Chapter XIV: Part I: Morphology and Physiology (10)

FIG. 251.—Sagittal section through the brain of the locust: _l. oc.
n_, lateral ocellus nerve; _a. t_, anterior tubercle of the mushroom
body; _i. t_, internal tubercle of the mushroom body; _c. l_,
cerebral lobes; _l. l_, lateral lobe of the middle protocerebrum;
_com_, commissural cord; _c. mol_, central mass of the olfactory
lobe; _ac. an. l_, fibres uniting the median lobe of the middle
protocerebrum with dorsal lobes of the deutocerebrum; _gc. trit. l_,
ganglionated cortex of the tritocerebral lobe; _c. an. l_, cortex of
antennal (olfactory) lobe; _lab. fr_, labrofrontal nerve; _oe. com_,
œsophageal commissure; _tr. com_, transverse commissure of
œsophageal ring; other letters as in Fig. 250.—After Viallanes.
]

The stalked bodies are enveloped by the cortical layers of ganglion-cells, those filling the hollow of the calyx having little or no protoplasm around the nucleus.

=Structure of the mushroom bodies.=—By staining the brain of the
honey bee with bichromate of silver, Kenyon has worked out the
structure of the mushroom bodies, with their cells. The cup-shaped
bodies or calyces are composed of fibrillar substance
(_punktsubstanz_). Each of these cups, he says, is “filled to
overflowing with cells having large nuclei and very little
cytoplasm.” From the under surface of each of these cups there
descends into the general fibrillar substance of the brain “a column
of fibrillar substance, which unites with its fellow of the same
side to send a large branch obliquely downward to the median line of
the brain, and an equally large or larger branch straight forwards
to the anterior cerebral surface.”

The cells of the mushroom bodies, observes Kenyon, “stand out in
sharp contrast to all other nerve cells known, though they recall to
some extent the cells of Purkinje in the higher mammals. Each of the
cells contained within the fibrillar cup sends a nerve-process into
the latter, where it breaks up into a profusely arborescent system
of branchlets, which often appear with fine, short, lateral
processes, such as are characteristic of the dendrites of some
mammalian nerve-cells.” Just before entering the fibrillar
substance, a fine branch is given off that travels along the inner
surface of the cup along with others of the same nature, forming a
small bundle to the stalk of the mushroom body, down which it
continues until it reaches the origin of the anterior and the inner
roots above mentioned. “Here it branches, one branch continuing
straight on to the end of the anterior root, while the other passes
to the end of the inner root. Throughout its whole course the fibre
and its two branches are very fine. Nearly the whole stalk and
nearly the whole of each root is made up of these straight, parallel
fibres coming from the cells within the cup of the mushroom bodies.
What other fibres there are enter these bodies from the side, and
branch between the straight fibres very much as the dendrites of the
cells of Purkinje branch among the parallel fine fibres from the
cells of the granular layer in the mammalian cerebellum. These
fibres are of the nature of association fibres.”

Viallanes showed that from the olfactory or antennal lobes, as well
as from the optic ganglia, there are tracts of fibres which finally
enter the cups of the mushroom bodies, and Kenyon has confirmed this
observation. Kenyon has also, by the Golgi method, detected another
tract, before unknown, “passing down the hinder side of the brain,
from the cups to the region above the œsophagus, where it bends
forward and comes in contact with fibres from the ventral cord,
which exists, although Binet was unable to discover any growth of
fibres connecting the cord with the brain.

“The fibres entering the cups from the antennal lobe, the optic
ganglia, and the ventral region, spread out and branch among the
arborescent endings of the mushroom-body cells. The fibres branching
among the parallel fibres of the roots and the stalk lead off to
lower parts of the brain, connecting with efferent or motor-fibres,
or with secondary association fibres, that in their turn make such
connections. This portion of the circuit has not been perfectly made
out, though there seems to be sufficient data to warrant the
assumption just made.

