THE NERVOUS SYSTEM AND SENSE ORGANS.
_SPECIAL REFERENCES._
NEWPORT. Nervous System of Sphinx Ligustri. Phil. Trans. (1832–4).
Todd’s Cyclopædia, Art. “Insecta” (1839).
LEYDIG. Vom Bau des Thierischen Körpers. Bd. I. (1864). Tafeln zur.
vergl. Anat. Hft. I. (1864).
BRANDT (E.) Various memoirs on the Nervous System of Insects in
Horæ Soc. Entom. Ross., Bd. XIV., XV. (1879).
MICHELS. Nervensystem von Oryctes nasicornis im Larven--, Puppen--,
und Käferzustande. Zeits. f. wiss. Zool., Bd. XXXIV. (1881).
DIETL. Organisation des Arthropodengehirns. Zeits. f. wiss. Zool.,
Bd. XXVII. (1876).
FLÖGEL. Bau des Gehirns der verschiedenen Insektenordnungen. Zeits.
f. wiss. Zool., Bd. XXX. Sup. (1878).
NEWTON. On the Brain of the Cockroach. Q. J. Micr. Sci. (1879).
Journ. Quekett Club (1879).
GRENACHER. Sehorgan der Arthropoden. (1879). [Origin, Structure,
and Action of the Compound Eye.]
CARRIERE. Sehorgane der Thiere, vergl.-anat. dargestellt (1885).
[Comparative Structure of various Simple and Compound Eyes.]
_General Anatomy of Nervous Centres._
The nervous system of the Cockroach comprises ganglia and
connectives,[100] which extend throughout the body. We have first, a
supra-œsophageal ganglion, or brain, a sub-œsophageal ganglion, and
connectives which complete the œsophageal ring. All these lie in the
head; behind them, and extending through the thorax and abdomen, is a
gangliated cord, with double connectives. The normal arrangement of the
ganglia in Annulosa, one to each somite, becomes more or less modified
in Insects by coalescence or suppression, and we find only eleven
ganglia in the Cockroach--viz., two cephalic, three thoracic, and six
abdominal.
[100] Yung (“Syst. nerveux des Crustacées Décapodes, Arch. de Zool.
exp. et gén.,” Tom. VII., 1878) proposes to name _connectives_ the
longitudinal bundles of nerve-fibres which unite the ganglia, and
to reserve the term _commissures_ for the transverse communicating
branches.
[Illustration: Fig. 39.--Nervous System of Female Cockroach, × 6. _a_,
optic nerve; _b_, antennary nerve; _c_, _d_, _e_, nerves to first,
second, and third legs; _f_, to wing-cover; _g_, to second thoracic
spiracle; _h_, to wing; _i_, abdominal nerve; _j_, to cerci.]
The nervous centres of the head form a thick, irregular ring,
which swells above and below into ganglionic enlargements, and
leaves only a small central opening, occupied by the œsophagus. The
tentorium separates the brain or supra-œsophageal ganglion from the
sub-œsophageal, while the connectives traverse its central plate. Since
the œsophagus passes above the plate, the investing nervous ring also
lies almost wholly above the tentorium.
[Illustration: Fig. 40.--Side view of Brain of Cockroach, × 25. _op_,
optic nerve; _oe_, œsophagus; _t_, tentorium; _sb_, sub-œsophageal
ganglion; _mn_, _mx_, _mx_′, nerves to mandible and maxillæ. Copied
from E. T. Newton.]
The brain is small in comparison with the whole head; it consists of
two rounded lateral masses or hemispheres, incompletely divided by
a deep and narrow median fissure. Large optic nerves are given off
laterally from the upper part of each hemisphere; lower down, and on
the front of the brain, are the two gently rounded antennary lobes,
from each of which proceeds an antennary nerve; while from the front
and upper part of each hemisphere a small nerve passes to the so-called
“ocellus,” a transparent spot lying internal to the antennary socket
on each side in the suture between the clypeus and the epicranium. The
sub-œsophageal ganglion gives off branches to the mandibles, maxillæ,
and labrum. While, therefore, the supra-œsophageal is largely sensory,
the sub-œsophageal ganglion is the masticatory centre.
The œsophageal ring is double below, being completed by the connectives
and the sub-œsophageal ganglion; also by a smaller transverse
commissure, which unites the connectives, and applies itself closely to
the under-surface of the œsophagus.[101]
[101] This commissure, which has been erroneously regarded as
characteristic of Crustacea, was found by Lyonnet in the larva of
Cossus, by Straus-Dürckheim in Locusta and Buprestis, by Blanchard
in Dytiscus and Otiorhynchus, by Leydig in Glomeris and Telephorus,
by Dietl in Gryllotalpa, and by Liénard in a large number of other
Insects and Myriapods, including Periplaneta. See Liénard, “Const. de
l’anneau œsophagien,” Bull. Acad. Boy. de Belgique, 2^e Sér., Tom.
