to be the _hardest_ part of the crust: and it is so for very good
reasons. In the first place, as it has to make way for the rest of
the body when the animal moves, it is thereby best fitted to overcome
such resistance as may be opposed by the medium through which it has
to pass; in the next, as it bears the organs of manducation, it was
requisite that it should be sufficiently firm and solid to support
their action, which is often upon very hard substances; and besides
this, as no motion of its parts _inter se_, as in the case of the
trunk, is requisite to facilitate the play of its organs, a thin
integument was not wanted.
ii. The most general law relative to the _figure_ or shape of the
head seems to be, that it should approach to that of an equilateral
triangle, with its angles rounded, and the vertex being the mouth;
and that the vertical diameter should be less than the horizontal,
whether longitudinal or transverse. But the infractions of this
law are numerous and various. Thus, in some insects an isosceles
triangle is represented by the head, the length being greater than
the breadth; in others, instead of being flat it is compressed, so
that the horizontal diameter is less than the vertical; in others,
again, it is orbicular, or round and depressed; in others nearly
spherical: occasionally it is rather cylindrical. In many instances
it is very long; in others the width exceeds the length. Though often
narrowest before, in some cases the reverse takes place. Its anterior
end is often attenuated into a long or short snout or rostrum, and
its posterior into a long or short neck. Its contour, though usually
regular, is sometimes either cut into lobes, or scooped out into
sinuosities. But to enumerate minutely all the variations of form
which take place in the head of insects would be endless; I shall
therefore proceed to the next particular.
iii. The _composition_ of the head is very simple; for, exclusive
of its organs, it consists only of a single piece or box, without
suture or segment, with an aperture at the end below to receive the
instruments of manducation, others for the eyes and stemmata when
present, and also for the antennæ. In the _Arachnida_, &c., in which
the head is not separated from the thorax, it is merely a plate, the
under-side or cavity of which is occupied and filled by the above
instruments.
iv. With regard to its _superficies_, the head of insects is generally
more or less uneven, though in some cases it presents no inequalities.
In many of the Lamellicorn tribes, and a few other individuals, in
one sex at least, as has been before observed[1173], it is armed with
long horns, or prominent tubercles; it is often covered with numerous
puncta, or pores; and some of its parts, as the nose, after-nose, &c.
in particular groups, marked out by an impressed line[1174]. In many
_Hymenoptera_, _Diptera_, &c. its upper surface is convex, and the
lower concave; in others both surfaces are convex.
v. It is the most general rule, as to its _proportion_, that it shall
be smaller than either trunk or abdomen; but in some instances, as in
the S. American ant, _Atta megacephala_, it is much larger than either.
vi. By the _direction_ of the head, I mean its inclination with
respect to the prothorax. The most natural direction, or that
which obtains most generally, is for it to form an angle more or
less obtuse with the part just mentioned. This seems to obtain
particularly in _Coleoptera_; but in some, as _Mylabris_, it is
inflexed, forming an acute angle with it. In the Heteropterous
_Hemiptera_ (_Cimex_ L. &c.) it is generally in the same line with
the body, or horizontal; and in many _Diptera_ it is vertical.
vii. We now come to a circumstance which will detain us longer,
namely, its _articulation with the trunk_, or rather with its anterior
segment, the _prothorax_.--M. Cuvier makes _two_ principal kinds of
articulation of the head upon the prothorax, in one of which the points
of contact are solid, and the movement subordinate to the configuration
of the parts; in the other, the articulation is ligamentous, the head
and the thorax being united and kept together by membranes.
1. The _first_ of these kinds of articulation--that by the contact
of solid parts--takes place, he says, in _four_ different ways.
"In the most common conformation, in the part that corresponds
to the neck, the head bears one or two smooth tubercles, which
receive corresponding cavities of the anterior part of the prothorax
observable in the Lamellicorn and Capricorn beetles. In this case
the head can move backwards, and the mouth forwards and downwards.