FIG. 252.—Section 17, showing the central body (_centr. b_) and
mushroom body, optic and antennal lobes (_a. l_), and procerebral
lobes (_pc. l_); _o. cal_, outer division of the calyx; _op. n_,
optic nerve; _trab_, trabeculum; _tc. n_, transverse nerve.
]

“Such fibres existing as described, there is then a complete circuit
for sensory stimuli from the various parts of the body to the cells
of the mushroom bodies. The dendritic or arborescent branches of
these cells take them up and pass them on out along the parallel
fibres or neurites in the roots of the mushroom bodies as motor or
other efferent impulses.

“This, however, is not all. For there are numerous fibres evident in
my preparations, the full courses of which I have not been thus far
able to determine, but which are so situated as to warrant the
inference that they may act as association fibres between the
afferent fibres from the antennæ, optic ganglia, and ventral system,
and the efferent fibres. There is then a possibility of a stimulus
entering the brain and passing out as a motor impulse without going
into the circuit of the fibres of the mushroom bodies; or, in other
words, a possibility of what may be compared to reflex action in
higher animals.”

FIG. 253.—Enlarged view of the trabeculum (the dotted lines _tcn_ and
_obt. n_ pass through it) and its nerves, of the mushroom body,—its
calices and stalk, and the origin of the optic nerve × 225
diameters: _atn_, ascending trabecular nerve; _obt. n_, oblique
trabecular nerve; _tcn_, transverse nerve; _lat. n_, lateral nerve;
_cent. n_, central nerve.
]

The mushroom bodies have not yet been found to be present in the Synaptera, but occur in the larvæ, at least of those of most metamorphic insects (Lepidoptera and Hymenoptera), though not yet found in the larvæ of Diptera. The writer has found these bodies in the nymphs of the locust (_Melanoplus spretus_), but not in the embryo just before hatching. They occur in the third larval or nymph stage of this insect. It is evident that by the end of the first larval stage the brain attains the development seen in the third larval state of the two-banded species (_C. bivittatus_).

FIG. 254.—Section through the brain of _Caloptenus bivittatus_ in the
third larval stage, showing the two hemispheres or sides of the
brain, and the ocelli and ocellar nerves, which are seen to arise
from the top of the hemispheres directly over the calices (compare
Fig. 251): _o. cal_, outer division of calyx of left mushroom body.
]

The result of our studies on the brain of the embryo locust was that from the embryonic cerebral lobes are eventually developed the central body and the two mushroom bodies. Fig. 254 shows the early condition of the mushroom bodies and their undoubted origin from the cerebral ganglia. Hence these bodies appear to be differentiations of the cerebral ganglia or lobes, having no connection with the optic or antennal lobes.

=The central body= (Fig. 252, _centr. b_).—This is the only single or unpaired organ in the brain. Dietl characterizes it as a median commissural system. Viallanes describes it as formed entirely of a very fine and close fibrillar web, like a thick hemispherical skull-cap, situated on the median line and united with the cerebral lobes. “It is like a central post towards which converge fibres passing from all points of the brain; being bound to the cerebral lobes, to the stalked bodies, to the optic ganglia, and to the olfactory lobes by distinct fibrous bundles.”

=The antennal or olfactory lobes (Deutocerebrum).=—This portion of the brain consists of two hemispherical lobes, highly differentiated for special sensorial perceptions, and connected by a slightly differentiated medullary mass, the dorsal lobe (Figs. 248, 249 _lo_), from which arise the motor fibres and those of general sensibility. The antennal lobes are in part attached to the optic ganglia, and partly to the stalked body on the same side, by the optic olfactory chiasma (Fig. 250 _fch_, _choo_), a system of fibres partially intercrossed on the median line.

=The œsophageal lobes (Tritocerebrum)= (Figs. 249, 250).—From this region the labrum and viscera are innervated, the nerves to the latter being called the visceral, sympathetic, or stomatogastric system. As Viallanes remarks, though plainly situated in front of the mouth, they are in fact post-œsophageal centres. The two lobes are situated far apart, and are connected by a bundle of fibres passing behind the œsophagus, called the transverse commissure of the œsophageal ring (Lienard). The œsophageal ganglia, besides giving rise to the labral nerves, also give origin to the root of the frontal ganglion.