XLIX., 1880.
Two long connectives issue from the top of the sub-œsophageal
ganglion, and pass between the tentorium and the submentum on their
way to the neck and thorax. The three thoracic ganglia are large (in
correspondence with the important appendages of this part of the
body) and united by double connectives. The six abdominal ganglia
have also double connectives, which are bent in the male, as if to
avoid stretching during forcible elongation of the abdomen. The sixth
abdominal ganglion is larger than the rest, and is no doubt a complex,
representing several coalesced posterior ganglia; it supplies large
branches to the reproductive organs, rectum, and cerci.
_Internal Structure of Ganglia._
Microscopic examination of the internal structure of the nerve-cord
reveals a complex arrangement of cells and fibres. The connectives
consist almost entirely of nerve-fibres, which, as in Invertebrates
generally, are non-medullated. The ganglia include (1) rounded,
often multipolar, nerve-cells; (2) tortuous and extremely delicate
fibres collected into intricate skeins; (3) commissural fibres, and
(4) connectives. The chief fibrous tracts are internal, the cellular
masses outside them. A relatively thick, and very distinct neurilemma,
probably chitinous, encloses the cord. Its cellular matrix, or
chitinogenous layer, is distinguished by the elongate nuclei of its
constituent cells.[102] Tracheal trunks pass to each ganglion, and
break up upon and within it into a multitude of fine branches.
[102] We have not been able to distinguish in the adult Cockroach the
_double_ layer of neurilemmar cells noticed by Leydig and Michels in
various Coleoptera.
[Illustration: Fig. 41.--Transverse section of Third Thoracic Ganglion.
_neu_, neurilemmar cells; _gc_, ganglionic cells; _tr_, tracheal tubes;
_A_, ganglionic cells, highly magnified. × 75.]
[Illustration: Fig. 42.--Longitudinal vertical section of Third
Thoracic Ganglion. _n_, connective. The other references as in fig. 41.
× 75.]
Bundles of commissural fibres pass from the ganglion cells of one
side of the cord to the peripheral nerves of the other. There are
also longitudinal bands which blend to form the connectives, and send
bundles into the peripheral nerves. Of the peripheral fibres, some are
believed to pass direct to their place of distribution, while others
traverse at least one complete segment and the corresponding ganglion
before separating from the cord.
[Illustration: Fig. 43.--Longitudinal horizontal section of Third
Thoracic Ganglion. _n_, peripheral nerves. The other references as
before. × 75.]
Many familiar observations show that the ganglia of an Insect possess
great physiological independence. The limbs of decapitated Insects, and
even isolated segments, provided that they contain uninjured ganglia,
exhibit unmistakable signs of life.
_Median Nerve-Cord._
Lyonnet,[103] Newport,[104] and Leydig[105] have found in large
Insects a system of median nerves, named _respiratory_ (Newport) or
_sympathetic_ (Leydig). These nerves do not form a continuous cord
extending throughout the body, but take fresh origin in each segment
from the right and left longitudinal commissures alternately. The
median nerve lies towards the dorsal side of the principal nerve-cord,
crosses over the ganglion next behind, and receives a small branch
from it. Close behind the ganglion it bifurcates, the branches passing
outwards and blending with the peripheral nerves. Each branch, close
to its origin, swells into a ganglionic enlargement. The median nerve
and its branches differ in appearance and texture from ordinary
peripheral nerves, being more transparent, delicate, and colourless.
They are said to supply the occlusor muscles of the stigmata. In the
Cockroach the median nerves are so slightly developed in the thorax
and abdomen (if they actually exist) that they are hardly discoverable
by ordinary dissection. We have found only obscure and doubtful traces
of them, and these only in one part of the abdominal nerve-cord. The
stomato-gastric nerves next to be described appear to constitute a
peculiar modification of that median nerve-cord which springs from the
circum-œsophageal connectives.
[103] Traité Anat., p. 201, pl. ix., fig. 1.
[104] Phil. Trans., 1834, p. 401, pl. xvi.
[105] Vom Bau des Thierischen Körpers, pp. 203, 262; Taf. z. vergl.
Anat., pl. vi., fig. 3.
_Stomato-gastric Nerves._
In the Cockroach the stomato-gastric nerves found in so many of the
higher Invertebrates are conspicuously developed. From the front of
each œsophageal connective, a nerve passes forwards upon the œsophagus,
outside the chitinous crura of the tentorium. Each nerve sends a branch
downwards to the labrum, and the remaining fibres, collected into two
bundles, join above the œsophagus to form a triangular enlargement,
the frontal ganglion. From this ganglion a recurrent nerve passes
backwards through the œsophageal ring, and ends on the dorsal surface
of the crop (·3 inch from the ring), in a triangular ganglion, from
which a nerve is given off outwards and backwards on either side.
