment of _eucharis_ a silver

compound which contains a percentage of metals exactly equal to that

from the pigment of _G. rhamni_. (Nature, April 2, 1892.)

“The scales of the wings of the white butterflies (Pieridæ) are also

shown by Hopkins to contain uric acid, this substance practically

acting as a white pigment in these insects. A yellow pigment, widely

distributed in the same family, is shown to be a derivative of uric

acid, and its artificial production as a by-product of the

hydrolysis of uric acid is demonstrated. That this yellow pigment is

an ordinary excretory product of the butterfly is indicated by the

fact that an identical substance is voided from the rectum on

emergence from the pupa. These excretory pigments, which have

well-marked reactions, are apparently confined to the Pieridæ, and

are not found in other Rhopalocera. This fact shows that when a

Pierid mimics an insect belonging to another group, the pigments of

the mimicked and mimicking insects, respectively, are chemically

quite distinct. Other pigments existing, not in the scales, but

between the wing-membranes, are shown to be of use for ornament.”

(Proc. Royal Soc., London, 1894.)

Griffiths (1892) claims that the green pigment found in several

species of Papilio, Hesperia, and Limenitis, also in Noctuidæ,

Geometridæ, and Sphingidæ likewise consists of a derivative of uric

acid, which he calls lepidopteric acid. By prolonged boiling in HCl

it is converted into uric acid.

Spuler, however, finds that green does not depend on pigmentation,

but is an optical color. As remarked by Spuler, either the chitin of

the scales itself is colored reddish (yellow grayish), or the

pigment is secreted in the nuclei.

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.

[Illustration:

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.

]

[Illustration:

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.”

[Illustration:

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).

[Illustration:

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.

]

[Illustration:

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.

[Illustration:

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).

[Illustration:

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.

[Illustration:

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

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.

[Illustration:

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_).

[Illustration:

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.

[Illustration:

FIG. 251.—Sagittal section through the brain of the locust: _l. oc.

n_, lateral ocellus nerve; _a. t_, anterior tubercle of the mushroom

body; _i. t_, internal tubercle of the mushroom body; _c. l_,

cerebral lobes; _l. l_, lateral lobe of the middle protocerebrum;

_com_, commissural cord; _c. mol_, central mass of the olfactory

lobe; _ac. an. l_, fibres uniting the median lobe of the middle

protocerebrum with dorsal lobes of the deutocerebrum; _gc. trit. l_,

ganglionated cortex of the tritocerebral lobe; _c. an. l_, cortex of

antennal (olfactory) lobe; _lab. fr_, labrofrontal nerve; _oe. com_,

œsophageal commissure; _tr. com_, transverse commissure of

œsophageal ring; other letters as in Fig. 250.—After Viallanes.

]

The stalked bodies are enveloped by the cortical layers of

ganglion-cells, those filling the hollow of the calyx having little or

no protoplasm around the nucleus.

=Structure of the mushroom bodies.=—By staining the brain of the

honey bee with bichromate of silver, Kenyon has worked out the

structure of the mushroom bodies, with their cells. The cup-shaped

bodies or calyces are composed of fibrillar substance

(_punktsubstanz_). Each of these cups, he says, is “filled to

overflowing with cells having large nuclei and very little

cytoplasm.” From the under surface of each of these cups there

descends into the general fibrillar substance of the brain “a column

of fibrillar substance, which unites with its fellow of the same

side to send a large branch obliquely downward to the median line of

the brain, and an equally large or larger branch straight forwards

to the anterior cerebral surface.”

The cells of the mushroom bodies, observes Kenyon, “stand out in

sharp contrast to all other nerve cells known, though they recall to

some extent the cells of Purkinje in the higher mammals. Each of the

cells contained within the fibrillar cup sends a nerve-process into

the latter, where it breaks up into a profusely arborescent system

of branchlets, which often appear with fine, short, lateral

processes, such as are characteristic of the dendrites of some

mammalian nerve-cells.” Just before entering the fibrillar

substance, a fine branch is given off that travels along the inner

surface of the cup along with others of the same nature, forming a

small bundle to the stalk of the mushroom body, down which it

continues until it reaches the origin of the anterior and the inner

roots above mentioned. “Here it branches, one branch continuing

straight on to the end of the anterior root, while the other passes

to the end of the inner root. Throughout its whole course the fibre

and its two branches are very fine. Nearly the whole stalk and

nearly the whole of each root is made up of these straight, parallel

fibres coming from the cells within the cup of the mushroom bodies.