_c._ Histological elements of the brain

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

1. The outer slightly darker, usually pale grayish white portion consists of cortical or ganglion-cells differing in size. This portion is stained red by carmine, the cells composing it readily taking the stain.

The large ganglion cells (represented in Figs. 252 and 253) are oval, and send off usually a single nerve-fibre; they have a thin fibrous cell-wall, and the contents are finely granular. The nucleus is very large, often one-half the diameter of the entire cell, and is composed of large round refractive granules, usually concealing the nucleolus.

2. The medullary or inner part of the brain consists of matter which remains white or unstained after the preparation has remained thoroughly exposed to the action of the carmine. It consists of minute granules and interlacing fibres. The latter often forms a fine irregular network inclosing masses of finely granulated nerve matter.

This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des
thierischen Körpers, p. 89, thus refers to it:—

“In the brain and ventral ganglia of the leech, of insects, and
in the brain of the gastropods (Schnecken) I observe that the
stalks (stiele) of the ganglion-cells in nowise immediately
arise as nerve-fibres, but are planted in a molecular mass or
_punktsubstanz_, situated in the centre of the ganglion, and
merged with this substance. It follows, from what I have seen,
that there is no doubt that _the origin of the nerve-fibres
first takes place from this central punktsubstanz_.”

“This relation is the rule. But there also occur in the
nerve-centres of the invertebrates single, definitely situated
ganglion-cells, whose continuations become nerve-fibres without the
intervention of a superadded _punktsubstanz_.” We may, with Kenyon,
call it the fibrillar substance.

Leydig subsequently (p. 91) further describes this fibrillar
substance, stating that the granules composing it form a reticulated
mass of fibrillæ, or, in other words, a tangled web of very fine
fibres:—

“We at present consider that by the passage of the continuation of
the ganglion-cells into the _punktsubstanz_ this continuation
becomes lost in the fine threads, and on the other side of the
_punktsubstanz_ the similar fibrillar substance forms the origin of
the axis-cylinders arranged parallel to one another; so it is quite
certain _that the single axis-cylinder derives its fibrillar
substance as a mixture from the most diverse ganglion-cells_.”

_d._ The visceral (sympathetic or stomatogastric) system

This system in insects is composed (1) of a series of three unpaired ganglia (Fig. 249, _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_).

FIG. 255.—Anterior portion of the paired and unpaired visceral nervous
system of _Blatta orientalis_ seen from above. The outlines of the
brain (_g_) and the roots of the antennal nerve (_na_), which cover
a portion of the sympathetic nervous system, are given by dotted
lines. Lettering as in Fig. 247. _nsd_, nerve to salivary gland. The
nervus recurrens (_nr_) enters an unpaired stomach ganglion farther
back.—After Hofer, from Lang.
]

2. Of two pairs of lateral ganglia (Fig. 255, _ga_, _gp_) situated two on each side of the œsophagus. They are connected both with the antennal lobes by a nerve (_rvd_), and to the chain of unpaired ganglia by a special connective. The first pair of these ganglia sends nerves to the heart and aorta; the second pair to the tracheæ of the head.

The unpaired median or recurrent nerve (_nr_) extends back from under the brain along the upper side of the œsophagus, and (in Blatta), behind the origin of the nerves to the salivary glands, enters an unpaired ganglion, called the stomachic ganglion (_ganglion ventriculare_), situated in front of the proventriculus. The number of these stomachic ganglia varies in different orders of insects.

In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller,
Brandt, _ex_ Kolbe) that the nerve to each salivary gland arises
from three different centres: the anterior end situated under the
œsophagus is innervated by the paired visceral nerves from the
hinder paired ganglia; the remaining part by nerves arising from
each side of the recurrent nerve; and thirdly by a pair of nerves
arising from the subœsophageal ganglion which accompanies the common
salivary duct, and ends in branches which partly innervate the
salivary glands and in part their muscles.