Each nerve bifurcates, and then breaks up into branches which are
distributed to the crop and gizzard.[106] Just behind the œsophageal
ring, the recurrent nerve forms a plexus with a pair of nerves which
proceed from the back of the brain. Each nerve forms two ganglia, one
behind the other, and each ganglion sends a branch inwards to join the
recurrent nerve. Fine branches proceed from the paired nerves of the
œsophageal plexus to the salivary glands.
[106] The stomato-gastric nerves of the Cockroach have been carefully
described by Koestler (Zeits. f. wiss. Zool., Bd. XXXIX., p. 592).
[Illustration: Fig. 44.--Stomato-gastric Nerves of Cockroach. _fr.g._,
frontal ganglion; _at._, antennary nerve; _conn._, connective; _pa.g._,
paired ganglia; _r.n._, recurrent nerve; _v.g._, ventricular ganglion.]
The stomato-gastric nerves differ a good deal in different insects;
Brandt[107] considers that the paired and unpaired nerves are
complementary to each other, the one being more elaborate, according
as the other is less developed. A similar system is found in Mollusca,
Crustacea, and some Vermes (_e.g._, Nemerteans). When highly developed,
it contains unpaired ganglia and nerves, but may be represented only by
an indefinite plexus (earthworm). It always joins the œsophageal ring,
and sends branches to the œsophagus and fore-part of the alimentary
canal. The system has been identified with the sympathetic, and also
with the vagus of Vertebrates, but such correlations are hazardous; the
first, indeed, may be considered as disproved.
[107] “Mem. Acad. Petersb.,” 1835.
_Internal Structure of Brain._
The minute structure of the brain has been investigated by Leydig,
Dietl, Flögel, and others, and exhibits an unexpected complexity. It
is as yet impossible to reduce the many curious details which have
been described to a completely intelligible account. The physiological
significance, and the homologies of many parts are as yet altogether
obscure. The comparative study of new types will, however, in time,
bridge over the wide interval between the Insect-brain and the
more familiar Vertebrate-brain, which is partially illuminated by
physiological experiment. Mr. E. T. Newton has published a clear and
useful description[108] of the internal and external structure of the
brain of the Cockroach, which incorporates what had previously been
ascertained with the results of his own investigations. He has also
described[109] an ingenious method of combining a number of successive
sections into a dissected model of the brain. Having had the advantage
of comparing the model with the original sections, we offer a short
abstract of Mr. Newton’s memoir as the best introduction to the
subject. He describes the central framework of the Cockroach brain as
consisting of two solid and largely fibrous trabeculæ, which lie side
by side along the base of the brain, becoming smaller at their hinder
ends; they meet in the middle line, but apparently without fusion or
exchange of their fibres. Each trabecula is continued upwards by two
fibrous columns, the cauliculus in front, and the peduncle behind; the
latter carries a pair of cellular disks, the calices (the cauliculus,
though closely applied to the calices, is not connected with them);
these disks resemble two soft cakes pressed together above, and bent
one inwards, and the other outwards below. The peduncle divides above,
and each branch joins one of the calices of the same hemisphere.
[108] “Q. J. Micr. Sci.,” 1879, pp. 340–350, pl. xv., xvi.
[109] “Journ. Quekett Micr. Club,” 1879.
[Illustration: Fig. 45.--A, lobes of the brain of the Cockroach, seen
from within; _c_, cauliculus; _p_, peduncle; _t_, trabecula. B, ditto,
from the front; _ocx_, outer calyx; _icx_, inner calyx. C, ditto, from
above. Copied from E. T. Newton.]
This central framework is invested by cortical ganglionic cells,
which possess distinct nuclei and nucleoli. A special cellular mass
forms a cap to each pair of calices, and this consists of smaller
cells without nucleoli. Above the meeting-place of the trabeculæ is
a peculiar laminated mass, the _central body_, which consists of a
network of fibres continuous with the neighbouring ganglionic cells,
and enclosing a granular substance. The antennary lobes consist of a
network of fine fibres enclosing ganglion cells, and surrounded by a
layer of the same. It is remarkable that no fibrous communications
can be made out between the calices and the cauliculi, or between the
trabeculæ and the œsophageal connectives.
[Illustration: Fig. 46.--Model of Cockroach Brain, constructed from
slices of wood representing successive sections.]
