name and notice, which I conceive in some sort to be analogous to
the _nostrils_ of quadrupeds, and which I have therefore named the
_Rhinarium_ or nostril-piece. I had originally distinguished it by
the plural term _nares_, nostrils; but as it is usually a single
piece, I thought it best to denote it by one in the singular. When
I treat of the senses of insects, I shall give you my reasons, as I
have before said, for considering this part as the organ of scent,
or connected with it, which you will then be able to appreciate.
I shall only here observe, that the piece in question is in the
usual situation of the nostrils--between the nose and the lip.
In a large number of insects this part may be regarded as nearly
obsolete; or at least it is merely represented by the very narrow
membranous line that intervenes between the nose and the lip and
connects them; which, as in the case of the head of _Harpali_
before noticed, may be capable of tension and relaxation, and so
present a greater surface to the action of the atmosphere. But I
offer this as mere conjecture. In the lady-bird (_Coccinella_) this
line is a little wider, and becomes a distinct _Rhinarium_; as it
does also in _Geotrupes_. With respect to its _insertion_, the
_rhinarium_ is a piece that either entirely separates the nose from
the lip, or only partially: the former is the most common structure.
It is particularly remarkable in a New Holland genus of chafers
(_Anoplognathus_ Leach). In _A. viridiæncus_ it is very ample, and
forms the under side of the recurved nose, so that a large space
intervenes between the margin of the latter and the base of the
_labrum_. In _Macropus_ Thunb., of the Capricorn tribe (_Cerambyx_
L.), the nostril-piece, which forms a distinct segment, is narrower
than the nose, and the upper-lip than the nostril-piece, forming as
it were a triple gradation from the front to the mouth. Again, in
others the part in question is received into a sinus of the nose.
This is the case with the dragon-flies (_Libellulina_), in which this
sinus is very wide; in the burying-beetle (_Necrophorus_)[1414], in
some species of which it is deep but narrow; and in a species of
_Tenebrio_ from New Holland, which perhaps would make a subgenus.
If you examine with a common glass any of the larger rove-beetles
(_Staphylinidæ_), you will find that the nose itself seems lost in
the nostril-piece, both together forming a very narrow line across
the head above the _labrum_, without any apparent distinction between
them; but if you have recourse to a higher magnifier, you will find
this divided into an upper and lower part, the former of the hard
substance of the rest of the head, and the latter membranous. I once
was of opinion that the prominent transversely furrowed part, so
conspicuous in the face of _Tettigonia_ F.[1415], was the _front_:
but upon considering the situation of this, chiefly below the eyes
and antennæ, and comparing it with the analogous piece in _Fulgora
laternaria_ and other insects of the Homopterous section of the
_Hemiptera_, I incline to think that it represents the _nose_, and
that the longitudinal ridge below it is the nostril-piece[1416].
In the Heteropterous section it is merely the vertical termination
of their narrow nose. In other insects again, this part approaches
in some measure to the common idea of nostrils; there being _two_,
either one on each side the nose, or two approximated ones. If you
catch the first humble-bee that you see busy upon a flower, you will
discover a minute membranous protuberance under each angle of the
nose. Something similar may be observed in some species of _Asilus_
L. In the _Orthoptera_, especially in _Blatta_, _Phasma_, and some
_Locustæ_, two roundish or square pieces, close to each other on the
lower part of the nose, represent the nostrils[1417].--With regard to
_substance_, in the chafer-tribes, at least those that feed on leaves
or living vegetable matter, as the _Melolonthidæ_, _Anoplognathidæ_,
and in many other insects, the _rhinarium_ is of the same substance
with the rest of the head; but in _Macropus_ Thunb., _Staphylinus_,
_Necrophorus_, &c., it consists of membrane.