What other fibres there are enter these bodies from the side, and

branch between the straight fibres very much as the dendrites of the

cells of Purkinje branch among the parallel fine fibres from the

cells of the granular layer in the mammalian cerebellum. These

fibres are of the nature of association fibres.”

Viallanes showed that from the olfactory or antennal lobes, as well

as from the optic ganglia, there are tracts of fibres which finally

enter the cups of the mushroom bodies, and Kenyon has confirmed this

observation. Kenyon has also, by the Golgi method, detected another

tract, before unknown, “passing down the hinder side of the brain,

from the cups to the region above the œsophagus, where it bends

forward and comes in contact with fibres from the ventral cord,

which exists, although Binet was unable to discover any growth of

fibres connecting the cord with the brain.

“The fibres entering the cups from the antennal lobe, the optic

ganglia, and the ventral region, spread out and branch among the

arborescent endings of the mushroom-body cells. The fibres branching

among the parallel fibres of the roots and the stalk lead off to

lower parts of the brain, connecting with efferent or motor-fibres,

or with secondary association fibres, that in their turn make such

connections. This portion of the circuit has not been perfectly made

out, though there seems to be sufficient data to warrant the

assumption just made.

[Illustration:

FIG. 252.—Section 17, showing the central body (_centr. b_) and

mushroom body, optic and antennal lobes (_a. l_), and procerebral

lobes (_pc. l_); _o. cal_, outer division of the calyx; _op. n_,

optic nerve; _trab_, trabeculum; _tc. n_, transverse nerve.

]

“Such fibres existing as described, there is then a complete circuit

for sensory stimuli from the various parts of the body to the cells

of the mushroom bodies. The dendritic or arborescent branches of

these cells take them up and pass them on out along the parallel

fibres or neurites in the roots of the mushroom bodies as motor or

other efferent impulses.

“This, however, is not all. For there are numerous fibres evident in

my preparations, the full courses of which I have not been thus far

able to determine, but which are so situated as to warrant the

inference that they may act as association fibres between the

afferent fibres from the antennæ, optic ganglia, and ventral system,

and the efferent fibres. There is then a possibility of a stimulus

entering the brain and passing out as a motor impulse without going

into the circuit of the fibres of the mushroom bodies; or, in other

words, a possibility of what may be compared to reflex action in

higher animals.”

[Illustration:

FIG. 253.—Enlarged view of the trabeculum (the dotted lines _tcn_ and

_obt. n_ pass through it) and its nerves, of the mushroom body,—its

calices and stalk, and the origin of the optic nerve × 225

diameters: _atn_, ascending trabecular nerve; _obt. n_, oblique

trabecular nerve; _tcn_, transverse nerve; _lat. n_, lateral nerve;

_cent. n_, central nerve.

]

The mushroom bodies have not yet been found to be present in the

Synaptera, but occur in the larvæ, at least of those of most metamorphic

insects (Lepidoptera and Hymenoptera), though not yet found in the larvæ

of Diptera. The writer has found these bodies in the nymphs of the

locust (_Melanoplus spretus_), but not in the embryo just before

hatching. They occur in the third larval or nymph stage of this insect.

It is evident that by the end of the first larval stage the brain

attains the development seen in the third larval state of the two-banded

species (_C. bivittatus_).

[Illustration:

FIG. 254.—Section through the brain of _Caloptenus bivittatus_ in the

third larval stage, showing the two hemispheres or sides of the

brain, and the ocelli and ocellar nerves, which are seen to arise

from the top of the hemispheres directly over the calices (compare

Fig. 251): _o. cal_, outer division of calyx of left mushroom body.

]

The result of our studies on the brain of the embryo locust was that

from the embryonic cerebral lobes are eventually developed the central

body and the two mushroom bodies. Fig. 254 shows the early condition of

the mushroom bodies and their undoubted origin from the cerebral

ganglia. Hence these bodies appear to be differentiations of the

cerebral ganglia or lobes, having no connection with the optic or

antennal lobes.

=The central body= (Fig. 252, _centr. b_).—This is the only single or

unpaired organ in the brain. Dietl characterizes it as a median

commissural system. Viallanes describes it as formed entirely of a very

fine and close fibrillar web, like a thick hemispherical skull-cap,

situated on the median line and united with the cerebral lobes. “It is

like a central post towards which converge fibres passing from all

points of the brain; being bound to the cerebral lobes, to the stalked

bodies, to the optic ganglia, and to the olfactory lobes by distinct

fibrous bundles.”