Hofer considers that the function of this complex system of paired and unpaired ganglia, with their nerves, is a double one, viz. serving both as a centre for the peristaltic action of the œsophagus, and as innervating the salivary glands.

Besides these a second portion of the visceral system arises from the thoracic and abdominal ventral cord. It may be seen in the simplest condition yet known in the nervous system of Machilis (Fig. 239 _s_). It consists of a fine, slender nerve, which extends along the surface of the ventral chain of ganglia, and sending off a pair of branches (accessory transverse nerves) in front of each ganglion. These accessory nerves receive nerve-twigs from the upper cord of the ventral chain, dilating near their origins into a minute elongated ganglion, and then passing partly outwards to the branches of the tracheæ and the muscles of the spiracles, uniting in the middle line of each segment of the body behind the head, _i.e._ of those segments containing a pair of ganglia.

_e._ The supraspinal cord

In the adult Lepidoptera has been detected, continuous with and on the upper side of the abdominal portions of the ventral cord, a longitudinal cord of connective tissue forming a white or yellowish band, and which seems to be an outgrowth of the dorsal portion of the neurilemma of the ventral cord. Muscles pass from it to the neighboring ventral portions of the integument. Its use is unknown, and attention was first called to it by Treviranus, who called it “an unknown ventral vessel” (_Bauchgefäss_). Afterwards it was re-discovered by Newport, who described it as “a distinct vascular canal.” But Burger has proved by cross-sections that it is not tubular, but a comparatively solid cord composed, however, of loose connective tissue. Newport found it in the larva of _Sphinx ligustri_, but Cattie states that it is not present in that of _Acherontia atropos_. It has not yet been observed in insects of other orders, but its homologue exists in the scorpion and in the centipede, and it may prove to correspond with the far more complete arterial coat which, with the exception of the brain, envelops the nervous system of Limulus.

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

There are different grades of cerebral development in insects, and Viallanes claimed that it was no exaggeration to say that the brain of the locust (Melanoplus) differs as much from that of the wasp as that of the frog differs from that of man. He insists that the physiological conditions which determine the anatomical modifications of the brain are correlated with 1, the food; 2, the perfection of the senses; and 3, with the perfection of the psychic faculties. For example, in those which feed on solid food and whose œsophagus is large (Orthoptera and Coleoptera), the connectives are elongated, the subœsophageal commissure free in all its extent, and the tritocerebrum is situated quite far from the preceding segment of the brain.

On the other hand, in insects which feed on fluid food (Hymenoptera, Lepidoptera, Diptera, Hemiptera), the œsophagus is slender and the nervous centres which surround them are very much condensed; the connectives are short, and the tritocerebrum is closely fused, partly to a portion of the antennal lobes (deutocerebrum) and partly to the mandibular ganglion.

As regards the perfection of the senses, where, as in dragon-flies, the eyes are very large, the optic ganglia are correspondingly so, and in the same insects the antennæ being very small, the antennal lobes are almost rudimentary. The ants exhibit inverse conditions; in their brain the antennal lobes are well developed, while the optic ganglia are reduced, and where, as in Typhlopone, the eyes are wanting, they are completely atrophied.

FIG. 256.—Head of _Anophthalmus tellkampfii_, showing the brain,—the
optic ganglia, nerves, and eyes totally atrophied.
]

FIG. 257.—Head of another Carabid, with the brain and eyes normal:
_op_, optic ganglion; _pcl_, brain.
]

In certain cave insects where the eyes are wanting, the optic ganglia are also absent. In the eyeless cave species of Anophthalmus the optic ganglia and nerves are entirely atrophied, as they are in Adelops, which, however, has vestiges of the facets (ommatidia). Fig. 257 represents the brain of _Chlænius pennsylvanicus_, a Carabid beetle, with its eyes and optic ganglia (_op_) which may be compared with Anopthalmus, in which these parts are totally atrophied.