ii. _Postnasus_[1418].--This is a part that appears to have been
confounded by Entomologists with the front of insects; in general,
indeed, it may be regarded as included in the nose, and does not
require separate notice: but there are many cases in which it is
distinctly marked out and set by itself, and in which it forms a
useful diagnostic of genera or subgenera. There is a very splendid and
beautiful Chinese beetle, to be seen in most collections of foreign
insects (_Sagra purpurea_), in which this part forms a striking
feature, and helps to distinguish the genus from its near neighbour
_Donacia_. If you examine its face, you will discover a triangular
piece, below the antennæ and above the _nasus_, separated from the
latter and from the front by a deeply-impressed line: this is the
_postnasus_ or _after-nose_. Again: if you examine any specimens of a
Hymenopterous genus called by Fabricius _Prosopis_ (_Hylæus_ Latr.),
remarkable for its scent of baum, you will find a similar triangle
marked out in a similar situation[1419]. In many Coleopterous
insects, besides _Sagra_, you will discover traces of the part we are
considering: as in _Anthia_, _Dytiscus_, and several others of the
Predaceous beetles. In _Cistela_ it is larger than the nose itself;
but it is more conspicuous in the _Orthoptera_, particularly in
_Locusta_ (_Gryllus_ F.), in which it is the space below the antennæ,
distinguished by two or four rather diverging ridges[1420]. In the
_Libellulina_, _Myrmeleonina_, &c. it is a distinct transverse piece.
In _Dasyga_ Latr., a kind of bee, it is armed with a transverse ridge
or horn--But enough has been said to render you acquainted with it; I
shall therefore proceed to the next piece.
iii. _Frons_[1421].--The _Front_ of insects may be denominated the
_middle_ part of the face between the eyes, bounded anteriorly by the
nose, or after-nose, where it exists, and the cheeks; laterally by
the eyes; and posteriorly by the vertex. Speaking properly, it is the
region of the _antennæ_; though when these organs are placed before
the eyes, under the margin of the nose, as in many Lamellicorn and
Heteromerous beetles, they seem to be rather _nasal_ than _frontal_.
This part is often elevated, as in the elastic beetles (_Elater_),
whose faculty of jumping, by means of a pectoral spring, has been
related to you[1422]. In _Anthia_, a Predaceous beetle, it has
often three longitudinal ridges. In many of the Capricorn beetles
(_Cerambyx_ L.), it is nearly in the shape of a Calvary cross, with
the arms forming an obtuse angle, and then terminating at the sinus
of the eyes in an elevation for the site of the antennæ. In the
ants also (_Formicidæ_), the front is often elevated between those
organs. In _Ponera_, one tribe of them, this elevation is bilobed,
and receives between its lobes the vertex of the _postnasus_. In
the hornet (_Vespa Crabro_) the elevation is a triangle, with its
vertex towards the mouth. In _Sagra_ it is marked out into three
triangles, the _postnasus_ making a fourth, with the vertexes
meeting in the centre. In the _Dynastidæ_ and _Scarabæidæ_ the
horns are often _frontal_ appendages, as is that of _Empusa_ Latr.,
a leaf-insect, and probably those of _Sphinx Iatrophæ_ F., which
affords a singular instance of a horned Lepidopterous one. Sometimes
it is an ample space, reducing the nose to a very narrow line, as in
the _Staphylinidæ_, or sending forth a lobe on each side, as before
mentioned, which embraces the nose. In a species of bug from Brazil,
related to _Aradus_ F., these lobes are dilated, foliaceous, and
meet before the nose, so as to form a remarkable extended frontlet
to the head. In others this part is extremely minute: thus in many
_male_ flies and other insects, as the _Libellulina_, where the
eyes touch each other, the front is cut off from the vertex and
reduced to a small angle. In the female flies the communication
with the vertex is kept open, and the front consequently longer.
In the horse-flies (_Tabanidæ_), in _Hæmatopota_, and _Heptatoma_,
the frontal space is wider than in the rest of that tribe. Many
of these are distinguished by a levigated area behind the antennæ
in the part we are treating of. In the _Libellulina_, and in the
drone-bee, whose eyes are confluent, the stemmata are in the front.