=The antennal or olfactory lobes (Deutocerebrum).=—This portion of the

brain consists of two hemispherical lobes, highly differentiated for

special sensorial perceptions, and connected by a slightly

differentiated medullary mass, the dorsal lobe (Figs. 248, 249 _lo_),

from which arise the motor fibres and those of general sensibility. The

antennal lobes are in part attached to the optic ganglia, and partly to

the stalked body on the same side, by the optic olfactory chiasma (Fig.

250 _fch_, _choo_), a system of fibres partially intercrossed on the

median line.

=The œsophageal lobes (Tritocerebrum)= (Figs. 249, 250).—From this

region the labrum and viscera are innervated, the nerves to the latter

being called the visceral, sympathetic, or stomatogastric system. As

Viallanes remarks, though plainly situated in front of the mouth, they

are in fact post-œsophageal centres. The two lobes are situated far

apart, and are connected by a bundle of fibres passing behind the

œsophagus, called the transverse commissure of the œsophageal ring

(Lienard). The œsophageal ganglia, besides giving rise to the labral

nerves, also give origin to the root of the frontal ganglion.

_c._ Histological elements of the brain

The brain and other ganglia are composed of two kinds of tissue.

1. The outer slightly darker, usually pale grayish white portion

consists of cortical or ganglion-cells differing in size. This portion

is stained red by carmine, the cells composing it readily taking the

stain.

The large ganglion cells (represented in Figs. 252 and 253) are oval,

and send off usually a single nerve-fibre; they have a thin fibrous

cell-wall, and the contents are finely granular. The nucleus is very

large, often one-half the diameter of the entire cell, and is composed

of large round refractive granules, usually concealing the nucleolus.

2. The medullary or inner part of the brain consists of matter which

remains white or unstained after the preparation has remained thoroughly

exposed to the action of the carmine. It consists of minute granules and

interlacing fibres. The latter often forms a fine irregular network

inclosing masses of finely granulated nerve matter.

This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des

thierischen Körpers, p. 89, thus refers to it:—

“In the brain and ventral ganglia of the leech, of insects, and

in the brain of the gastropods (Schnecken) I observe that the

stalks (stiele) of the ganglion-cells in nowise immediately

arise as nerve-fibres, but are planted in a molecular mass or

_punktsubstanz_, situated in the centre of the ganglion, and

merged with this substance. It follows, from what I have seen,

that there is no doubt that _the origin of the nerve-fibres

first takes place from this central punktsubstanz_.”

“This relation is the rule. But there also occur in the

nerve-centres of the invertebrates single, definitely situated

ganglion-cells, whose continuations become nerve-fibres without the

intervention of a superadded _punktsubstanz_.” We may, with Kenyon,

call it the fibrillar substance.

Leydig subsequently (p. 91) further describes this fibrillar

substance, stating that the granules composing it form a reticulated

mass of fibrillæ, or, in other words, a tangled web of very fine

fibres:—

“We at present consider that by the passage of the continuation of

the ganglion-cells into the _punktsubstanz_ this continuation

becomes lost in the fine threads, and on the other side of the

_punktsubstanz_ the similar fibrillar substance forms the origin of

the axis-cylinders arranged parallel to one another; so it is quite

certain _that the single axis-cylinder derives its fibrillar

substance as a mixture from the most diverse ganglion-cells_.”

_d._ The visceral (sympathetic or stomatogastric) system

This system in insects is composed (1) of a series of three unpaired

ganglia (Fig. 249, _gv^1_, _gv^2_, _gv^3_), 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_).

[Illustration:

FIG. 255.—Anterior portion of the paired and unpaired visceral nervous

system of _Blatta orientalis_ seen from above. The outlines of the

brain (_g_) and the roots of the antennal nerve (_na_), which cover

a portion of the sympathetic nervous system, are given by dotted

lines. Lettering as in Fig. 247. _nsd_, nerve to salivary gland. The

nervus recurrens (_nr_) enters an unpaired stomach ganglion farther

back.—After Hofer, from Lang.

]

2. Of two pairs of lateral ganglia (Fig. 255, _ga_, _gp_) situated two

on each side of the œsophagus. They are connected both with the antennal

lobes by a nerve (_rvd_), and to the chain of unpaired ganglia by a

special connective. The first pair of these ganglia sends nerves to the

heart and aorta; the second pair to the tracheæ of the head.

