to consist usually of an infinite number of convex _hexagonal_
pieces. If you examine with a good glass the eye of any fly, you
will find it traversed by numberless parallel lines, with others
equally numerous cutting them at right angles, so as apparently to
form myriads of little squares, with each a lens of the above figure
set in it. The same structure, though often not so easily seen,
obtains in the eyes of _Coleoptera_ and other insects. When the eye
is separated and made clean, these hexagons are as clear as crystal.
Reaumur fitted one eye to a lens, and could see through it well, but
objects were greatly multiplied[1466]. In Coleopterous insects they
are of a hard and horny substance; but in _Diptera_, &c. more soft
and membranous. The number of lenses in an eye varies in different
insects. Hooke computed those in the eye of a horse-fly to amount
to nearly 7,000[1467]; Leeuwenhoeck found more than 12,000 in that
of a dragon-fly[1468]; and 17,325 have been counted in that of a
butterfly[1469]. But of all insects they seem to be most numerous in
the beetles of Mr. W. S. MacLeay's genus _Dynastes_. In the eyes of
these the lenses are so small as not to be easily discoverable even
under a pocket microscope, except the eye has turned white[1470]: it
is not, therefore, wonderful, that Fabricius should call these eyes
_simple_[1471]. In some insects, however, as in the _Strepsiptera_
Kirby, the lenses are not numerous: in _Xenos_ they do not exceed
fifty, and are distinctly visible to the naked eye[1472]. These
lenses vary in magnitude, not only in different, but sometimes in
the same eyes. This is the case in those of male horse-flies and
flies, those of the upper part of the eye being much larger than
those of the lower[1473]. The partitions that separate the lenses,
or rather bezels, in which they are set, are very visible in the
eyes just mentioned, and those of _Xenos_; but in many insects they
are only discernible at the intersecting lines of separation between
the lenses. In hairy eyes, such as those of the hive-bee, the hairs
emerge from these _septa_. Every single lens of a compound eye may be
considered as a _cornea_, or a _crystalline humour_, it being convex
without and concave within, but thicker in the middle than at the
margin: it is the only transparent part to be found in these most
remarkable eyes. Immediately under the _cornea_ is an opaque varnish,
varying according to the species, which produces sometimes in one
and the same eye spots or bands of different colours. These spots
and bands form a distinguishing ornament of many of the _Tabani_ and
other flies. And to this varnish the lace-winged flies (_Hemerobius_,
&c.) are indebted for the beautiful metallic hues that often adorn
them. When insects are dead, this varnish frequently loses its
colour, and the eye turns white: hence many species are described
as having _white_ eyes which when alive had _black_ ones. The
consistence of this covering is the same with that of the varnish
of the _choroid_ in the eyes of vertebrate animals; but it entirely
covers the underside of the lens, without leaving any passage for the
light. Below this varnish there are numbers of short white hexagonal
prisms[1474], every one of which enters the concavity of one of the
lenses of the cornea, and is only separated from it by the varnish
just described: this may be considered as the _retina_ of the lens
to which it is attached; but at present it has not been clearly
explained how the light can act upon a _retina_ of this description
through an opaque varnish. Below this multitude of threads (for such
the bodies appear), perpendicular to the _cornea_, is a _membrane_
which serves them all for a base, and which consequently is nearly
parallel with that part. It is very thin, of a black colour, not
produced by a varnish; and in it may be seen very fine white
_tracheæ_, which send forth branches still finer, that penetrate
between the prisms of the _cornea_: this membrane may be called
the _choroid_. Behind this is a thin expansion of the optic nerve,
which is a true nervous membrane, precisely similar to the _retina_
of red-blooded animals. It appears that the white pyramidal threads
which form the _retina_ of each lens are sent forth by this general
_retina_, and pierce the choroid by a number of almost imperceptible
holes[1475]. From this description it appears that the eyes of
insects have nothing corresponding with the _uvea_ or _humours_ of
those of vertebrate animals, but are of a type peculiar to themselves.