The _second_ mode of solid articulation takes place when the
posterior part of the head is rounded, and turns upon its axis in a
corresponding cavity of the anterior part of the prothorax; as may
be seen in _Curculio_, _Reduvius_, &c. The axis of motion is then
at the centre of articulation, and the mouth of the insect moves
equally backwards and forwards, upwards and downwards, to right and
left.--The _third_ sort of articulation, by solid surfaces, takes
place when the head, truncated posteriorly, and presenting a flat
surface, is articulated, sometimes upon a tubercle of the thorax,
and sometimes upon another flat and corresponding surface, as in
almost all the _Hymenoptera_ and the majority of the _Diptera_. The
disposition of the _fourth_ kind of articulation allows the head only
the movement of the angular hinge (_le seul mouvement de charnière
angulaire_). The only examples at present known are in some species
of _Attelabus_ F. The head of these insects terminates behind in a
round tubercle, received in a corresponding cavity of the thorax: the
lower margin of this cavity has a notch, and permits the movement of
the head only in one direction[1175]."
2. The _second_ kind of articulation, the ligamentous, he affirms takes
place only in _orthopterous_ and some _neuropterous_ insects: "The head
in this kind of articulation is only impeded in its movements towards
the back, because it is stopped there by the advance of the prothorax;
but below it is quite free. The membranes or ligaments extend from the
circuit of the occipital cavity to that of the anterior part of the
prothorax, which gives a great extent to the movement[1176]."
When I consider the well-deserved celebrity of the great author whose
words I have here quoted, and the great and useful light that the
genius and learning which conducted his patient labours and researches
have thrown over every department of comparative anatomy,--a science he
may be almost said to have founded,--I feel the most intire reluctance
to differ in any thing from an authority so justly venerable to all
lovers of that interesting study. But, however great my diffidence and
hesitation to express an opinion at all opposed to his, the interests
of truth and science require that I should state those particulars in
which my own observations, made upon a careful examination of various
insects of every Order, have led to results in some respects different
from the above. "_Aliquando bonus dormitat Homerus_;" and if the
Genius of Comparative Anatomy ever nodded, it sometimes happened when
he was examining the structure of insects. An instance of this with
regard to the mouth of the bee has been noticed by Mr. Savigny[1177];
and indeed it is not wonderful that in so extensive an undertaking, in
which the number of examples to be examined upon every branch of his
subject must be immense, that he did not always scrutinize minutely
those that seemed less important. Every writer on every department
of Natural History, especially where the objects of research, as in
the insect world, are so infinite in number, will be liable to such
mistakes; but these will meet with due allowance from every candid
mind--
"Hanc veniam damus, petimusque vicissim:"
and I shall express my trust that you will overlook any errors of
mine; and doubtless I shall not be free from them--
"---------Quas aut incuria fudit,
Aut humana parum cavit natura----"
The two kinds of articulation of the head which our learned author
has stated as the principal ones, will, I think, be found upon
examination not so widely distant as his expressions seem to
indicate; for in fact in all insects, as well as the _Orthoptera_,
this part is _suspended_ by a membrane or ligament which unites
the margins of the occipital cavity with those of the anterior one
of the prothorax: thus forming all round some protection to the
organs that are transmitted from the head through the latter part to
the rest of the body. Though the head in most _Orthoptera_ is not
partly received into the cavity of the prothorax, as is the case in
the Order _Coleoptera_, but is rather suspended to it, yet in some
instances, for example in the mole-cricket (_Gryllotalpa vulgaris_),
it is partially inserted.
Again: when, in his _first_ mode of articulation by contact of solid
parts, he speaks of one or two smooth tubercles of the neck, with
their corresponding cavities in the prothorax, as forming the most
common conformation, you would expect to find examples of this in
very many insects; yet upon a close examination, unless in _Oryctes
nasicornis_[1178], and perhaps in others of the _Dynastidæ_ MacLeay,
you would scarcely meet with any thing that could be called a
tubercle and its corresponding cavity in the neck or prothorax of any
Lamellicorn or Capricorn beetle that you might chance to examine.