The unpaired median or recurrent nerve (_nr_) extends back from under

the brain along the upper side of the œsophagus, and (in Blatta), behind

the origin of the nerves to the salivary glands, enters an unpaired

ganglion, called the stomachic ganglion (_ganglion ventriculare_),

situated in front of the proventriculus. The number of these stomachic

ganglia varies in different orders of insects.

In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller,

Brandt, _ex_ Kolbe) that the nerve to each salivary gland arises

from three different centres: the anterior end situated under the

œsophagus is innervated by the paired visceral nerves from the

hinder paired ganglia; the remaining part by nerves arising from

each side of the recurrent nerve; and thirdly by a pair of nerves

arising from the subœsophageal ganglion which accompanies the common

salivary duct, and ends in branches which partly innervate the

salivary glands and in part their muscles.

Hofer considers that the function of this complex system of paired and

unpaired ganglia, with their nerves, is a double one, viz. serving both

as a centre for the peristaltic action of the œsophagus, and as

innervating the salivary glands.

Besides these a second portion of the visceral system arises from the

thoracic and abdominal ventral cord. It may be seen in the simplest

condition yet known in the nervous system of Machilis (Fig. 239 _s_). It

consists of a fine, slender nerve, which extends along the surface of

the ventral chain of ganglia, and sending off a pair of branches

(accessory transverse nerves) in front of each ganglion. These accessory

nerves receive nerve-twigs from the upper cord of the ventral chain,

dilating near their origins into a minute elongated ganglion, and then

passing partly outwards to the branches of the tracheæ and the muscles

of the spiracles, uniting in the middle line of each segment of the body

behind the head, _i.e._ of those segments containing a pair of ganglia.

_e._ The supraspinal cord

In the adult Lepidoptera has been detected, continuous with and on the

upper side of the abdominal portions of the ventral cord, a longitudinal

cord of connective tissue forming a white or yellowish band, and which

seems to be an outgrowth of the dorsal portion of the neurilemma of the

ventral cord. Muscles pass from it to the neighboring ventral portions

of the integument. Its use is unknown, and attention was first called to

it by Treviranus, who called it “an unknown ventral vessel”

(_Bauchgefäss_). Afterwards it was re-discovered by Newport, who

described it as “a distinct vascular canal.” But Burger has proved by

cross-sections that it is not tubular, but a comparatively solid cord

composed, however, of loose connective tissue. Newport found it in the

larva of _Sphinx ligustri_, but Cattie states that it is not present in

that of _Acherontia atropos_. It has not yet been observed in insects of

other orders, but its homologue exists in the scorpion and in the

centipede, and it may prove to correspond with the far more complete

arterial coat which, with the exception of the brain, envelops the

nervous system of Limulus.

_f._ Modifications of the brain in different orders of insects

There are different grades of cerebral development in insects, and

Viallanes claimed that it was no exaggeration to say that the brain of

the locust (Melanoplus) differs as much from that of the wasp as that of

the frog differs from that of man. He insists that the physiological

conditions which determine the anatomical modifications of the brain are

correlated with 1, the food; 2, the perfection of the senses; and 3,

with the perfection of the psychic faculties. For example, in those

which feed on solid food and whose œsophagus is large (Orthoptera and

Coleoptera), the connectives are elongated, the subœsophageal commissure

free in all its extent, and the tritocerebrum is situated quite far from

the preceding segment of the brain.

On the other hand, in insects which feed on fluid food (Hymenoptera,

Lepidoptera, Diptera, Hemiptera), the œsophagus is slender and the

nervous centres which surround them are very much condensed; the

connectives are short, and the tritocerebrum is closely fused, partly to

a portion of the antennal lobes (deutocerebrum) and partly to the

mandibular ganglion.

As regards the perfection of the senses, where, as in dragon-flies, the

eyes are very large, the optic ganglia are correspondingly so, and in

the same insects the antennæ being very small, the antennal lobes are

almost rudimentary. The ants exhibit inverse conditions; in their brain

the antennal lobes are well developed, while the optic ganglia are

reduced, and where, as in Typhlopone, the eyes are wanting, they are

completely atrophied.

[Illustration:

FIG. 256.—Head of _Anophthalmus tellkampfii_, showing the brain,—the

optic ganglia, nerves, and eyes totally atrophied.

]

[Illustration:

FIG. 257.—Head of another Carabid, with the brain and eyes normal:

_op_, optic ganglion; _pcl_, brain.