Having explained to you the wonderful and complex structure with
which it has pleased the CREATOR to distinguish the organs of vision
of these minute beings, proving, what I have so often asserted,
that when animals seem approaching to nonentity, where one would
expect them to be most _simple_, we find them in many cases most
_complex_, I shall now call your attention to the next thing I am
to consider--the _number_ of the eyes in question. Most insects
have only _two_; but there are several exceptions to this rule.
Those that have occasion to see both above and below the head,
the eyes of all being immovable, must have them so placed as to
enable them to do this. This end is accomplished in many beetles,
for instance _Scarabæus_ L., _Helæus_ Latr., &c., by having these
organs fixed in the _side_ of the head, so that part looks upward
and part downward; but in others _four_ are given for this purpose.
If you examine the common whirlwig (_Gyrinus Natator_) that I have
so often mentioned[1476], which has occasion, at the same time,
to observe objects in the air and in the water, you will find it
is gifted with this number of eyes. _Lamia Tornator_ (_Cerambyx
tetrophthalmus_ Forst.) and some others, of which I make a genus,
under the appellation of _Tetrops_, are also so distinguished. In
these insects, one eye is above and the other below the base of
the antennæ; in fact, in these the _canthus_, instead of dividing
the eye partially, as in the other Capricorn-beetles, runs quite
through it at considerable width[1477]. In _Ryssonotus_ MacLeay
(_Lucanus nebulosus_ K.) the eye appears also to be divided in two
by the _canthus_. In the _Neuroptera_ Order there is more than one
instance of the same kind. In _Ascalaphus_ there are two considerable
eyes on each side of the head, which, though clearly distinct, meet
like those of many male flies and the drone. The male, likewise,
of more than one species of _Ephemera_, besides the common lateral
eyes and the _stemmata_ on the back of the head, have a pair of
compound eyes on the top of a short columnar process[1478]. In the
_Hemiptera_ Order, also, an instance occurs of four eyes in the
genus _Aleyrodes_[1479]. Amongst the vertebrate animals, there
is an example of eyes with two pupils in _Anableps_, a genus of
fishes[1480], but no vertebrate animal has _four_ of these organs.
That many insects should have more than _two_ eyes, will not seem
to you so extraordinary as that any should be found that, like the
Cyclops of old, have only _one_. There is, however, an insect,
before celebrated for its agility[1481] (_Machilis polypoda_ Latr.),
which has a single eye in its forehead; or we may say, its eyes
are confluent, without any line of distinction between them except
a small notch behind. Now that I am treating of the _number_ of
eyes, I must not forget to observe to you, that in some insects no
eyes at all have been discovered. In _Polydesmus complanatus_, on
each side of the head there is an eye-shaped portion separated by a
suture, in which under a powerful lens I cannot satisfy myself that
I can discern any thing like the facets that usually distinguish
compound eyes. In _Geophilus electricus_, another myriapod, they
certainly do not exist[1482]. Whence we may conclude, as was before
observed[1483], that the faculty of emitting light is rather given it
as a means of defence than to guide it in its path.
The _situation_ of compound eyes differs in different tribes. In
some, as in the _Staphylinidæ_, they are planted laterally in the
_anterior_ part of the head; in others, the _Carabi_ &c., in the
_middle_; in others again, _Locusta_ Leach &c., in the _posterior_
part. In some, their station is more in the upper surface, either
before or behind; so that a very narrow space separates them, or
perhaps none at all. Instances of this position of the eyes occur
in a minute weevil (_Ramphus_ Clairv.[1484]), and many _Diptera_,
&c. Of those that form an union on the top of the head, some are
placed obliquely, so as to leave a diverging space below them,
as in many _Libellulina_[1485], the drone[1486], &c. Others, as
_Atractocerus_, in which the eyes occupy nearly the whole head,
and unite anteriorly, have this diverging space _above_ their
conflux. In _Rhina barbirostris_ Latr., another kind of weevil,
they are confluent _below_ the head, at the base of the rostrum,
and a very narrow interval separates them above. In a large number
of the _Heteromerous_ beetles, they are set _transversely_, in the
_Capricorn_ ones _longitudinally_. Their surface, when they are
lateral, has usually two aspects, one _prone_ to see below, the other
_supine_ to see above. In general the eyes are situated behind the
antennæ, so that their position, whether it shall be anterior or
posterior, depends upon that of those organs. Often, indeed, as in
the last-named beetles, part of the eye is behind and part before the
antennæ; but except where there are _four_ eyes, as in _Tetrops_,
they are never placed before or below them.