[Illustration: Fig. 47.--Right half of Model-brain seen from the
inner side, with the parts dissected away, so as to show the anterior
nervous mass (_cauliculus_), _a_; the median mass (_trabecula_), _m_;
the mushroom-bodies (_calices_), _mb_; and their stems (_peduncles_),
_st_. The cellular cap, _c_, has been raised, so as to display the
parts below: _com_, is a part of the connective uniting the brain
and infra-œsophageal ganglia. [Figs. 45–48 are taken from Mr. E. T.
Newton’s paper in “Journ. Quekett Club,” 1879.]]
[Illustration: Fig. 48.--Diagrammatic outlines of sections of the Brain
of a Cockroach. Only one side of the brain is here represented. The
numbers indicate the position in the series of thirty-four sections
into which this brain was cut. _al_, antennary lobe; _mb_, mushroom
bodies (_calices_), with their cellular covering, _c_, and their stems
(_peduncles_), _st_; _a_, anterior nervous mass (_cauliculus_); _m_,
median nervous mass (_trabecula_). From E. T. Newton.]
[Illustration: Fig. 49.--Frontal section of Brain of Cockroach. _C_,
cellular layer beneath neurilemma; _ICx_, inner calix; _OCx_, outer
calix; _GC_, ganglion-cells; _P_, peduncle; _T_, trabecula; _Op_, optic
nerve; _AnL_, antennary lobe. × 24.]
_Sense Organs. The Eye of Insects._
The sense organs of Insects are very variable, both in position and
structure. Three special senses are indicated by transparent and
refractive parts of the cuticle, by tense membranes with modified
nerve-endings, and by peculiar sensory rods or filaments upon the
antennæ. These are taken to be the organs respectively of sight,
hearing, and smell. Other sense organs, not as yet fully elucidated,
may co-exist with these. The maxillary palps of the Cockroach,
for example, are continually used in exploring movements, and may
assist the animal to select its food; the cerci, where these are
well-developed, and the halteres of Diptera, have been also regarded as
sense organs of some undetermined kind, but this is at present wholly
conjecture.[110]
[110] It is to be remarked that unusually large nerves supply the
cerci of the Cockroach.
[Illustration: Fig. 50.--Plan of Eye of Cockroach, showing the number
of facets along the principal diameters. _as_, antennary socket.]
The compound eyes of the Cockroach occupy a large, irregularly oval
space (see fig. 50) on each side of the head. The total number of
facets may be estimated at about 1,800. The number is very variable in
Insects, and may either greatly exceed that found in the Cockroach, or
be reduced to a very small one indeed. According to Burmeister, the
Coleopterous genus Mordella possesses more than 25,000 facets. Where
the facets are very numerous, the compound eyes may occupy nearly the
whole surface of the head, as in the House-fly Dragon-fly, or Gad-fly.
Together with compound eyes, many Insects are furnished also with
simple eyes, usually three in number, and disposed in a triangle on
the forehead. The white fenestræ, which in the Cockroach lie internal
to the antennary sockets, may represent two simple eyes which have
lost their dioptric apparatus. In many larvæ only simple eyes are
found, and the compound eye is restricted to the adult form; in larval
Cockroaches, however, the compound eye is large and functional.
[Illustration: Fig. 51.--One element of the Compound Eye of the
Cockroach, × 700. _Co. F_, corneal facets; _Cr_, crystalline cones;
_Rm_, nerve-rod (rhabdom); _Rl_, retinula of protoplasmic fibrils.
To the right are transverse sections at various levels. Copied from
Grenacher.]
[Illustration: Fig. 52.--Diagram of Insect Integument, in section.
_bm_, basement-membrane; _hyp_, hypodermis, or chitinogenous layer;
_ct_, _ct_′, chitinous cuticle; _s_, a seta.]
Each facet of the compound eye is the outermost element of a series of
parts, some dioptric and some sensory, which forms one of a mass of
radiating rods or fibres. The facets are transparent, biconvex, and
polygonal, often, but not quite regularly, hexagonal. In many Insects
the deep layer of each facet is separable, and forms a concavo-convex
layer of different texture from the superficial and biconvex lens.
The facets, taken together, are often described as the cornea; they
represent the chitinous cuticle of the integument. The subdivision of
the cornea into two layers of slightly different texture suggests an
achromatic correction, and it is quite possible, though unproved, that
the two sets of prisms have different dispersive powers. Beneath the
cornea we find a layer of crystalline cones, each of which rests by
its base upon the inner surface of a facet, while its apex is directed
inwards towards the brain. The crystalline cones are transparent,
refractive, and coated with dark pigment; in the Cockroach they
are comparatively short and blunt. Behind each cone is a nerve-rod
(rhabdom), which, though outwardly single for the greater part of
its length, is found on cross-section to consist of four components
(rhabdomeres)[111]; these diverge in front, and receive the tip of
a cone, which is wedged in between them; the nerve-rods are densely
pigmented. The rhabdom is invested by a protoplasmic sheath, which
is imperfectly separated into segments (retinulæ), corresponding in
number with the rhabdomeres. Each retinula possesses at least one
nucleus. The retinulæ were found by Leydig to possess a true visual
purple. To the hinder ends of the retinulæ are attached the fibres of
the optic nerve, which at this point emerges through a “fenestrated
membrane.”