]

In certain cave insects where the eyes are wanting, the optic ganglia

are also absent. In the eyeless cave species of Anophthalmus the optic

ganglia and nerves are entirely atrophied, as they are in Adelops,

which, however, has vestiges of the facets (ommatidia). Fig. 257

represents the brain of _Chlænius pennsylvanicus_, a Carabid beetle,

with its eyes and optic ganglia (_op_) which may be compared with

Anopthalmus, in which these parts are totally atrophied.

Dujardin claimed that the degree of complication of the stalked body of

the Hymenoptera was in direct relation with their mental powers. This

has been proved by Forel, who has shown that in the honey bee and ants

the mushroom bodies are much more developed in the workers than in the

males or females and Viallanes adds that these bodies are almost

rudimentary in the dragon-flies, whose eyes are so large; while on the

contrary in the blind ants (Typhlopone), these bodies are as perfect and

voluminous as in the ants with eyes.

[Illustration:

FIG. 258.—Diagrammatic outlines of sections of the upper part of the

brain of a cockroach. Only one side of the brain is here

represented. The numbers indicate the position in the series of 34

sections into which this brain was cut. _mb_, mushroom bodies, with

their cellular covering (_c_) and their stems (_st_); _a_, anterior

nervous mass; _m_, median nervous mass.—After Newton.

]

Within the limits of the same order the stalked bodies are most perfect

in the most intelligent forms. Thus in the Orthoptera, says Viallanes,

the Blattæ, Forficulæ, and the crickets, the mushroom bodies are more

perfect than in the locusts, which have simpler herbivorous habits. This

perfection of the mushroom bodies is seen not only in the increase in

size, but also in the complication of its structures. Thus in the groups

with lower instincts (Tabanus, Æschna) the stalk does not end in a calyx

projecting from the surface of the brain, but its end, simply truncated,

is indicated externally only by an accumulation of the ganglionic nuclei

which cover it.[43]

In types which Viallanes regards as more advanced, _i.e._ Œdipoda and

Melanoplus, the end of the stalk projects and is folded into a calyx.

The brain of the cockroach (Periplaneta, Fig. 258) is a step higher than

that of the locusts, each calyx being divided into two adjacent calices,

although the cockroaches are an older and more generalized type than

locusts.

The stalked bodies of cockroaches are thus complex, like those of the

higher Hymenoptera, the calices in Xylocopa, Bombus, and Apis being

double and so large as to cover almost the entire surface of the brain.

Finally, in what Viallanes regards as the most perfect type (Vespa), the

sides of the calices are folded and become sinuous, so as to increase

the surface, thus assuming an appearance which, he claims, strongly

recalls that of the convolutions of the brain of the mammals.

Cheshire also calls attention to a progression in the size of these

appendages, as well as in mental powers as we rise from the

cockchafer (_Melolontha vulgaris_) to the cricket, up to the

ichneumon, then to the carpenter bee, and finally to the social hive

bee, “where the pedunculated bodies form the ⅕ part of the volume of

the cerebral mass, and the 1⁄870 of the volume of the entire

creature, while in the cockchafer they are less than 1⁄2300 the

part. The size of the brain is also a gauge of intelligence. In the

worker bee the brain is 1⁄174 of the body; in the red ant, 1⁄296; in

the Melolontha, 1⁄3500; in the Dyticus beetle, 1⁄4400.” (Bees and

bee-keeping, p. 54.)

_g._ Functions of the nerve-centres and nerves

As we have seen, the central seat of the functions of the nervous system

is not the brain alone (supraœsophageal ganglion), but each ganglion is

more or less the seat of vital movements, those of the abdomen being

each a distinct motor and respiratory centre. The two halves of a

ganglion are independent of each other.

According to Faivre, the brain is the seat of the will and of the power

of coördinating the movements of the body, while the infraœsophageal

ganglion is the seat of the motive power and also of the will.

The physiological experiments of Binet, which are in the line of those

of Faivre, but more thorough, demonstrate that an insect may live for

months without a brain, if the subœsophageal ganglion is left intact,

just as a vertebrate may exist without its cerebrum. As Kenyon says:

“Faivre long ago showed that the subœsophageal ganglion is the seat of

the power of coördination of the muscular movements of the body. Binet

has shown that the brain is the seat of the power directing these

movements. ‘A debrained hexapod will eat when food is placed beneath its

palpi, but it cannot go to its food even though the latter be but a very

small space removed from its course or position. Whether the insect

would be able to do so if the mushroom bodies only were destroyed, and

the antennal lobes, optic lobes, and the rest of the brain were left

intact, is a question that yet remains to be answered’” (Kenyon).