Though the eyes of insects are generally _sessile_, yet to give them
a wider range they are sometimes, but it rarely occurs, placed, like
those of many _Crustacea_, on a _footstalk_, but not a _moveable_
one. An instance of this in certain male _Ephemeræ_ has already been
mentioned. In the _Hemiptera_ De Geer has figured two species of bugs
(_Cimicidæ_) that are so circumstanced[1487]; as are also all the known
_Strepsiptera_ K., though in these the footstalk is very short[1488]:
but the most remarkable example of columnar eyes is afforded by that
curious _Dipterous_ genus _Diopsis_, in which both eyes and antennæ
stand upon a pair of branches, vastly longer than the head, which
diverge at a very obtuse angle from its posterior part[1489].
In their _figure_ eyes vary much. Sometimes they are so prominent as
to be nearly _spherical_: this is the case with some aquatic bugs,
as _Ranatra_, _Hydrometra_, and several male _Ephemeræ_[1490]. Very
often they are _hemispherical_, as in the tiger-beetles (_Cicindela_
L.), and the clocks or dors (_Carabus_ L.); but in a large number
of insects they are flat, and do not rise above the surface of the
head.--With regard to their _outline_, they are often perfectly
_round_, as in many weevils; _oval_, as in various bees; _ovate_, as
in other bees (_Andrena_ F.); _triangular_, as in the water-boatman
(_Notonecta_). They are also often _oblong_, and occasionally narrow
and _linear_; as in that singular beetle _Helæus_. In many of the
_Muscidæ_ they form nearly a _semicircle_, or rather, perhaps, the
quadrant of a sphere. The eyes of the Capricorn-beetles (_Cerambyx_
L.) have a sinus on their inner side, as it were, taken out of them;
so that they more than half surround the antennæ, before which is the
longest portion of them. An approach to this shape is more or less
observed in the darkling-beetles (_Tenebrio_ L.); but in these the
sinus is not so deep. I may under this head observe, that in those
_Mantidæ_ that represent dry leaves, and some others, these organs
usually terminate in a spine[1491].
Though not distinguished by the beauty and animation that give such
interest to the eye of vertebrate animals, and exhibiting no trace of
iris or pupil, yet from the variety of their _colours_ the compound
eyes of insects, though most commonly black or brown, are often very
striking. Look at those of one of the lace-winged flies that commit
such havoc amongst the _Aphides_[1492], and it will dazzle you with
the splendour of the purest gold, sometimes softened with a lovely
green. The lenses of those of _Xenos_ blaze like diamonds set in
jet[1493]. You have often noticed the fiery eyes of many horse-flies
(_Tabanus_ L.) with vivid bands of purple and green[1494]. Others
are spotted[1495]; and Schellenberg has figured one (_Thereva
hemiptera_)[1496], that exhibits the figure of a flower painted in
red on a black ground. These colours and markings are all most vivid
and brilliant in the _living_ insect, and often impart that fire
and animation to the eyes for which those of the higher animals are
remarkable. Take one of the large dragon-flies that you see hawking
about the hedges in search of prey, examine its eyes under a lens, and
you will be astonished at the brilliance and crystalline transparency
which its large eyes exhibit, and by the remarkable vision of larger
hexagons which appear in motion under the _cornea_, being reflected by
the _retina_--all which give it the appearance of a living eye. This
moving reflexion of the hexagonal lenses in living insects was noticed
long since in some bees (_Nomada_ F., _Cœlioxys_ Latr.)[1497]
Compound eyes differ greatly in their _size_. In some insects,
as _Atractocerus_, the drone-bee, many male _Muscidæ_, &c., they
occupy nearly the whole of the head; while in others, as numerous
_Staphylinidæ_, _Locusta_ Leach, &c., they are so small as to be
scarcely larger than some simple eyes of spiders: and they exhibit
every intermediate difference of magnitude in different tribes,
genera, and species.