[111] The number in Insects varies from eight to four, but seven is
usual; four is the usual number in Crustacea.
[Illustration: Fig. 53.--Section through Eye of Dytiscus-larva, showing
the derivation of the parts from modified hypodermic cells. _L_, lens;
_Cr_, crystalline cones; _R_, nerve-rods; _N. Op._ optic nerve. From
Grenacher.]
In the simple eye the non-faceted cornea and the retinula are readily
made out, but the crystalline cones are not developed as such. The
morphological key to both structures is found in the integument, of
which the whole eye, simple or compound, is a modification. A defined
tract of the chitinous cuticle becomes transparent, and either swells
into a lens (fig. 53), or becomes regularly divided into facets
(fig. 55), which are merely the elaboration of imperfectly separated
polygonal areas, easily recognised in the young cuticle of all parts
of the body. Next, the chitinogenous layer is folded inwards, so as to
form a cup, and this, by the narrowing of the mouth, is transformed
into a flask, and ultimately into a solid two-layered cellular
mass (fig. 53). The deep layer undergoes conversion into a retina,
its chitinogenous cells developing the nerve-rods as interstitial
structures, while the superficial layer, which loses its functional
importance in the simple eye, gives rise by a similar process of
interstitial growth to the crystalline cones of the compound eye
(fig. 55). The basement-membrane, underlying the chitinogenous cells,
is transformed into the fenestrated membrane. The nerve-rods stand upon
it, like organ pipes upon the sound-board, while fibrils of the optic
nerve and fine tracheæ pass through its perforations. The mother-cells
of the crystalline cones and nerve-rods are largely replaced by the
interstitial substances they produce, to which they form a sheath; they
are often loaded with pigment, and the nuclei of the primitive-cells
can only be distinguished after the colouring-matter has been
discharged by acids or alkalis.
[Illustration: Fig. 54.--Section through Simple Eye of Vespa. The
references as above. Simplified from Grenacher.]
Dr. Hickson[112] has lately investigated the minute anatomy of the
optic tract in various Insects. He finds, in the adult of the higher
Insects, three distinct ganglionic swellings, consisting of a network
of fine fibrils, surrounded by a sheath of crowded nerve-cells. Between
the ganglia the fibres usually decussate. In the Cockroach, and some
other of the lower Insects, the outermost ganglion is undeveloped.
The fibres connecting the second ganglion with the eye take a straight
course in the young Cockroach, but partially decussate in the adult.
[112] “Q. J. Micr. Sci.,” 1885.
All the parts between the crystalline cones and the true optic nerve
are considered by Hickson to compose the retina of Insects, which,
instead of ending at the fenestrated-membrane, as has often been
assumed, includes the ganglia and decussating fibres of the optic
tract. The layer of retinulæ and rhabdoms does not form the whole
retina, but merely that part which, in the vertebrate eye, is known as
the layer of rods and cones.
[Illustration: Fig. 55.--Diagrammatic section of Compound Eye. The
references as above.]
As to the way in which the compound eye renders distinct vision
possible, there is still much difference of opinion. A short review
of the discussion which has occupied some of the most eminent
physiologists and histologists for many years past will introduce the
reader to the principal facts which have to be reconciled.
The investigation, like so many other trains of biological inquiry,
begins with Leeuwenhoeck (Ep. ad Soc. Reg. Angl. iii.), who ascertained
that the cornea of a shardborne Beetle, placed in the field of a
microscope, gives images of surrounding objects, and that these
images are inverted. When the cornea is flattened out for microscopic
examination, the images (_e.g._, of a window or candle-flame) are
similar, and it has been too hastily assumed that a multitude of
identical images are perceived by the Insect. The cornea of the
living animal is, however, convex, and the images formed by different
facets cannot be precisely identical. No combined or collective image
is formed by the cornea. When the structure of the compound eye had
been very inadequately studied, as was the case even in Cuvier’s time
(Leçons d’Anat. Comp., xii., 14), it was natural to suppose that all
the fibres internal to the cornea were sensory, that they formed a
kind of retina upon which the images produced by the facets were
received, and that these images were transmitted to the brain, to be
united, either by optical or mental combination, into a single picture.