In insects which are beheaded, however readily they respond to

stimulation of the nerves, they are almost completely wanting in will

power. Yet insects which have been decapitated can still walk and fly.

Hymenoptera will live one or two days after decapitation, beetles from

one to three days, and moths (Agrotis) will show signs of life five days

after the loss of their head.

That the loss of will power is gradual was proved by decapitating

_Polistes pallipes_. A day after the operation she was standing on her

legs and opening and closing her wings; 41 hours after the operation she

was still alive, moving her legs, and thrusting out her sting when

irritated. _Ichneumon otiosus_, after the removal of its head, remained

very lively, and cleaned its wings and legs, the power of coördination

in its wings and legs remaining. A horse-fly, a day after decapitation,

was lively and flew about in a natural manner.[44]

When the abdomen is cut off, respiration in that region is not at first

interrupted. The seat of respiratory movements was referred by Faivre to

the hinder thoracic ganglion, but Plateau says that this view must be

entirely abandoned, remarking: “All carefully performed experiments on

the nervous system of Arthropoda have shown that each ganglion of the

ventral chain is a motor centre, and in insects a respiratory centre,

for the somite to which it belongs” (Miall and Denny’s The Cockroach, p.

164).

The last pair of abdominal ganglia serve as the nervous centre of the

nerves sent to the genital organs.

The recurrent or stomatogastric nerve, which, through the medium of the

frontal ganglion, regulates digestion, has only a slight degree of

sensibility; the insect remains quiet even when a powerful allurement is

presented to the digestive tract (Kolbe).

Faivre states that the destruction of the frontal ganglion, or a section

of the commissures connecting it with the brain, puts an end to

swallowing movements; on the other hand, stimulation results in

energetic movements of this nature.

Yersin, by cutting through the commissure in different places, and thus

isolating the ganglia of the nervous cord of _Gryllus campestris_,

arrived at the following results:—

1. The section of a nerve near its origin rendered the organ supplied by

this nerve incapable of performing its functions.

2. If the connectives between two ganglia, _i.e._ the second and third

thoracic ganglia, are cut through, the fore as well as hinder parts of

the body retain their power of motion and sensation; but a stimulus

applied to the anterior part of the body does not pass to the hinder

portion.

3. Insects with an incomplete metamorphosis after section of the

connectives are not in every case unable to moult and to farther

develop.

4. If only one of the two connectives be cut through, the appendages of

the side cut through which take their origin between the place injured

and the hinder end of the body, often lose sensation and freedom of

motion, or the power of coördination of movements becomes irregular.

Sometimes this is shown by an unsteadiness in the gait, so that the

insect walks around in a circle; after a while these irregularities

cease, and the movements of the limbs on the injured side are only

slightly restrained. By a section of both connectives in any one place

the power of coördination of movements is not injured.

5. The section of the connectives appear to have no influence on

nutrition, but affects reproduction, the attempt at fertilization on the

part of the male producing no result, and the impregnated female laying

no eggs.

6. Injury to the brain, or to the subœsophageal, or one of the thoracic

ganglia, is followed by a momentary enfeeblement of the ganglion

affected. Afterwards there results a convulsive trembling, which either

pervades the whole body or only the appendages innervated by the injured

ganglion.

7. As a result of an injury to the brain there is such a lack of

steadiness in the movements that the insect walks or flies in a circle;

for instance, a fly or dragon-fly thus injured in flying describes a

circle or spiral. Steiner, in making this experiment, observed that the

insect circled on its uninjured side. The brain is thus a motor centre.

8. By injuring a thoracic ganglion, one or all the organs which receive

nerves from the ganglion are momentarily weakened. Afterwards the

functions become restored. Sometimes, however, the insect walks in a

circle. Faivre observed that after the destruction of the metathoracic

ganglion of _Dyticus marginalis_ the hind wings and hind legs were

partially paralyzed (Kolbe, _ex_ Yersin).

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, 3_a_, 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.

THE SENSORY ORGANS

_a._ The eyes and insect vision

[Illustration:

FIG. 259.—Different forms of compound eyes. _A_, a bug (Pyrrhocoris).

_B_, worker bee. _C_, drone. _D_, male Bibio, a holoptic

insect.—From Judeich and Nitsche.

]

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).