You would find, indeed, that the occiput was usually smooth and very
slippery, as if lubricated; that in its margin were one or two notches
(_Myoglyphides_), with muscles attached to them; that in the former of
these tribes, the _Lamellicorns_, it projected on each side so as to
form a more or less prominent angle; and that the throat (_jugulum_)
was very convex, and lodged in a cavity of the lower margin of the
prothorax: but further appearances of tubercles &c. you would in vain
look for even in this tribe; and in the _Capricorns_ you would find
that the general conformation in this respect belonged to our learned
author's _second_ mode of solid articulation, resembling that of the
majority of the weevils (_Curculio_ L.), in which the head has no
projecting angles or tubercles, or other elevation, but is received
usually into the circular cavity of the prothorax.
His _third_ mode of this articulation, that of the _Hymenoptera_
and _Diptera_, is so peculiar, that it ought to be considered as a
_primary_ kind; since in this the head moves _upon_ the prothorax as
upon a pivot, and has a kind of versatile motion.
With regard to his _fourth_ mode, which from his description appears
to be that of _Apoderus_ Oliv., he allows motion to the head only in
_one_ direction, observing that the lower margin of the prothoracic
cavity has a notch. But M. Latreille calls the articulation of the
head in this genus an _Enarthrosis_[1179], which admits motion in
every direction; and if you examine the common species (_A. Coryli_),
you will find that the prothorax has a sinus taken out of its upper
margin, as well as out of its lower one--which at any rate will allow
a motion _upwards_.
I merely mention these little inaccuracies, with due diffidence,
as some apology for giving you a different and at least a more
popular and general view of this part of my subject, which I shall
now proceed to state to you. It seems to me most convenient for the
Entomologist, and most consonant to nature, to divide insects, with
respect to the articulation of the head with the trunk, into three
_primary_ sections, each admitting one or more subdivisions.
1. The _first_ consists of those whose head inosculates more or less in
the anterior cavity of the chest; and whose articulation, therefore,
seems to partake in a greater or less degree of the ball and socket
(_Enarthrosis_). The head, however, is often capable of being protruded
from this cavity. If you take into your hand any common _Harpalus_
that you may find under a stone, you will see, if pressed, that it
can shoot forth its head, so as to be entirely disengaged from the
prothorax: a neck of ligament intervening between them[1180]: of course
this power of protruding the head enables the animal to disengage
it at its will from the restriction imposed upon its motions by the
surrounding margin of the prothoracic cavity. To this section belong
all the _Coleoptera_, the Heteropterous _Hemiptera_ (_Cimex_ L., &c.),
and some of the _Neuroptera_ (_Raphidia_, _Semblis_, &c.).--It may
be further divided into _two_ subsections--those, namely, whose head
inosculates in the prothorax by means of a _neck_: as for instance
Latreille's _Trachelides_, _Apoderus_, and the _Staphylinidæ_, amongst
the beetles; the _Reduviadæ_ amongst the _Heteropterous_ insects, and
_Raphidia_ in the _Neuroptera_; and those whose head inosculates in the
prothorax without the intervention of a neck; as, the _Petalocera_,
the aquatic beetles (_Dytiscus_, _Hydrophilus_, &c.), and most of the
genus _Curculio_ L. in the first of these orders, the great body of the
_Cimicidæ_ in the second, and _Semblis_, _Corydalis_, &c. in the third.
2. The _second_ section consists of those insects whose head does not
inosculate in the chest, but is merely suspended to it by ligament or
membrane. To this belong most of the tribes of the _Orthoptera_ Order,
with the exception of the _Mantidæ_, the _Dermaptera_, the _Homopterous
Hemiptera_, and such of the _Aptera_ as have the head distinct from
the prothorax.--This section admits of a _triple_ subdivision: those,
namely, whose head is _wholly_ covered by the shield of the prothorax,
as in _Blatta_ L.; those whose head is _partly_ covered by it, as
_Gryllotalpa_, and other _Gryllina_; and those whose head is quite
free, not being at all impeded in its motion by the prothorax, as the
_Dermaptera_, _Nirmus_, _Pediculus_, &c.