Müller,[113] in 1826, pointed out that so simple an explanation was
inadmissible. He granted that the simple eye, with its lens and
concave retina, produces a single inverted image, which is able to
affect the nerve-endings in the same manner as in Vertebrates. But the
compound eye is not optically constructed so as to render possible the
formation of continuous images. The refractive and elongate crystalline
cones, with their pointed apices and densely pigmented sides, must
destroy any images formed by the lenses of the cornea. Even if the
dioptric arrangement permitted the formation of images, there is no
screen to receive them.[114] Lastly, if this difficulty were removed,
Müller thought it impossible for the nervous centres to combine a
great number of inverted partial images. How then can Insects and
Crustaceans see with their compound eyes? Müller answered that each
facet transmits a small pencil of rays travelling in the direction
of its axis, but intercepts all others. The refractive lens collects
the rays, and the pigmented as well as refractive crystalline cone
further concentrates the pencil, while it stops out all rays which
diverge appreciably from the axis. Each element of the compound eye
transmits a single impression of greater or less brightness, and the
brain combines these impressions into some kind of picture, a picture
like that which could be produced by stippling. It may be added that
the movements of the insect’s head or body would render the distance
and form of every object in view much readier of appreciation. No
accommodation for distance would be necessary, and the absence of all
means of accommodation ceases to be perplexing. Such is Müller’s theory
of what he termed “mosaic vision.” Many important researches, some
contradictory, some confirmatory of Müller’s doctrine,[115] have since
been placed on record, with the general result that some modification
of Müller’s theory tends to prevail. The most important of the new
facts and considerations which demand attention are these:--
[113] Exner has since determined by measurement and calculation the
optical properties of the eye of Hydrophilus. He finds that the focus
of a corneal lens is about 3mm. away, and altogether behind the eye.
[114] Zur vergl. Phys. des Gesichtsinnes.
[115] A critical history of the whole discussion is to be found in
Grenacher’s “Sehorgan der Arthropoden” (1879), from which we take
many historical and structural details.
Reasons have been given for supposing that images are formed by the
cornea and crystalline cones together. This was first pointed out by
Gottsche (1852), who used the compound eyes of Flies for demonstration.
Grenacher has since ascertained that the crystalline cones of Flies
are so fluid that they can hardly be removed, and he believes that
Gottsche’s images were formed by the corneal facets alone. He finds,
however, that the experiment may be successfully performed with
eyes not liable to this objection, _e.g._, the eyes of nocturnal
Lepidoptera. A bit of a Moth’s eye is cut out, treated with nitric acid
to remove the pigment, and placed on a glass slip in the field of the
microscope. The crystalline cones, still attached to the cornea, are
turned towards the observer, and one is selected whose axis coincides
with that of the microscope. No image is visible when the tip of the
cone is in focus, but as the cornea approaches the focus, a bristle,
moved about between the mirror and the stage, becomes visible. This
experiment is far from decisive. No image is formed where sensory
elements are present to receive and transmit it. Moreover, the image is
that of an object very near to the cornea, whereas all observations of
living Insects show that the compound eye is used for far sight, and
the simple eye for near sight. Lastly, the treatment with acid, though
unavoidable, may conceivably affect the result. It is not certain that
the cones really assist in the production of the image, which may be
due to the corneal facets alone, though modified by the decolorised
cones.
Grenacher has pointed out that the composition of the nerve-rod
furnishes a test of the mosaic theory. According as the percipient
rod is simple or complex, we may infer that its physiological action
will be simple or complex too. The adequate perception of a continuous
picture, though of small extent, will require many retinal rods; on
the other hand, a single rod will suffice for the discrimination of
a bright point. What then are the facts of structure? Grenacher has
ascertained that the retinal rods in each element of the compound eye
rarely exceed seven, and often fall as low as four--further, that the
rods in each group are often more or less completely fused so as to
resemble simple structures, and that this is especially the case with
Insects of keen sight.[116]
[116] Flies, whose eyes are in several respects exceptional, have
almost completely separated rods, notwithstanding their quick sight.
Certain facts described by Schultze tell on the other side. Coming
to the Arthropod eye, fresh from his investigation of the vertebrate
retina, Schultze found in the retinal rods of Insects the same lamellar
structure which he had discovered in Vertebrata. He found also that
in certain Moths, Beetles, and Crustacea, a bundle of extremely fine
fibrils formed the outer extremity of each retinal or nerve-rod. This
led him to reject the mosaic theory of vision, and to conclude that a
partial image was formed behind every crystalline cone, and projected
upon a multitude of fine nerve-endings. Such a retinula of delicate
fibrils has received no physiological explanation, but it is now known
to be of comparatively rare occurrence; it has no pigment to localise
the stimulus of light; and there is no reason to suppose that an image
can be formed within its limits.