[Illustration:

FIG. 260.—Section through the ocellus of a young Dyticus larva: _ct_,

cuticula; _l_, corneal lens; _gh_, cells of the vitreous body, being

modified hypodermal cells (_hy_); _st_, rods; _re_, retinal cells;

_no_, optic nerve.—After Grenacher, from Lang.

]

=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]

[Illustration:

FIG. 261.—Section through the eye of a fly (_Musca vomitoria_): _c_,

cornea, or facet; _pc_, pseudocone; _r_, retinula; _Rh_, rhabdom;

_pg^1_, _pg^2_, _pg^3_, pigment cells; _b.m_, basilar membrane; _T_,

_Tt_{1}_, _Tt_{2}_, trachea; _tv_, tracheal vesicle; _t.a_, terminal

anastomosis; _op_, opticon; _c.op_, epiopticon; _p.op_, periopticon;

_n.c_, nuclei; _n.c.s_, nerve-cell sheath; _N.f_, decussating

nerve-fibres.—After Hickson, from Lubbock.

]

=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).

[Illustration:

FIG. 262.—Ommatidium of cockroach (Periplaneta): _lf_, cornea; _kk_,

crystalline cone; _pg′_ pigment cell; _rl_, retinula; _rm_,

rhabdom.—After Grenacher, from Lubbock.

]

[Illustration:

FIG. 263.—Two separate elements of the eucone eye of a bee; _Lf_,

cornea; _n_, nucleus of Semper; _Kk_, crystalline cone; _Pg_,

pigment cells; _Rl_, retinula; _Rm_, rhabdom.—After Grenacher, from

Lubbock.

]

[Illustration:

FIG. 264.—Three ommatidia of a pseudocone eye, diagrammatic: _A_, a

separate ommatidium of _Musca vomitoria_, semi-diagrammatic: _c_,

cornea; _p.c_, pseudocone; _pg′_, pigmented cells surrounding the

pseudocone; _p.g_{2}_, additional pigment cells; _p.g_{3}_, basal

pigment cells; _n.p.c_, nuclei of pseudocone; _r_, retinulæ; _n.r_,

_n.r′_, nucleus of retinulæ; _R_, rhabdom; _b.m_, basal membrane;

_t.a_, terminal anastomosis sending nerve-fibrils to the retinulæ.

_B_, section through a retinula and rhabdom near the basal membrane,

the six retinulæ (_r_) fused into a tube ensheathing the rhabdom

(_R_).—After Hickson.

]

_a._ Pseudocone eyes; in which, instead of the crystalline lens or cone,

there are four cells filled with a transparent fluid medium, and a

smaller protoplasmic portion containing a nucleus (Muscidæ, Fig. 264,

_pc_). Hickson states that the difference between the eucone and

pseudocone eyes lies in the fact that in the pseudocone eye “the

refracting body formed by the cone-cell lies behind the nuclei,” and in

the eucone eye in front of it.

2. _Acone eyes_, where the cone or refracting body is wanting, but is

represented by the four primitive cone-cells. Acone eyes occur in

Forficulidæ, Hemiptera (except Cicadidæ), the nematocerous Diptera

(Tipula, etc.), and those Coleoptera which have less than five tarsal

joints.

=The retinula and rod.=—The retinula is morphologically a nerve-end

cell, situated at the end of a nerve-fibril arising from the optic

nerve. The elements of the retinula of Musca are six in number and

surround the _rhabdom_ (Fig. 264), which consists of a bundle of six

long, delicate chitinous rods, more or less firmly united together (Fig.

264, _R_).

The six elements of the retinula of Musca are in their outer or distal

portion free from one another, but towards their base are fused into a

sheath (Fig. 264, _r_). They are true nerve-end cells, as shown by

Müller and by Max Schultze, their views having been confirmed by

Grenacher and by Hickson. The relations of the nerves to the rods after

passing through the basal membrane is seen in Fig. 266.

=The pigment.=—The cones or pseudocones are mostly buried in pigment, as

well as the rods; and the pigment forms two layers. The outer of the two

layers is called the iris pigment (Fig. 265, _e_, _iris tapetum_), and

the inner (_f_) the retinal pigment.

Between the ommatidia internally there occur, according to Hickson,

pigment cells (Fig. 264, _p.g_{3}_), each of which stands on the basilar

membrane and sends a fine process outwards towards the internal process

of the external pigment-cell (_p.g_{2}_). A long, slender tracheal

vesicle also passes in between the retinulæ.

