articulation take place:--in the first the abdomen is united to the
trunk by the _whole_ diameter of its base, without any appearance of
incision; in the other only a _small part_ of that diameter, with
a very visible incision. All the Orders, except the majority of
the _Hymenoptera_ and _Diptera_, and the _Araneidæ_, belong to the
_first_ of these sections; for in all these the aperture by which the
abdomen is suspended to the trunk, occupies the whole of the base;
I say _suspended_, because, though in many cases it inosculates in
the posterior cavity of the latter part, it does not in all, and the
margins of the orifice are united by ligament to those of that cavity.
Indeed, in the _Coleoptera_ and others that have a somewhat prominent
metaphragm[2125], the trunk may with more propriety be said to
inosculate in the abdomen. With regard to the _second_ section,--those
in which the orifice is of less diameter than the base, occupying
only a portion of it,--it may be further subdivided into those whose
abdomen is _sessile_, and those in which it is united to the trunk
by the intervention of a long or short _pedicle_ or footstalk: to
the first of these subdivisions belong all those _Diptera_ that have
an incision between the trunk and abdomen--for many tribes of this
Order, as the _Tipulidæ_, _Asilidæ_, &c., belong rather to the _first_
section--and the _Araneidæ_; the abdomen, however, in _all_ is merely
suspended, without any inosculation. To the _second_ subdivision belong
all the _Hymenoptera_, except the _Tenthredinidæ_ and _Siricidæ_,
the abdomen of which is united to the trunk by the whole diameter
of its base; these may be further subdivided into those that have a
very _short_ pedicle and those that have a _long_ one; but as the
mode of articulation in both these is the same, there will be no
necessity to consider them separately. M. Cuvier has included the
_Diptera_ and _Araneidæ_ in the same tribe with such _Hymenoptera_
as have a petiolate abdomen[2126]; but as the manner in which the
latter articulates with the trunk is widely different from that of
the _Diptera_ &c., I thought it best to consider them as distinct;
especially as in the _Diptera_ there is no tendency to a pedicle,
while only the above two tribes of _Hymenoptera_ are wholly without
it. This learned author thus describes the articulation where the
abdomen is connected by a pedicle. "They have," says he, "a real solid
articulation, a kind of hinge in which the first segment is emarginate
above, and receives a saliant portion of the trunk upon which it moves;
this articulation is rendered solid by elastic and powerful ligaments;
muscles which have their attachment in the interior of the trunk
are inserted in this first segment, and determine the extent of its
movement[2126]." But this passage by no means conveys an adequate idea
of the singular mechanism by which the _Divine Artificer_ has enabled
these little creatures to impart the necessary movements to an organ
so bulky compared with its very diminutive point of attachment. As
no author that has fallen in my way has examined the articulation of
the abdomen with the trunk in these _Hymenoptera_ with the attention
which it merits[2127], I shall enlarge a little upon it. You would
be surprised, and not without reason incredulous, were I seriously
to assert that these insects lift their weighty posteriors by means
of a _rope_ and _pulley_; yet something like this really does take
place, though not with all in a manner equally striking. The point of
articulation in the insects in question, except in _Evania_, is at the
base of the metathorax just above the posterior pair of legs: here
you see a small orifice, either insulated or connected by a narrow
opening with the larger one, when the abdomen is removed, which in
many instances, as in the common wasp, is surmounted by another still
smaller, through which, if you examine it attentively, you will find
there is transmitted a flat and sometimes broadish ligament or rather
_tendon_, in which the levator muscles of the abdomen, attached by
their other end to the metaphragm[2128], terminate: another minute
orifice above the base of the pedicle affords a point of attachment to
the tendon, so as to give it prize upon the abdomen. Here the upper
orifice in the trunk is the _pulley_ (_trochlea_)[2129], the tendon is
the _rope_ (_funiculus_)[2129], and the abdomen is the weight to be
lifted. When the muscles contract, the tendon, running over the edge of
the aperture, is pulled in, and the part just named is elevated; and
when they are relaxed the tendon is let out, and it falls. Some little
variation in the structure takes place in different tribes: thus, in
the _Formicidæ_, _Scoliadæ_, &c., instead of a separate orifice, the
part I call the pulley is merely an upper sinus of the large orifice
that receives the pedicle of the abdomen. The shape of these orifices,
both of the trunk and abdomen, varies in different genera: thus, in the
bee it is triangular, with the vertex reversed; and in the wasp the
upper one is circular, and the lower one transversely oblong; but in
all, the apertures of the trunk correspond with those of the abdomen.