3. The _third_ section consists of those whose head is truncated
posteriorly, and flat or concave, with a very small occipital
aperture, and is attached to a neck of the prothorax upon which
it turns, or is merely suspended to that part. This includes
the _Lepidoptera_, _Hymenoptera_, _Diptera_, the _Libellulina_,
&c. in the _Neuroptera_, and the _Mantidæ_ in the _Orthoptera_.
_Three_ subsections at least, if not more, present themselves in
this section: the first is, of those whose head is united to the
prothorax, without the latter forming any _neck_. To this belong the
_Lepidoptera_, _Trichoptera_? The _second_ is of those the upper
side of whose thoracic neck is _ligamentous_; and here you may range
most of the _Hymenoptera_. The _third_ is of those in whom it is a
_continuation_ of the ordinary _integument_. In this subsection the
_Diptera_, _Libellulina_ and _Mantidæ_ will find their place. In this
last section the head appears to turn upon the thorax as upon a pivot.
Before I finish what I have to say on the _articulation_ of the
head, I must direct your attention to the _analogies_ that hold in
this respect between the different tribes. Thus the _Coleoptera_
are analogous to the _Heteropterous Hemiptera_; the _Orthoptera_,
with the exception of the _Mantidæ_, to the _Homopterous Hemiptera_;
the _Mantidæ_ to the _Libellulina_; the _Lepidoptera_ to the
_Trichoptera_; the _Hymenoptera_ to the _Diptera_, with a slight
variation, and probably others might be traced.
viii. A word or two upon the _motions_ of which the head of insects
is capable. M. Cuvier, in the extracts lately laid before you,
speaks of different powers of movement as the result of different
configurations of the parts of the head. This probably is correct
with regard to many cases; but a great deal will depend upon the
power the insect has of protruding its head and disengaging its base
from the restriction of the prothorax; for where, like the _Harpali_,
_Staphylini_, &c. it is able to do this, it can probably move its
head in every direction. It is only where the ligaments are less
elastic, or allow of little tension, that its movements are confined;
and few living insects have been sufficiently examined to ascertain
how far this takes place. In those cases belonging to the _third_
section of articulations, in which the head moves _upon_ the thorax
as upon a pivot, as is the case with _Hymenoptera_ and _Diptera_,
the movement is nearly versatile. I have seen a fly turn its head
completely round, so that the mouth became supine and the vertex
prone; and from the form and fixing of the head, it should seem that
those of the _Mantidæ_ were endued with the same faculty.
ix. The _parts_ and _appendages_ of the head are now in the last
place to be considered. I shall begin with the _Mouth_, or rather
the orifice in which the _trophi_ or organs of manducation are
inserted. In some insects, as was before observed, they occupy all
the under-side of the head, as in the _Arachnida_, _Myriapoda_,
&c; but in the great majority they fill an orifice in its anterior
part, which in some instances, as in _Lampyris_, the _Lepidoptera_,
_Cimex_ L., _Truxalis_, appears to be nearly under the head; but in
general it terminates that part, though it extends further below than
above. In _Chermes_, a Homopterous genus, the promuscis is stated
to be in the _Antepectus_, and consequently the _mouth_; but, as I
shall endeavour to prove to you hereafter, this is a fallacy. In the
males of the species of _Coccus_ there is no mouth at all. In that
of the elm (_C. Ulmi_) in lieu there are ten little shining points,
arranged two before and two behind in a line, and three on each side
in a triangle[1181]. It is to be observed that the orifice of which
I am speaking is usually much smaller in those insects which take
their food by suction, the _Hemiptera_, _Lepidoptera_, _Diptera_,
&c., than in the masticating tribes. With regard to the real mouth,
or that through which the food enters, I have nothing at present to
observe, except that it lies between the upper-lip and tongue, is
sometimes covered by a valve, as in _Apis_, _Vespa_[1182], &c., and
is different in the masticators and suckers.