Under this head I must say something of the _Canthus_ of the eye;
by which I mean an elevated process of the cheek, which in almost
all the genera of the Lamellicorn beetles enters the eye more or
less, dividing the upper portion from the lower. Though usually
only a _process_ of the cheek, yet in the _Scarabæidæ_ the whole
of that part forms the _canthus_[1498]. It only _enters_ the eye
in the _Rutelidæ_, _Cetonidæ_, &c.; it extends through _half_ of
it in _Copris_; it goes _beyond_ the half in _Ateuchus_; and in
_Ryssonotus_ MacLeay (_Lucanus nebulosus_ K.) it quite divides the
eye into two[1499], as I before observed. In _Lucanus_, _Passalus_
&c. it projects before the eye into an angle; in _Lucanus femoralis_
nearly into a spine; but in _Lamprima_ and _Œsalus_ it does not
exist. The part, also, that enters the eye in the Capricorn-beetles
may be regarded as a kind of _canthus_, though it is merely a
dilatation of the _front_.
4. _Stemmata_[1500].--Having given so full an account of the kinds
and structure of the ordinary eyes of insects, you may perhaps expect
that I should now dismiss the subject: you would, however, have great
cause to blame me, did I not make you acquainted with a kind of
auxiliary eyes with which a large portion of them are gifted; I mean
those pellucid spots often to be found on the posterior part of the
front of these animals, or upon the vertex, frequently arranged in a
triangle. These, Linné, from his regarding them as a kind of coronet,
called _Stemmata_. They have been of late denominated _Ocelli_; but as
this latter term is also in general use for the eyelets on the wings
of _Lepidoptera_, I have adhered to that of the illustrious Swede.
Neither he nor Fabricius has expressed any opinion as to the _use_ of
these organs; but Swammerdam and Reaumur were aware that they were
real _eyes_. The former found that there are nerves that diverge to
them though not easily traced, and that they have a _cornea_, and
what he takes for the _uvea_[1501]; and the latter has supposed that
the compound eyes and these simple ones have, the one the power of
magnifying objects much, and the other but little, so that the former
are for surveying those that are distant, and the latter those that
are near[1502]. The same author relates some experiments that he tried
with the common hive bee, by which he ascertained that the _stemmata_,
as well as the compound eyes, were organs of vision. He first smeared
the _latter_ over with paint, and the animals, instead of making for
their hive, rose in the air till he lost sight of them. He next did the
same with the _former_, and placing the bees whose _stemmata_ he had
painted within a few paces of their hive, they flew about on all sides
among the neighbouring plants, but never far: he did not observe that
these ever rose in the air like the others[1503]. From this experiment
it seems as if the compound eyes were for _horizontal_ sight, and the
_stemmata_ for _vertical_.
The definition of them by Linné and Fabricius as smooth, shining,
elevated or hemispheric puncta, conveys a very inadequate idea of
them; for, except in a very few instances, they are perfectly clear
and transparent, and their appearance is precisely the same as that
of the simple eyes of _Arachnida_ &c., under which head they might
very well have been arranged; but as the last are _primary_ eyes, and
the stemmata _secondary_, it seemed to me best that they should stand
by themselves. The structure of both is probably the same, and their
internal organization that of one of the lenses of a compound eye,
and both are set in a socket of the head.
Though a large number of insects have them, they are by no means
universal, since some Orders, as the _Strepsiptera_, _Dermaptera_, and
_Aptera_, are altogether without them. The _Coleoptera_, also, have
been supposed to afford no instance of species furnished with them;
but in the last number of Germar and Zincken Sommer's _Magasin_, it is
affirmed that they are discoverable in Gravenhorst's genus _Omalium_,
but not in the kindred genera _Micropeplus_ and _Anthophagus_[1504].