In _Evania_, in which the minute abdomen is inserted in the upper side
of the metathorax, there is scarcely any trace of this structure.
With regard to the articulation of the pedicle itself with the lower
orifice of the trunk, it appears simply suspended, with little or no
inosculation. I may observe under this head, that though the abdomen in
almost all insects is wholly clear of the cavity of the trunk, yet in
some _Phalangidæ_ (_Gonyleptes_ K.) it appears almost retracted within
it[2130].
iii. _Composition._ I shall next consider the _segments_ into which
the abdomen is usually divided, their number, and other circumstances
connected with them. In the _Hippoboscidæ_, _Acaridæ_, _Phalangidæ_,
and _Araneidæ_, the part we are considering is not divided into
segments, though in some instances, as in _Gonyleptes_ and the
cancriform _Epeiræ_[2131], they are represented by _folds_; but in the
great majority of insects it consists of several dorsal and ventral
pieces or segments, forming by their union the _annuli_ or rings into
which it appears divided. The _number_ of these abdominal segments
varies in different insects; I have noticed more than _twenty_ such
variations, and probably there are many more. Before I give you them
in detail, I must first observe that the dorsal and ventral segments,
though sometimes they correspond in number, yet very often do not,
the dorsal most commonly exceeding the ventral by a segment; in a few
cases however the reverse takes place. In the sexes also there is
frequently a difference in the number of segments, as has been before
observed[2132]. I shall express the variations in question by two
figures, the _first_ representing the number of _dorsal_ segments, and
the _second_ the _ventral_--they usually only express the _apparent_
segments: perhaps a very general examination and dissection might bring
many of them nearer to a common type.
1:1. _Chelonus._
3:3. _Chrysidæ_[2133].
4:2. _Leucospis_[2134].
5:5. _Syrphus_.
5:6. _Halictus_ ♀.
6:5. _Nepa._
6:6. { _Halictus_ ♂.
{ _Belostoma._
7:5. { _Curculio_ L.
{ _Cerambyx_ L.
7:6. _Dytiscus_ ♀.
7:7. _Ammophila_, &c.
8:6. { _Dytiscus_ ♂.
{ _Lucanus_, &c.
8:7. _Geotrupes_ Latr.
8:8. _Pimpla._
8:10. _Euchlora_ M^cL.[2135]?
8:13. _Scutigera._
9:5. _Carabus_ Latr.
9:6. _Gymnopleurus_ Ill.
9:7. _Perga_ Leach ♀.
9:8. _Perga_ ♂.
10:7. _Locusta_ Leach ♀.
10:8. --------------- ♂.