The optical possibility of such an eye as that interpreted to us by
Müller has been conceded by physicists and physiologists so eminent
as Helmholz and Du Bois Reymond. Nevertheless, the competence of any
sort of mosaic vision to explain the precise and accurate perception
of Insects comes again and again into question whenever we watch the
movements of a House-fly as it avoids the hand, of a Bee flying from
flower to flower, or of a Dragon-fly in pursuit of its prey. The sight
of such Insects as these must range over several feet at least, and
within this field they must be supposed to distinguish small objects
with rapidity and certainty. How can we suppose that an eye without
retinal screen, or accommodation for distance, is compatible with sight
so keen and discriminating? The answer is neither ready nor complete,
but our own eyesight shows how much may be accomplished by means of
instruments far from optically perfect. According to Aubert, objects,
to be perceived as distinct by the human eye, must have an angular
distance of from 50″ to 70″, corresponding to several retinal rods.
Our vision is therefore mosaic too, and the retinal rods which can be
simultaneously affected comprise only a fraction of those contained
within the not very extensive area of the effective retina. Still
we are not conscious of any break in the continuity of the field of
vision. The incessant and involuntary movements of the eyeball, and
the appreciable duration of the light-stimulus partly explain the
continuity of the image received upon a discontinuous organ. Even more
important is the action of the judgment and imagination, which complete
the blanks in the sensorial picture, and translate the shorthand of the
retina into a full-length description. That much of what we see is seen
by the mind only is attested by the inadequate impression made upon us
by a sudden glimpse of unfamiliar objects. We need time and reflection
to interpret the hints flashed upon our eyes, and without time and
reflection we see nothing in its true relations. The Insect-eye may be
far from optical perfection, and yet, as it ranges over known objects,
the Insect-mind, trained to interpret colour, and varying brightness,
and parallax, may gain minute and accurate information. Grant that the
compound eye is imperfect, and even rude, if regarded as a camera; this
is not its true character. It is intended to receive and interpret
flashing signals; it is an optical telegraph.
Plateau[117] has recently submitted the seeing powers of a number of
different Insects to actual experiment. The two windows of a room five
metres square were darkened. An aperture fitted with ground glass was
then arranged in each window. At a distance of four metres from the
centre of the space between the windows captive Insects were from time
to time liberated. One of the windows was fenced with fine trellis, so
as to prevent the passage of the Insect, or otherwise altered in form,
but the size of the aperture could be increased at pleasure, so as
exactly to make up for any loss of light caused thereby, the brightness
of the two openings being compared by a photometer.
[117] Bull. de l’Acad. Roy. de Belgique, 1885.
It was found that day-flying Insects require a tolerably good light; in
semi-obscurity they cannot find their way, and often refuse to fly at
all. By varnishing one or other set in Insects possessing both simple
and compound eyes, it was found that day-flying Insects provided with
compound eyes do not use their simple eyes to direct their course.
When the light from one window was sensibly greater than that from
the other, the Insect commonly chose the brightest, but the existence
of bars, close enough to prevent or to check its passage, had no
perceptible effect upon the choice of its direction. Alterations in
the shape of one of the panes seemed to be immaterial, provided that
the quantity of light passing through remained the same, or nearly the
same. Plateau concludes that Insects do not distinguish the forms of
objects, or distinguish them very imperfectly.
It is plain, and Plateau makes this remark himself, that such
experiments upon the power of unaided vision in Insects, give a very
inadequate notion of the facility with which an Insect flying at large
can find its way. There the animal is guided by colour, smell, and the
actual or apparent movements of all visible objects. Exner has pointed
out how important are the indications given by movement. Even in man,
the central part of the retina is alone capable of precise perception
of form, but a moving object is observed by the peripheral tract.
Plateau (from whom this quotation is made) adds that most animals are
very slightly impressed by the mere form of their enemies, or of their
prey, but the slightest movement attracts their notice. The sportsman,
the fisherman, and the entomologist cannot fail to learn this fact by
repeated and cogent proofs.
_Sense of Smell in Insects._
The existence of a sense of smell in Insects has probably never been
disputed. Many facts of common observation prove that carrion-feeders,
for example, are powerfully attracted towards putrid animal substances
placed out of sight. The situation of the olfactory organs has only
been ascertained by varied experiments and repeated discussion.
Rosenthal, in 1811, and Lefebvre, in 1838, indicated the antennæ as
the organs of smell, basing their conclusions upon physiological
observations made upon living insects. Many entomologists of that
time were inclined to regard the antennæ as auditory organs.[118]
Observations on the minute structure of the antennæ were made by
many workers, but for want of good histological methods and accurate
information concerning the organs of smell in other animals, these
proved for a long time indecisive. It was by observation of living
insects that the point was actually determined.
[118] References to the literature of the question are given by
Hauser in Zeits. f. wiss. Zool., Bd. XXXIV., and by Plateau in Bull.
Soc. Zool. de France, Tom. X.