Dujardin claimed that the degree of complication of the stalked body of the Hymenoptera was in direct relation with their mental powers. This has been proved by Forel, who has shown that in the honey bee and ants the mushroom bodies are much more developed in the workers than in the males or females and Viallanes adds that these bodies are almost rudimentary in the dragon-flies, whose eyes are so large; while on the contrary in the blind ants (Typhlopone), these bodies are as perfect and voluminous as in the ants with eyes.

FIG. 258.—Diagrammatic outlines of sections of the upper part of the
brain of a cockroach. Only one side of the brain is here
represented. The numbers indicate the position in the series of 34
sections into which this brain was cut. _mb_, mushroom bodies, with
their cellular covering (_c_) and their stems (_st_); _a_, anterior
nervous mass; _m_, median nervous mass.—After Newton.
]

Within the limits of the same order the stalked bodies are most perfect in the most intelligent forms. Thus in the Orthoptera, says Viallanes, the Blattæ, Forficulæ, and the crickets, the mushroom bodies are more perfect than in the locusts, which have simpler herbivorous habits. This perfection of the mushroom bodies is seen not only in the increase in size, but also in the complication of its structures. Thus in the groups with lower instincts (Tabanus, Æschna) the stalk does not end in a calyx projecting from the surface of the brain, but its end, simply truncated, is indicated externally only by an accumulation of the ganglionic nuclei which cover it.[43]

In types which Viallanes regards as more advanced, _i.e._ Œdipoda and Melanoplus, the end of the stalk projects and is folded into a calyx.

The brain of the cockroach (Periplaneta, Fig. 258) is a step higher than that of the locusts, each calyx being divided into two adjacent calices, although the cockroaches are an older and more generalized type than locusts.

The stalked bodies of cockroaches are thus complex, like those of the higher Hymenoptera, the calices in Xylocopa, Bombus, and Apis being double and so large as to cover almost the entire surface of the brain.

Finally, in what Viallanes regards as the most perfect type (Vespa), the sides of the calices are folded and become sinuous, so as to increase the surface, thus assuming an appearance which, he claims, strongly recalls that of the convolutions of the brain of the mammals.

Cheshire also calls attention to a progression in the size of these
appendages, as well as in mental powers as we rise from the
cockchafer (_Melolontha vulgaris_) to the cricket, up to the
ichneumon, then to the carpenter bee, and finally to the social hive
bee, “where the pedunculated bodies form the ⅕ part of the volume of
the cerebral mass, and the 1⁄870 of the volume of the entire
creature, while in the cockchafer they are less than 1⁄2300 the
part. The size of the brain is also a gauge of intelligence. In the
worker bee the brain is 1⁄174 of the body; in the red ant, 1⁄296; in
the Melolontha, 1⁄3500; in the Dyticus beetle, 1⁄4400.” (Bees and
bee-keeping, p. 54.)

_g._ Functions of the nerve-centres and nerves

As we have seen, the central seat of the functions of the nervous system is not the brain alone (supraœsophageal ganglion), but each ganglion is more or less the seat of vital movements, those of the abdomen being each a distinct motor and respiratory centre. The two halves of a ganglion are independent of each other.

According to Faivre, the brain is the seat of the will and of the power of coördinating the movements of the body, while the infraœsophageal ganglion is the seat of the motive power and also of the will.

The physiological experiments of Binet, which are in the line of those of Faivre, but more thorough, demonstrate that an insect may live for months without a brain, if the subœsophageal ganglion is left intact, just as a vertebrate may exist without its cerebrum. As Kenyon says: “Faivre long ago showed that the subœsophageal ganglion is the seat of the power of coördination of the muscular movements of the body. Binet has shown that the brain is the seat of the power directing these movements. ‘A debrained hexapod will eat when food is placed beneath its palpi, but it cannot go to its food even though the latter be but a very small space removed from its course or position. Whether the insect would be able to do so if the mushroom bodies only were destroyed, and the antennal lobes, optic lobes, and the rest of the brain were left intact, is a question that yet remains to be answered’” (Kenyon).