10:10. _Æshna._
11:7. _Phasma._
11:9. _Chelifer._
12:11. _Thelyphonus._
Many: _Myriapoda._
I shall next explain the articulation of the segments with each other,
both that of the rings formed by the union of the dorsal and ventral
pieces, and that of those pieces themselves. In general it may be
stated with respect to the former, that each ring is suspended by
ligament to that which precedes it; but this takes place in _three_
ways--in some the margins of the suspended rings _touch_ each other
only, with little or no _inosculation_; in others the _dorsal_ segments
only touch, and the base of each _ventral_ is covered more or less by
the apex of the preceding one; and in others again the base of the
whole ring, both above and below, is so covered, or inosculates. The
first kind here mentioned you will find exemplified in _Melolontha_,
_Geotrupes_, _Musca_, &c.; the second in _Scorpio_; and the third in
_Staphylinus_, the _Hymenoptera_, and many others. In the _Coleoptera_,
says M. Cuvier, speaking of the movements of the abdomen, the rings
only touch each other at the margin, and the movement is very limited;
whilst in the _Hymenoptera_ they are so many little hoops, which
inosculate in each other as the tubes of a telescope, one third only of
their extent often appearing uncovered[2136]. We see the reason of this
structure when we consider the calls they have for greater powers of
movement in this part in laying their eggs, and annoying their enemies
and assailants; and also in the _Staphylinidæ_ to enable them to turn
up their abdomen like a scorpion, both as a posture of attack, and to
fold their wings: in all cases, however, as far as my observation goes,
these animals, when they want to lengthen this part, can disengage the
rings from almost all inosculation, so that no impediment remains to
any movement.
The articulation of the dorsal and ventral segments with each
other is next to be considered. In _Iulus_ and some _Centroti_ the
ring appears to be formed of a single piece, with scarcely any
trace of the existence of any such division; it is however almost
universal, and is of _three_ descriptions; in the first the dorsal
segments are united to the ventral _at_ the lateral margin or edge
of the abdomen; in the second it is _above_ this margin, and in
the third _below_ it. You will find that in _Fulgora_ and many
other Homopterous _Hemiptera_ these segments unite at the margin,
as they do likewise in _Cimex lectularius_ belonging to the other
Hemipterous section; but in the rest of the Heteropterous tribes,
the ventral segments turn upwards, and their union with the dorsal
is in the back of the abdomen; in these the _Hemelytra_ and wings
only cover the dorsal segments, leaving the edge, formed of the
ends of the ventral, uncovered. The Lamellicorn beetles also, and
many other _Coleoptera_, exhibit the same structure. To the last
description, in which the dorsal segments turn down to meet the
ventral, belong the _Lepidoptera_, _Locusta_ Leach; likewise _Sirex_,
_Chrysis_, and many other _Hymenoptera_. The articulation between
these segments is by means of an elastic membranous ligament, which
usually is not externally visible; but in many instances, in which
the connecting ligament is of a firmer substance, as in _Scorpio_,
_Thelyphonus_, and _Phrynus_, it is very conspicuous, and in the
latter genus exhibits many longitudinal folds, as it does likewise in
_Gryllotalpa_, which must permit a vast extension of the abdomen. In
this membrane, in some cases, as in _Dynastes_ M^cL., _Melolontha_,
&c., the two or three first spiracles are fixed[2137]. In the
_Hymenoptera_ and many other insects the dorsal segments do not unite
by their _margin_ with the ventral, but the _end_ of each dorsal laps
over that of the corresponding ventral.
_Dorsal segments_[2138]. I shall next notice the segments _seriatim_,
in the order of their occurrence, beginning with the _dorsal_ ones.
The most remarkable circumstance with respect to these that occurs
to my recollection takes place in the Cancroid spiders (_Epeira
cancriformis_, _aculeata_, &c.), in which the back of the abdomen is
formed by a plate, in some extended in a transverse direction (_E.
cancriformis_), in others in a longitudinal one (_E. aculeata_),
of a much harder substance than the under side and quite flat, set
with strong sharp spines, in the former species apparently moveable,
and terminating behind in a piece resembling in some measure the
scutellum of the _Stratyomidæ_ and similarly armed with a pair of
spines[2139]: in _E. aculeata_ the sides of the abdomen, under the
plate, have a number of longitudinal folds like those of _Phrynus_.