Upon examining the former genus, I find, that although _Omalium
planum_ and affinities, _O. striatulum_, and some others, appear
not to have them, yet with the aid of a good magnifier they may be
discovered in most species of that genus; as likewise in _Evœsthetus_
Grav. I find them also very conspicuous in _A. Caraboides_ and other
_Anthophagi_, but some species appear to want them. In these insects
they are two in number, situated in the vertex a little behind the
eyes but within them, and either at each end of a transverse furrow,
or at the posterior termination of two longitudinal ones. Nor are they
found in all the genera of the other Orders. In the _Orthoptera_, the
_Blattidæ_, unless a white smooth spot on the inner and upper side of
the eyes may be regarded as representing them, have them not; but in
all the other genera of that Order they are to be found[1505]. In the
_Hemiptera_ all the _Cicadiadæ_ are gifted with them; as are likewise
_Tetyra_, _Pentatoma_, with many other _Cimicidæ_, and the _Reduviadæ_
very remarkably; but many others in both sections of this order, as
_Thrips_, _Coccus_, _Aphis_, _Capsus_, _Miris_, _Naucoris_, _Nepa_, and
_Notonecta_, &c. are deprived of them[1506]. Of the _Neuroptera_ the
_Libellulina_ add _stemmata_ to their large eyes, in the anterior angle
of which they are stationed[1507]; but many other genera of that Order
are without them; as _Myrmeleon_, _Ascalaphus_, _Hemerobius_, &c. The
_Trichoptera_ and _Lepidoptera_ universally have them; though in the
latter, except in _Castnia_ and the _Sphingidæ_, they are not easily
seen. In the _Hymenoptera_ they are usually very conspicuous, but in
_Larra_ and _Lyrops_, two genera of this order, the posterior pair are
scarcely discernible; and in the neuter ants they are quite obsolete.
In the _Diptera_, though many genera are furnished with them, yet many
also want them; amongst the rest Latreille's _Tipulariæ_, and all the
horse-flies (_Tabanus_ L.). The _Pupiparæ_ (_Hippobosca_ L.) usually
have none; but in _Ornithomyia avicularia_, one of that tribe, though
extremely minute they are visible, arranged in a triangle, in the
polished space of their vertex.
As to the _Number_ of the _stemmata_, _three_ appears to be most
universal. Reaumur mentions an instance in which he counted _four_ in
a fly with two threads at its tail; but great doubt rests upon this
statement[1508]. Some Orthopterous genera, as _Gryllotalpa_, and many
Hemipterous, as _Tetyra_, _Pentatoma_, _Reduvius_[1509], _Cercopis_,
_Fulgora_[1510], &c., have no more than _two_; and in _Larra_ and its
affinities, as just observed, the posterior ones are obsolete, so as
to leave only _one_ discernible.
Where there are _three_ of these organs, they are usually arranged in
an obverse _triangle_ in the space behind the antennæ, at a greater
or less distance from them. In those male flies (_Muscidæ_) whose
eyes are confluent, the _stemmata_ are in a little area _behind_
their conflux; but, as before observed, in the drone-bee and the
_Libellulina_ they are _before_ it. This triangle is in some cases
nearly _equilateral_, as in _Perla_ related to the may-flies, and
many _Hymenoptera_; in others it is _acutangular_, as in _Locusta_
&c., in which the _stemma_ forming the vertex of the triangle is
before the antenna[1511]: in others, again, it is _obtusangular_,
as you will see in _Pepsis_ and various _Hymenoptera_. In the
humble-bees (_Bombus_), a line drawn through them would form a slight
curve. Their _situation_ also varies. In insects that have only
_two_, they are sometimes placed a little _behind_ the eyes, or in
the back part of the space between them: this is the case with most
of the bugs (_Cimex_ L.) that have them.--They are often _distant_,
as in _Tetyra_ F., _Edessa_ F.; and sometimes _approximated_, as
in _Reduvius_ F.[1512] In many of the Homopterous _Hemiptera_, as
_Cercopis_, _Ledra_, &c. they are planted in the _upper_ part of
the head[1513], but in _Iassus_ their situation is on the _under_
part; and in a North American subgenus, as yet without a name, they
are exactly _between_ the two, being placed in the frontal angle.