In many _Orthoptera_ also, as _Locusta_ Leach, one of them is below
the antennæ; and in the lanthorn-fly tribe (_Fulgoridæ_), both these
organs, which are situate between them and the eyes, as they do also
in _Truxalis_, appear to be in it[1423]. In this tribe the rostrum is
an elongation of the part in question; and perhaps you would think
at first that what I have considered as the _nose_ in _Tettigonia_
F. was also a tendency to this kind of rostrum; but if you examine
the great lanthorn-fly (_Fulgora laternaria_), you will find besides,
at the lower base of the lanthorn, a triangular piece analogous to
the nose of _Tettigonia_, and below it another representing its
nostril-piece:--the horizontal part of the nose in that genus may
perhaps be regarded as part of the front. In _Truxalis_ F. the face
consists of a supine and prone surface, and the latter is composed
of the front, after-nose, nose, and organs of the mouth. I may
notice here a most remarkable and singular tribe of bugs, of which
two species have been figured by Stoll[1424]: in these the head,
or rather those parts of it that we have now been describing, the
nose, namely, the after-nose, and front, are absolutely divided
longitudinally in two, each half having an eye and antenna planted in
it; or perhaps, as it is stated to be divided in one instance to the
commencement of the _promuscis_, the nose is left intire, and dips
down, as in cases before alluded to: so that in this the nose appears
to leave the lobes of the front, which in others embrace its sides.
iv. _Vertex_[1425].--We now come to the _vertex_, or crown of the head;
which is situated behind the front, and, except where the communication
is intercepted by confluent eyes, adjoins it. It is laterally bounded
by the hind part of the eyes and the temples; and _posteriorly_, where
that part exists, by the _occiput_. The vertex may be denominated the
ordinary region of the _stemmata_: for though in several cases, as
we have just seen, one or more of them are planted in the front; yet
this in the great majority, especially in the _Hymenoptera_, is their
natural station. In _Blatta_ and some other _Orthoptera_ the posterior
angle of the head is the vertex. In many dung-chafers of Latreille's
genus _Onthophagus_, which are said to have _occipital_ horns, as _O.
nutans_, _nuchicornis_, _Xiphias_, &c., the horn really arms the part
I regard as the vertex. In _Locusta_ Leach, this part is very ample,
and in _Truxalis_ very long; but more generally it is small, and not
requiring particular notice.
v. _Occiput_[1426].--The _occiput_, or _hind-head_, is that part of
the face that either forms an angle with the vertex posteriorly,
or slopes downwards from it. It has for its lateral boundaries the
temples, and behind it is either terminated by the orifice of the
head, or in many cases by the neck. In those beetles that have no
neck, as the Lamellicorn and Capricorn, the hind-head is merely a
declivity from the vertex, usually concealed by the shield of the
thorax, very lubricous, to facilitate its motion in the cavity of
that part, and at its posterior margin distinguished by one or two
notches, which I shall notice hereafter, for the attachment of the
levator muscles: but in those beetles or other insects that have
a neck, or a versatile head, the occiput forms an angle with the
vertex, often rounded, and sometimes acute. This structure may be
seen in Latreille's _Trachelides_, and several other beetles. In the
_Hymenoptera_, _Diptera_, and others with a versatile head, the part
now under consideration curves inwards from the vertical line, so
as with the temples and under parts of the head to form a concavity
adapted to its movement upon the trunk.
vi. _Genæ_[1427].--The _cheeks_ of insects (_Genæ_) usually surround
the anterior part of the eyes, and lie between them and the mandibles
or their representatives. Where they approach the latter, as in the
Predaceous beetles (_Cicindela_, _Carabus_ L. &c.), they are very
short, and of course longer where the eyes are further removed from
the mouth; as in the Rhyncophorous beetles (_Curculio_ L.), where
they form the sides of the rostrum, and often contain a channel which
receives the first joint of the antennæ, when they are unemployed.
In the _Scarabæidæ_ and many other Lamellicorn beetles, their
separation on each side from the nose is marked by a ridge[1428];
and in the wasps (_Vespa_) by an impressed line or channel. In an
African tribe at present arranged with _Cetonia_ F., to which _C.
bicornis_ Latr.[1429] and another, which he has named, I believe, _C.