In _Cryptocerus_, a genus of ants peculiar to South America, the
_first_ segment, not reckoning the pedicle, forms almost the whole
back of the abdomen, and the three last are so minute as scarcely
to be distinguishable. Nothing very remarkable is exhibited by the
other segments, except that in _Trichius_ the _penultimate_ is the
largest; in some _Staphylinidæ_ (_S. splendens_) and _Brachini_ (_B.
melanocephalus_) it is emarginate, and in the former tribe also often
terminating in a white membrane. The _dorsal_ segment most worthy
of notice is the last, which is called the _podex_; for though in
general it is a minute piece, often retracted within the abdomen
and invisible, as in many _Diptera_, yet sometimes it is the most
conspicuous of the dorsal segments. It is most commonly triangular,
and usually deflexed and forming an angle with a horizontal line;
but in _Clytra_, _Chlamys_, and _Oryctes_, it is inflexed; in many
Lamellicorns it is nearly vertical. In _Tettigonia_ F., many other
Homopterous _Hemiptera_, and some _Hymenoptera_ (_Cimbex_), its sides
turn down and become ventral; on its lower side it has in these a
longitudinal cavity which receives the ovipositor in repose[2140].
In many other insects it unites with the last ventral segment, the
_hypopygium_, to form a tube for that organ, as you will find in
_Callidium violaceum_[2141], many _Muscidæ_, and _Thelyphonus_. As to
its _termination_ the podex is sometimes bifid, _Blatta_; bipartite,
_Ranatra_; mucronate, _Sirex_; acuminate, _Melolontha vulgaris_,
_Trichius hemipterus_. Generally this part is flat; but the disk is
elevated or gibbous in _Oryctes_ and some other Lamellicorns. In the
majority of the _Coleoptera_ Order it is quite covered by the wings
and elytra; but in many of the last-mentioned tribe, and sometimes
the penultimate segment also, it is not covered by them[2142]. In
some insects the piece we are considering appears to consist of
_two_ segments; in the male of _Locusta morbillosa_ the whole podex
is rhomboidal, but it is formed by two triangular pieces which
articulate with each other; this structure permits the more easy
elevation of the terminal one for the extrusion of the feces.
_Ventral Segments_[2143]. We are now to turn our attention to the
_ventral_ segments of the abdomen. The first of them is what is
called the _epigastrium_[2144] in the table. This part, according
to M. Chabrier, is of considerable importance to the animal in
flight, as, by its pressure against the trunk, not only regulating
the movements of the abdomen, but as, in his opinion, contributing
to push forward the trunk[2145] in the descent of the animal. It is
remarkable only in the _Coleoptera_ and Heteropterous _Hemiptera_,
to which my observations upon it will be confined. It may be stated
as usually consisting of two articulations, that nearest the trunk
being narrow, and in the Predaceous beetles[2146], as also in
_Scutellera_, _Pentatoma_, &c., interrupted in the middle[2147].
In many _Lamellicorns_ this joint is concealed under the posterior
_coxæ_, and with the anterior part of the second forms a hollow
cavity for their reception; this last joint is what is properly
the _Epigastrium_, the former, especially when distinct, being
called in the table the _Hypochondria_. In _Sagra_ and _Brentus_
the _epigastrium_ is particularly conspicuous for its size, in the
former occupying half, and in the latter nearly two-thirds of the
under side of the abdomen; but in general it is distinguished from
the remaining segments only by the central mucro or point that
terminates it towards the trunk[2148], and which is received by
a sinus of the _metasternum_; this point is generally minute and
triangular, but in _Sagra_ it is large and rounded at the extremity,
and in _Calandra_ it terminates nearly in a transverse line somewhat
waving. It is most remarkable, however, in some species of the
Heteropterous genus _Edessa_ F.; for in _E. nigripes_ and affinities
it is a sharp _sterniform_ conical horn, which passing between the
four posterior legs covers the end of the _promuscis_. In fact, this
part appears a kind of abdominal _sternum_. In the _Cetoniadæ_,
&c., the _Hypochondria_ unite before this mucro, and form a ridge
which articulates with it, and dips towards the abdominal cavity;
in _Scolytus_ the _epigastrium_ is much elevated from the rest of
the ventral segments, so that the under side of the abdomen appears
as if it were suddenly cut off, whence Herbst's awkward though not
inexpressive name, _Ekkoptogaster_; this part in this genus has
something of a posterior mucro.