In insects which are beheaded, however readily they respond to stimulation of the nerves, they are almost completely wanting in will power. Yet insects which have been decapitated can still walk and fly. Hymenoptera will live one or two days after decapitation, beetles from one to three days, and moths (Agrotis) will show signs of life five days after the loss of their head.

That the loss of will power is gradual was proved by decapitating _Polistes pallipes_. A day after the operation she was standing on her legs and opening and closing her wings; 41 hours after the operation she was still alive, moving her legs, and thrusting out her sting when irritated. _Ichneumon otiosus_, after the removal of its head, remained very lively, and cleaned its wings and legs, the power of coördination in its wings and legs remaining. A horse-fly, a day after decapitation, was lively and flew about in a natural manner.[44]

When the abdomen is cut off, respiration in that region is not at first interrupted. The seat of respiratory movements was referred by Faivre to the hinder thoracic ganglion, but Plateau says that this view must be entirely abandoned, remarking: “All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and in insects a respiratory centre, for the somite to which it belongs” (Miall and Denny’s The Cockroach, p. 164).

The last pair of abdominal ganglia serve as the nervous centre of the nerves sent to the genital organs.

The recurrent or stomatogastric nerve, which, through the medium of the frontal ganglion, regulates digestion, has only a slight degree of sensibility; the insect remains quiet even when a powerful allurement is presented to the digestive tract (Kolbe).

Faivre states that the destruction of the frontal ganglion, or a section of the commissures connecting it with the brain, puts an end to swallowing movements; on the other hand, stimulation results in energetic movements of this nature.

Yersin, by cutting through the commissure in different places, and thus isolating the ganglia of the nervous cord of _Gryllus campestris_, arrived at the following results:—

1. The section of a nerve near its origin rendered the organ supplied by this nerve incapable of performing its functions.

2. If the connectives between two ganglia, _i.e._ the second and third thoracic ganglia, are cut through, the fore as well as hinder parts of the body retain their power of motion and sensation; but a stimulus applied to the anterior part of the body does not pass to the hinder portion.

3. Insects with an incomplete metamorphosis after section of the connectives are not in every case unable to moult and to farther develop.

4. If only one of the two connectives be cut through, the appendages of the side cut through which take their origin between the place injured and the hinder end of the body, often lose sensation and freedom of motion, or the power of coördination of movements becomes irregular. Sometimes this is shown by an unsteadiness in the gait, so that the insect walks around in a circle; after a while these irregularities cease, and the movements of the limbs on the injured side are only slightly restrained. By a section of both connectives in any one place the power of coördination of movements is not injured.

5. The section of the connectives appear to have no influence on nutrition, but affects reproduction, the attempt at fertilization on the part of the male producing no result, and the impregnated female laying no eggs.

6. Injury to the brain, or to the subœsophageal, or one of the thoracic ganglia, is followed by a momentary enfeeblement of the ganglion affected. Afterwards there results a convulsive trembling, which either pervades the whole body or only the appendages innervated by the injured ganglion.

7. As a result of an injury to the brain there is such a lack of steadiness in the movements that the insect walks or flies in a circle; for instance, a fly or dragon-fly thus injured in flying describes a circle or spiral. Steiner, in making this experiment, observed that the insect circled on its uninjured side. The brain is thus a motor centre.

8. By injuring a thoracic ganglion, one or all the organs which receive nerves from the ganglion are momentarily weakened. Afterwards the functions become restored. Sometimes, however, the insect walks in a circle. Faivre observed that after the destruction of the metathoracic ganglion of _Dyticus marginalis_ the hind wings and hind legs were partially paralyzed (Kolbe, _ex_ Yersin).

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

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).

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]

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).

FIG. 262.—Ommatidium of cockroach (Periplaneta): _lf_, cornea; _kk_,
crystalline cone; _pg′_ pigment cell; _rl_, retinula; _rm_,
rhabdom.—After Grenacher, from Lubbock.
]

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.
]

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.