vitticollis_, belong, the cheeks are porrected on each side of the
mouth into a horizontal horn. These horns have at first the aspect
of a pair of open mandibles. In the magnificent _Goliathi_ Lam.,
the horns of the male are rather a process of the _cheek_ than of
the _nose_. In _Alurnus_, _Hispa_, and other beetles, these parts,
by their elevation and conjunction with the lower side of the head,
form a kind of fence which surrounds and protects the oral organs;
in many _Cimicidæ_, by a similar elevation of the cheeks, the bed
of the _promuscis_ is formed. In the Homopterous _Hemiptera_ they
run parallel nearly with the _rhinarium_ or nostril-piece. In the
_Hymenoptera_ they are almost always ample, but they are confined
to the lower side of the eye. In _Sirex grandis_, and others of
that genus, the cheek at the base of the mandible is dilated so
as to form a rounded tooth below it. In the Capricorn-beetles it
is considerable, and sometimes terminates, at the base of the
mandible, in two or three notches. In _Scaurus_ and _Eurychora_,
darkling-beetles, the cheek below projects into a lobe that covers
the base of the _maxilla_. But the animal distinguished by the
most remarkable cheeks is a species of _Phryganea_ L. (_Phryganea
personata_ Spence); for from this part projects a spoon-shaped
process, which curves upwards, and uniting with that of the other
cheek, forms an ample mask before the face, the anterior and upper
margin of which, in the insect's natural state, are closely united;
and the posterior part being applied to the anterior part of the
eye, causes the face to appear much swoln. It looks as if it was a
single piece; but upon pressing the thorax it opens, both above and
in front, into two parts, each convex without and hollow within, and
each having attached to its inside a yellow tuft of hair resembling a
feather. The use of this machinery at present remains a mystery[1430].
vii. _Tempora_[1431].--The temples (_Tempora_) are merely a
continuation of the cheeks to the posterior limit of the head,
forming its sides and posterior angles, and including the hinder part
of the eyes, the vertex, and the occiput. They seldom exhibit any
tangible character, except in certain ants (_Atta_ Latr.), in which
their angle terminates in one or two strong spines, giving the animal
a most ferocious aspect; and in that remarkable genus _Corydalis_
they are armed below with a tooth or point, which was not overlooked
by De Geer[1432].
viii. _Oculi_[1433].--I must now call your attention to organs of more
importance and interest, and which indeed include a world of wonders: I
mean the eyes (_Oculi_) of insects. These differ widely from those of
vertebrate animals, being incapable of motion. They may be regarded as
of _three_ descriptions--_simple_, _conglomerate_, and _compound_.
1. _Simple Eyes_[1434]. We will consider them as to their _number_,
_structure_, _shape_, _colour_, _magnitude_, _situation_, and
_arrangement_.
As to their _number_, they vary from two to sixteen. In the flea,
the louse, the harvest-man (_Phalangium_), there are only _a pair_;
in the bird-louse of the goose (_Nirmus Anseris_), and probably in
others of the same genus, there are _four_[1435]; in some spiders
(_Scytodes_, _Dysdera_, and _Segestria_ Latr.[1436]), and some
scorpions[1437], there are _six_. In the majority of spiders and
_Scolopendra morsitans_, _Scorpio maurus_, &c. there are _eight_;
and in _Podura_ and _Sminthurus_ Latr. there are sixteen[1438].
As to their _structure_, nothing seems to have been ascertained;
probably their organization does not materially differ from that of one
of the lenses of a compound eye; which I shall soon explain to you.
Their _colour_ in the many is black and shining, but in the
bird-louse of the goose they are quite white and transparent.
In spiders they are often of a sapphirine colour, and clear as
crystal. In _Scolopendra morsitans_ and many spiders, scorpions,
and _phalangia_[1439], they appear to consist of iris and pupil,
which gives them a fierce glare, the centre of the eye being dark
and the circumference paler. In the celebrated _Tarantula_ (_Lycosa
Tarantula_), the pupil is transparent, and red as a ruby; and the
iris more opaque, paler, and nearly the colour of amber.
Where there are more than two, they vary in _magnitude_. In the
enormous bird-spider (_Mygale avicularia_) the four external eyes
are larger than the four internal[1440]; but in the Tarantula
and _Sphasus_, the _two_ or _four_ internal are the largest. In
_Clubiona_ and _Drassus_ they are all nearly of the same size[1441];
and in the _Micrommata_ family they are very small[1442].
They vary also in _shape_. In _Scolopendra morsitans_ the three
anterior ones are round, and the posterior one transverse, and
somewhat triangular. In _Mygale calpeiana_, a spider, the two
smallest are round and the rest oval[1443]. In the trapdoor or
mason spider (_Mygale cæmentaria_), the four small internal ones are
round, and the large external ones oval[1444]; and those that are
circumscribed posteriorly with an impressed semicircle, are shaped
like the moon when gibbous[1445].