The intermediate ventral segments exhibiting no very remarkable
peculiarities, I shall pass them without further notice, and call
your attention to the last, which is opposed to the _podex_,
and which I have named the _hypopygium_[2149]. Though usually a
_single_ small piece, in _Edessa_ and many _Pentatomæ_ it consists
of _several_ plates; and in _Trichius_ it is very large: it is
mostly _intire_, but in the male _Dytisci_ it is _cleft_; in _Lamia
ocellata_ trilobed; in _Edessa_ tripartite; in _Centrotus Taurus_
it is boat-shaped and hollowed out to receive the stalk of the
ovipositor. It is also generally in the same line with the body, but
in _Xenos_ it is turned up and bent inwards[2150].
iv. _Shape._ With regard to _shape_, in some Orders the abdomen
varies considerably; but the most general form is one that approaches
to trigonal, so that a transverse section will be a triangle, with
the vertex more or less obtuse, and the base more or less convex;
some tendency to this form will often be found even in those insects
whose abdomen appears almost as flat as a leaf, as in many _Aradi_.
In the hive-bee the transverse section is almost an equilateral
triangle; in _Belostoma grandis_ the disk of the under side of the
part in question is longitudinally elevated into a trigonal ridge,
the section of which is an equilateral triangle, the sides being
quite flat. In general, in the vertical section of an abdomen, the
vertex of the triangle points _downwards_, but in _Libellula_ F. it
points _upwards_. In _Blatta_ this section is nearly lanceolate; in
_Staphylinus olens_ it is a segment of a circle with the convex side
downwards; in _Æshna_ F. with that side upwards; and in _Agrion_
the section is circular. In _Copris_, _Ateuchus_, &c., the abdomen
is very short and thick; in _Staphylinus_ slender and long; in
_Aradus_, _Nepa_, &c., depressed and flat; compressed in _Ophion_
and _Evania_; conical in _Cœlyoxis_; rhomboidal in many _Mantes_;
boat-shaped in many _Lygæi_; fusiform in various _Papilionidæ_;
lanceolate in some _Ichneumonidæ_, falcate in others; nearly
round in _Diapria pupurascens_; ovate in _Lyrops_; elliptical in
_Andrena_; oblong in many _Xylocopæ_; heart-shaped in the naked
_Euglossæ_; triangular in _Dytiscus_; gibbous in _Flata_; and
vaulted in _Chrysis_. At its base it is truncated in _Sirex_; retuse
in most _bees_; forming the segment of a circle in _Andrena_; in
general sessile, but in the majority of _Hymenoptera_, as has been
already observed, terminating in a pedicle. The pedicle is very
short in the _Andrenidæ_ and _Apidæ_; long in the _Sphecidæ_; thick
in the _Formicidæ_; slender in _Evania_; fusiform in _Pelecinus_;
clavate in _Ammophila_; campanulate in many _Vespidæ_; nodose in
_Myrmica_[2151]; squamigerous in _Formica_[2152]: it sometimes also
consists of _two_ joints, as in _Ammophila_ and many _Vespidæ_.
As to _margin_, some have none, as _Centrotus_; in others, as
_Dytiscus_, it is very narrow; in others again, wide and flat, as in
the _Nepidæ_; in _Staphylinus_, &c., it is distinguishable only on
the upper side of the abdomen; in _Locusta_ Leach only on the under
side, though mostly intire; it is serrated in _Blatta_, sinuated in
_Acanthia paradoxa_, and crenated in _Cerceris_.
v. _Proportions._ These vary greatly in the different tribes; in some
the abdomen is long and slender, as in _Locusta_, and _Staphylinus_;
disproportionably so in a remarkable degree in some _Agrionidæ_ from
South America, as _A. lineare_, &c.[2153]; in others it is extremely
short and thick, as in _Copris_, &c.; a mere appendage in _Evania_;
it is shorter than the elytra in _Trox_; of the same length in most
beetles; longer in _Melolontha_, _Hister_, &c.; disproportionably so
in _Staphylinus_: though usually of the same width with the trunk,
in many _Mantidæ_ it is much wider[2154]; and more slender in the
_Libellulina_, _Myrmeleon_, &c.
vi. _Arms and Appendages_[2155]. These are various; and may be
considered under the following heads: _processes_; _organs_ of
_respiration_, _motion_ and _prehension_; _weapons_; and other _anal
appendages_ the use of which is unknown.