In _Fulgora_ their station is between the eyes and antennæ[1514].
They are most commonly _sessile_, and as it were _set_ in the head;
but in some, as _Fulgora candelaria_, they stand on a _footstalk_.
The _stemmata_ are set in the side of a frontal tubercle in that
four-winged fly of threatening aspect, _Corydalis_, which in its
_perfect state_ has mandibles, but longer and more tremendous,
like those that distinguish the _larva_ only of the kindred genus
_Hemerobius_[1515]. These organs differ little in _shape_, being
usually perfectly _round_ and somewhat _convex_; but occasionally
they vary in this respect. In _Fulgora serrata_ they are _oblong_,
with a longitudinal depression; in _F. Diadema_ they are also
umbilicated, but the _umbilicus_ is circular; in _Corydalis_ they are
_oval_; in other insects they are _ovate_; in some _semicircular_,
and in a few _triangular_. They vary much in _size_: in some of
these animals being so minute as to be scarcely visible, while in
others, as _Corydalis_, _Dorylus_, _Vespa pallida_ F., _Reduvius_,
&c.[1515], they are as large as some compound eyes. They differ also
in _colour_, though often _black_: in _Fulgora laternaria_ they
are of a beautiful _yellow_; in F. _candelaria_ they are _white_;
in many _Hymenoptera_ they are _crystalline_, in others _red_: the
fierce look of _Reduvius personatus_ is rendered more hateful by its
_stemmata_ having a pale iris round a dark pupil[1516].
Let us here stop and adore the goodness of a beneficent CREATOR, who,
though he has deprived these little beings of the _moveable_ eyes
with which he has gifted the higher animals, has made it up to them
by the variety and complex structure of their organs of vision, where
_we_ have only _two_ points of sight, giving _them_ more than as many
_myriads_.
5. _Antennæ._--But of all the organs of insects, none appear to be
of more importance to them than their _Antennæ_, and none certainly
are more wonderful and more various in their structure, and probably
uses. Upon this last particular I shall enlarge hereafter. Their
structure, as far as it differs in the sexes, I fully discussed in
a former letter[1517]; and the most remarkable kinds of them will
be included in a set of definitions which I shall draw up for you
before our correspondence on this part of my subject closes: I shall
therefore now confine myself to the following particulars--namely,
their _number_, _insertion_, _substance_, _situation_, _proportion_,
_general form_ and _structure_, _clothing_, _expansion_, _motions_,
and _station of repose_.
As to their _Number_, in the majority of _crustaceous_ animals the
antennæ amount to _four_, but no _insect_ has more than _two_. A
genus recently established (_Otiocerus_ Kirby[1518]) seems to afford
an exception to this rule, since the species composing it at first
sight appear to have _four_, and in some instances even _six_ antennæ;
but as only _two_ of them terminate in a bristle, the other, though
proceeding from the same bed of membrane, may perhaps be regarded
as merely appendages. Germar, who has described a species of this
genus[1519] under the name of _Cobax Wintheri_, considers these
appendages as analogous to _palpi_: but as they do not proceed from
the _oral_ organs, but from the bed of the _antennæ_ at the base of the
nose[1520], they ought certainly to be regarded rather as accessories
to the latter, than as representing the former. In the _Aptera_ order
the mites (_Acacus_ L.) appear to be without these organs. In the
pupiparous tribe _Hippobosca_ they seem about to disappear; and in
the _Arachnida_ &c., as has been more than once observed[1521], the
_mandibulæ_ have been thought to represent, not indeed the antennæ of
_insects_, but the _inner_ pair of those of the _Crustacea_.