The _situation_ and _arrangement_ of simple eyes are also various.
In many they are imbedded, as usual, in the head; but in the little
scarlet mite, formerly noticed[1446], (_Trombidium holosericeum_),
they stand upon a small foot-stalk[1447]: the hairiness of this
animal might otherwise have impeded its sight. In spiders they
are planted on the back of the part that represents the head,
sometimes four on a central elevation or tubercle, and the remaining
four below it--as in _Lycosa_; sometimes the whole eight are
on a tubercle, as in _Mygale_; and sometimes, as in the common
garden-spider (_Epeira Diadema_), upon three tubercles, four on the
central one and two on each of the lateral ones. Other variations
in this respect might be named in this tribe. In the scorpions a
pair are placed one on each side, on a dorsal tubercle, and the
other four or six on two lateral ones of the anterior part of the
head[1448]. In the _Phalangidæ_ the _frontal_ eyes of the scorpion
cease, and only a pair of _dorsal_ ones are inserted vertically
in the sides of a horn or tubercle, either bifid or simple, often
itself standing upon an elevation which emerges from the back of
the animal[1449]. If their eyes were not in a vertical and elevated
position, the sight of these insects would be very limited; but by
means of the structure just stated, they get a considerable range of
surrounding objects, as well as of those above them. With regard to
the _arrangement_ of the eyes we are considering, it varies much.
Sometimes they are placed nearly in the segment of a circle, as
in those spiders that have _six_ eyes only, before noticed[1450];
sometimes in two straight lines[1451]; at others in two segments of
a circle[1452]; at others, in _three_ lines[1453], and at others
in _four_[1454]. Again, in some instances they form a cross, or
two triangles[1455]; in others, two squares[1456]; in others, a
smaller square included in a large one[1457]; in others, a posterior
square and two anterior triangles[1458]; sometimes a square and two
lines. Though generally separate from each other, in several cases
two of the eyes touch[1459]; and in one instance three coalesce
into a triangle[1460]. But it would be endless to mention all the
variations, as to arrangement, in the eyes of spiders.
2. _Conglomerate Eyes_[1461] differ in nothing from simple eyes, except
that instead of being dispersed they are collected into a body, so
as at first sight to exhibit the appearance of a compound eye:--they
are, however, not hexagonal, and are generally convex. They occur
in _Lepisma_, the _Iulidæ_, and several of the _Scolopendridæ_. In
_Scolopendra forficata_ the eye consists of about twenty contiguous,
circular, pellucid lenses, arranged in five lines, with another larger
behind them, as a sentinel or scout, placed at some little distance
from the main body. In the common millepede (_Iulus terrestris_) there
are twenty-eight of these eyes, placed in seven rows, and forming a
triangle, thus [Illustration: triangular eye shape]--the posterior row
containing seven lenses, the next six, and so on, gradually losing one,
till the last terminates in unity. Each of these lenses is umbilicated,
or marked with a central depression. In _Craspedosoma_ Leach, you will
find a similar formation. In _Glomeris zonata_, a kind of wood-louse
that rolls itself into a ball, the lenses are arranged in a line curved
at the lower end, with a single one by itself at the posterior end on
the outside; they are oblong and set transversely, and their white
hue and transparency give them the appearance of so many minute gems,
especially as contrasted with the black colour of the animal[1462].
Between these eyes and the antennæ is another transverse linear white
body, but opaque, seemingly set in a socket, and surrounded by a white
elevated line, like the bezel of a ring. Whether it is an eye, or what
organ, I cannot conjecture[1463]. Its aspect is that of a spiracle.
3. _Compound Eyes_[1464].--These are the most common kind of eye
in hexapod insects, when arrived at their perfect state; in their
larva state, as we have seen, their eyes being usually simple[1465];
except, indeed, those whose metamorphosis is _semicomplete_, which
have compound eyes in every state.--In considering compound eyes, I
shall advert to their _structure_, _number_, _situation_, _figure_,
_clothing_, _colour_, and _size_.