Hauser’s experiments, though by no means the first, are the most
instructive which we possess. He found that captive insects, though not
alarmed by a clean glass rod cautiously brought near, became agitated
if the same rod had been first dipped in carbolic acid, turpentine, or
acetic acid. The antennæ performed active movements while the rod was
still distant, and after it was withdrawn the insect was observed to
wipe its antennæ by drawing them through its mouth. After the antennæ
had been extirpated or coated with paraffin, the same insects became
indifferent to strong-smelling substances, though brought quite near.
Extirpation of the antennæ prevented flies from discovering putrid
flesh, and hindered or prevented copulation in insects known to breed
in captivity.
Following up these experiments by histological investigation of many
insects belonging to different orders, Hauser clearly established the
following points, which had been partially made known before:--
The sensory elements of the antennæ are lodged in grooves or pits,
which may be filled with fluid. The nerve-endings are associated with
peculiar rods, representing modified chitinogenous cells. The number
of grooves or pits may be enormous. In the male of the Cockchafer,
Hauser estimates that there are 39,000 in each antenna. He remarks
that in all cases where the female Insect is sluggish and prone to
concealment, the male has the antennæ more largely developed than the
female.
_Sense of Taste in Insects._
F. Will[119] gives an account of many authors who have investigated
with more or less success the sense organs of various Insects. He
relates also the results of his own experiments, and gives anatomical
details of the sensory organs of the mouth in various Hymenoptera.
[119] Zeits. f. wiss. Zool., 1885.
Wasps, flying at liberty, were allowed to visit and taste a packet
of powdered sugar. This was left undisturbed for some hours, and
then replaced by alum of the same appearance. The Wasps attacked the
alum, but soon indicated by droll movements that they perceived the
difference. They put their tongues in and out and cleansed them from
the ill-tasted powder. Two persisted at the alum till they rolled on
the table in agony, but they soon recovered and flew away. In a few
hours the packet was quite deserted. After a day’s interval, during
which the sugar lay in its usual place, powdered, and of course
perfectly tasteless, dolomite was substituted. The wasps licked it
diligently and could not be persuaded for a long time that it could do
nothing for them. Similar experiments were made with other substances,
and Insects whose antennæ and palps had been removed were subjected to
trial. The result clearly proved that a sense of taste existed, and
that its seat is in the mouth.[120] Peculiar nerve-endings, such as
Meinert and Forel had previously found in Ants, were found in abundance
on the labium, the paraglossæ, and the inner side of the maxillæ of
the Wasp. Some lay in pits, through the bases of which single nerves
emerged, and swelled into bulbs, or passed into peculiar conical
sheaths. Interspersed among the gustatory nerve-endings were setæ of
various kinds, some protective, some tactile, and others intended to
act as guiding-hairs for the saliva.
[120] Will confirms, by his own experiments (p. 685), Plateau’s
conclusion (_Supra_, p. 46), that the maxillary and labial palps have
nothing to do with the choice of food.
Will observes that the organs described satisfy the essential
conditions of a sense of taste. The nerve-endings pass free to the
surface, and are thus directly accessible to chemical stimulus.
Further, they are so placed that they and the particles of food which
get access to them are readily bathed by the saliva. Moistened or
dissolved in this fluid, the sapid properties of food are most fully
developed.
The sensory pits and bulbs appropriated to taste are believed to be
unusually abundant in the social Hymenoptera.
_Sense of Hearing in Insects._
The auditory organs of Insects and other Arthropoda are remarkable
for the various parts of the body in which they occur. Thus they have
been found in the first abdominal segment of Locusts, and in the tibia
of the fore-leg of Crickets and Grasshoppers, and more questionable
structures with peculiar nerve-endings have been described as occurring
in the hinder part of the abdomen of various larvæ (_Ptychoptera_,
_Tabanus_, _&c_). The auditory organ of Decapod Crustacea is lodged
in the base of the antennule, that of Stomapods in the tail, while an
auditory organ has been lately discovered on the underside of the head
of the Myriopod _Scutigera_.
Auditory organs are best developed in such Insects as produce sounds as
a call to each other. The Cockroach is dumb, and it is, therefore, not
a matter of surprise that no structure which can be considered auditory
should have ever been detected in this Insect.[121]
[121] For a popular account of auditory organs in Insects, see
Graber’s Insekten, Vol. I., page 287; also J. Müller, Vergl. Phys.
d. Gesichssinn, p. 439; Siebold, Arch. f. Naturg., 1844; Leydig,
Müller’s Arch. 1855 and 1860; Hensen, Zeits. f. wiss. Zool., 1866;
Graber, Denkschr. der Akad. der wiss. Wien, 1875; and Schmidt, Arch.
f. mikr. Anat., 1875.
The sensory hairs of the skin have been already noticed (p. 31).