* * * * *
I shall next offer a few observations _seriatim_, as they stand
in the Table, upon the organs of manducation; which, to avoid
circumlocution, instead of _Instrumenta cibaria_, the name Fabricius
gave them, I shall call _trophi_ or _feeders_. It is upon these
parts, you are aware, that the system of the celebrated Entomologist
just mentioned is founded; and could they always, or even for the
most part, be inspected with ease, they would no doubt afford
characters as various and discriminative as those of the vertebrate
animals. Differences in these parts indicate a difference in the
mode in which the animal takes its food, and often in the kind of
food, and sometimes in its general economy and habits,--circumstances
which are powerful and weighty in supporting the claim of any set
of animals to be considered as forming a natural genus or group.
Trifling variations, however, of these parts, unless supported by
other characters and qualities, ought not to have much stress laid
upon them, since, if we insist upon these, in some tribes almost
every species might be made a genus.
With respect to their _trophi_ in general, insects of late have been
divided into two great tribes[1183], _masticators_ and _suckers_;
the _first_ including those that are furnished with instruments to
separate and _masticate_ their food; namely, an upper- and under-lip
(_labrum_ and _labium_), upper- and under-jaws (_mandibulæ_ and
_maxillæ_), labial and maxillary palpi, and a tongue (_lingua_): and
the _second_ those in which these parts are replaced by an articulate
or exarticulate machine, consisting of several parts and pieces
analogous to the above, which pierce the food of the animal, and
form a tube by which it _sucks_ its juices. If, however, the mode in
which insects take their food be strictly considered, it will be
found that in this view they ought rather to be regarded as forming
_three_ tribes; for the great majority of the _Hymenoptera_ order,
and perhaps some others, though furnished with mandibles and maxillæ,
never use them for mastication, but really lap their food with their
tongue: these, therefore, might be denominated _lappers_.
When a mouth is furnished with the _seven_ ordinary organs used in
taking the food and preparing it for deglutition--I mean the upper-lip
and the two upper-jaws; the under-lip and the two under-jaws, including
the labial and maxillary palpi; and the tongue--I denominate it a
_perfect mouth_; but when it is deficient in any of these organs, or
they exist merely as rudiments, or when their place is supplied by
others, (which, though they may be analogous parts, have little or no
connection with them in their use or structure,) I denominate it an
_imperfect mouth_. This last I would further distinguish, according to
the nature of its _trophi_, by other and more distinctive terms, as I
shall presently explain to you.
1. _Labrum_[1184].--I shall first consider the organs present in
a _perfect mouth_, beginning with the _upper-lip_ (_labrum_).
This part, which Fabricius sometimes confounded with the _nose_,
miscalling it _clypeus_, is usually a moveable[1185] piece, attached
by its base to the anterior margin of the part last named, and
covering the mouth, and sometimes the mandibles, from above. In
insects in their last state it is usually of a horny or shelly
_substance_; yet in some cases, as in _Copris_ and _Cetonia_, beetles
that imbibe juices, it is membranous. In _form_ and shape it varies
greatly, being sometimes nearly square, at others almost round; in
some insects representing a parallelogram, in others a triangle, and
in many it is oblong. In some instances it is long and narrow, but
more generally short and wide. It is often large, but occasionally
very minute. In the majority it has an intire _margin_, but it is
not seldom emarginate or bilobed, or even dentate. Its _surface_
is commonly even, but it is sometimes uneven, sometimes flat, at
others convex, and in a few species armed with a short horn or
tubercle[1186]. As to its _pubescence_, it is often naked, but now
and then fringed or clothed with down or hairs, or sprinkled with
bristles. It consists in almost every instance of a single piece;
but an exception to this occurs in _Halictus_, a little bee, in the
females of which it is furnished with a slender appendage[1187].--The
_direction_ of the upper-lip is various. It is rarely horizontal, or
in the same line with the nose, often vertical; it sometimes forms an
obtuse angle with the anterior part of the head, and occasionally an
acute one, when it is more or less inflexed. The _use_ of this part
is ordinarily to close the mouth from above, to assist in retaining
the food while undergoing the process of mastication; but in many
Hymenopterous insects its principal use seems to be, to keep the
_trophi_ properly folded; and in some cases where it is inflexed,
as in the leaf-cutter bees (_Megachile_ Latr.), to defend its base,
while the mandibles are employed, from injury by their action[1188].