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.

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.

Looking at the eye as a whole, Hickson regards all the nerve structure of the eye lying between the crystalline cone-layer and the true optic nerve to be analogous with the retina of other animals. With Ciaccio, Berger, and others, he does not regard the layer composed of the retinulæ and rhabdoms as the equivalent of the retina of vertebrates, etc.

=Origin of the facetted eye.=—The two kinds of eye, the simple and the compound, are supposed to have been derived from a primitive type, resembling the single eye (ommatidium) of the acone eye of Tipula. As stated by Lang, “an increase of the elements of this primitive eye led to the formation of the ocellus; an increase in number of the primitive eyes, and their approximation, led to the formation of the compound facet eye.” This view is suggested, he says, by the groups of closely contiguous single eyes of the myriopods, considered in connection with the compound eye of Scutigera. Grenacher looks upon simple (ocelli) and compound eyes as “sisters,” not derived from one another, but from a common parentage.

Immature insects rarely possess compound eyes; they are only known
to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ
and pupa of Corethra.

=Mode of vision by single eyes or ocelli.=—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. The ocelli of different arthropods differ, however, very much in degree of complexity.

Müller considered that the power of vision of ocelli “is probably confined to the perception of very near objects.”

“This may be inferred,” Müller states, “partly from their existing
principally in larvæ and apterous insects, and partly from several
observations which I have made relative to the position of these
simple eyes. In the genus Empusa the head is so prolonged over the
middle inferior eye that, in the locomotion of the animal, the
nearest objects can only come within the range. In _Locusta
cornuta_, also, the same eye lies beneath the prolongation of the
head.... In the Orthoptera generally, also, the simple eyes are, in
consequence of the depressed position of the head, directed
downwards towards the surface upon which the insects are
moving.”[46] Lowne considers that in the ocellus of Eristalis, the
great convexity of the lens must give it a very short focus, and the
comparatively small number of rods render the picture of even very
near objects quite imperfect and practically useless for purposes of
vision, and that the function of the ocelli is “the perception of
the intensity and the direction of light, rather than of vision, in
the ordinary acceptation of the term.”

Réaumur, Marcel de Serres, Dugès, and Forel have shown by
experiment, that in insects which possess both ocelli and compound
eyes, the former may be covered over without materially affecting
the movements of the animals, while if the facetted eyes are
covered, they act as if in the dark (Lubbock).

While Plateau regards the ocelli as of scarcely any use to the
insect, and Forel claims that wasps, humble bees, ants, etc., walk
or fly almost equally well without as with the aid of their ocelli,
Lubbock demurs to this view, and says the same experiments of
Forel’s might almost be quoted to prove the same with reference to
the compound eyes. Indeed, the writer has observed that in caves,
eyeless beetles apparently run about as freely and with as much
purpose, as their eyed relatives in the open air.

Plateau has recently shown that caterpillars which have ocelli alone
are very short-sighted, not seeing objects at a distance beyond one
or two centimetres, and it has been fully proved by Plateau and
others, that spiders, with their well-formed ocelli, are myopic, and
have little power of making out distinctly the shape of the objects
they see.

On the whole, we are rather inclined to agree with Lubbock and
Forel, that the ocelli are useful in dark places and for near
vision. They are, as Lubbock states, especially developed in
insects, such as ants, bees, and wasps, which live partly in the
open light and partly in the dark recesses of nests. Moreover, the
night-flying moths nearly all possess ocelli, while with one known
exception (Pamphila) they are wanting in butterflies.

Finally, remarks Lubbock, “Whatever the special function of ocelli
may be, it seems clear that they must see in the same manner as our
eyes do—that is to say, the image must be reversed. On the other
hand, in the case of compound eyes, it seems probable that the
vision is direct, and the difficulty of accounting for the existence
in the same animal of two such different kinds of eyes is certainly
enhanced by the fact that, as it would seem, the image given by the
medial eyes is reversed, while that of the lateral ones is direct”
(p. 181).