[Illustration:

FIG. 265.—Two ommatidia from the eye of _Colymbetes fuscus_, × 160:

_a_, cornea; _b_, cone; _c_, rhabdom; _d_, basal membrane, with

nerve filaments below it: _e_, iris pigment; _f_, retina

pigment.—After Exner, from Sharp.

]

=The basilar membrane.=—This is a thin fenestrate membrane (Fig. 261)

separating the cones and rods from the optic tract (Fig. 264, _b.m_). It

is perforated for the passage of tracheal diverticula and of the optic

nerve fibrils. It separates the dioptric or instrumental portion of the

eye from the percipient portion, _i.e._ the optic tract.

=The optic tract.=—This is the optic ganglion of earlier writers, and

appears to be the percipient portion of the eye, as opposed to the

dioptric portion. If the reader will examine Figs. 249 and 261, he will

see that it consists of three distinct ganglionic swellings, _i.e._ the

_opticon_, _epiopticon_, and _periopticon_, whose structure is very

complicated. In Musca (Fig. 261) the first ganglionic swelling (opticon)

is separated from the brain by a slight constriction, which Berger

regards as the homologue of the optic nerve of the other arthropods. It

consists of a very fine granular matrix traversed throughout by a fine

meshwork of minute fibrillæ, the neurospongium of Hickson. In the young

cockroach (Periplaneta) the optic nerve separating the cerebral ganglion

from the opticon is much longer in proportion than it is in the adult

blow-fly.

[Illustration:

FIG. 266.—Periopticon and terminal anastomosis of Agrion, showing the

character of the elements of the periopticon (_p.op_) and the

structure of the terminal anastomosis (_t.a_). 1. The first layer of

the terminal anastomosis, consisting of a plexus of fibrils and

nerve-cells (_n.c_). 2. The second layer, in which the fibrils are

collected together in bundles. 3. The final optic plexus and

nerve-cells. 4. The layer in which the optic fibrils are collected

in bundles to be distributed to the retinulæ (_r_); _b.m_, basal

membrane.—After Hickson.

]

The second ganglionic swelling (epiopticon, Fig. 261, _c.op_) is

separated from the opticon by a tract of fine nerve-fibrils, which

partially decussate; at the decussation two or three larger nerve-cells

may be seen. It also contains a few scattered nerve-cells (_n.c_). The

third ganglionic swelling (periopticon, _p.op_) is separated from the

others by a bundle of long optic nerve-fibrils, which cross one another.

It is composed of a number of cylindrical masses of neurospongium

arranged side by side (Fig. 261, _p.op_). Between these elements of the

periopticon, which do not seem to bear any relation to the number of

ommatidia, a single nerve-cell is very frequently seen. The periopticon

does not occur in Periplaneta and Nepa (Hickson). The three optic

ganglia thus described, together with the cerebral ganglia, are

surrounded by a sheath of densely packed nerve-cells.

Bearing in mind the fact that the retinulæ are the nerve-end cells

of the fibres passing through the periopticon, it will be well to

read the following account, by Hickson, of the terminal anastomosis

of the optic fibrils in the periopticon of _Agrion bifurcatum_, and

to examine his sketch (Fig. 266):

“The terminal anastomosis of Agrion may be conveniently divided into

four regions. First the region (1) lying nearest to the periopticon

in which the nerve-cells are numerous, and the fibrils leaving the

periopticon form a complicated plexus; the region (2) next to this,

in which the fibrils have collected into bundles separated by spaces

occupied by very thin-walled tracheæ in which there are no spiral

markings, and lymph-spaces; next, the region (3) in which the

fibrils form a final plexus, and in which there are again a

considerable number of nerve-cells; and, lastly, the region (4) in

which the fibrils are again collected into bundles, separated by

spaces containing tracheæ, which perforate the basement membrane to

supply the retinulæ.”

It would seem as if the decussation of the optic nerve-fibrils were

a matter of primary importance, as it so generally occurs, but in

the young of that most generalized of all pterygote insects, the

cockroach (Periplaneta), Hickson states that the optic nerve-fibrils

which leave the periopticon pass without decussating to the

ommateum, and in the adult there is only a partial decussation. In

Nepa there is no decussation, but the anastomosis is complicated by

the presence of looped and transverse anastomoses.

A Text-book of Entomology Including the Anatomy, Physiology, Embryology and Metamorphoses of Insects for Use in Agricultural and Technical Schools and Colleges as Well as by the Working Entomologist · 第 5 / 16 章

Coste, and more especially Urech, have shown that many of the pigments