1. _Processes._ Under this term I include all prominences of whatever
kind, whether _tubercles_, _teeth_, _spines_, or _horns_, that arm
any part of the abdomen. Many of these are _sexual_ characters, and
have been sufficiently described in a former letter[2156]; I need
not therefore detain you long on this head. Of the first kind is a
remarkable elevation that distinguishes the second ventral segment of
_Scolytus Destructor_ (_Ips Scolytus_ Marsh.) or of a species allied
to it[2157]; in _S. pygmæus_ (_I. multistriatus_ Marsh.) the same
segment is armed by a flat horizontal _tooth_ or horn; in an _Aradus_
from Brazil, before alluded to[2158] (_A. laminatus_ K. MS.), the
margin of the abdomen is surrounded by eight flat subquadrangular
laminæ; in another species figured by Stoll[2159], it is cut out into
bays by a number of denticulated teeth; and in _Acanthia paradoxa_
by long spinose lobes[2160]. In _Edessa_ F., another genus of bugs,
the abdomen usually terminates in four strong sharp dentiform spines,
the intermediate ones being the shortest, and in some the _margin_ is
also armed with spines[2161]; occasionally the anal spines are very
long[2162]. In addition to the ventral horns before mentioned that
distinguish the sexes of some insects[2163]; the males of the genus
_Conops_, a two-winged fly, have, on the antepenultimate ventral
segment, a singular process, varying in length and shape in the
different species, standing nearly at right angles with the belly,
convex towards the trunk, and concave towards the anus. De Geer
supposes that with the anal extremity this forms a forceps with which
this fly seizes the other sex[2164].
2. _Organs of respiration_[2165]. I shall defer my account of the
_spiracles_, and other external respiratory organs, till I come
to treat of the system of _respiration_ in insects, when every
thing connected with that subject will be most properly discussed;
but there are certain appearances in some insects, which at first
sight seem to partake of the same character, but which being really
independent of that vital function, may here have their place. If you
examine the abdomen of the mole-cricket (_Gryllotalpa vulgaris_),
you will easily discover the true spiracles in the folds of the
_pulmonarium_, which separates the back of that part from the belly;
if you next inspect the five intermediate segments of the latter,
you will discover on each nearer the base a pair of oblique little
channels, which precisely resemble closed spiracles. These may be
denominated false or blind spiracles. Again, if you examine the
pupa of any _Scutellera_ or _Pentatoma_, in which tribe the true
spiracles are _ventral_, you will discover, placed in a square on the
two or three intermediate _dorsal_ segments, four or six elevated
points resembling spiracles, but not perforated, connected often
by corrugations in the skin or crust[2166]; in the larvæ also of
some _Reduvii_ the first minute dorsal segment, at each lateral
extremity, has a similar elevation with a central umbilicus precisely
resembling a spiracle, but still not perforated: another instance of
false spiracles in this section of the _Hemiptera_, is furnished by
_Aradus laminatus_ before mentioned, in the perfect insect; between
the spiracle and the margin of each ventral segment is a white round
callus, with a dark point resembling a perforation on its exterior
side, and terminating internally in a channel covered by membrane
leading to the disk of the segment, so that the whole in shape
resembles a tobacco-pipe[2167]. A number of similar callosities with
a central impression, but without any channel, variously disposed,
are also to be found in another bug, _Rhinuchus compressipes_
K.[2168] In the Homopterous section of this Order, a series of
impressed points, which may be easily mistaken for spiracles, are
to be discovered on both sides of the abdomen, at the margin in
_Centrotus_, in which the real spiracles are quite concealed.