_Flata reticulata_ and affinities have them: a proof that these
tribes are distinct genera.
[1507] PLATE VI. FIG. 10. i.
[1508] Reaum. iv. 243. He refers for this insect to plate xiv. without
adding any number for the figure; but no such is in that plate.
[1509] PLATE XXVI. FIG. 40. i.
[1510] _Cercopis_, Ibid. FIG. 42; and _Fulgora_, FIG. 41. i.
[1511] PLATE VI. FIG. 4. i.
[1512] PLATE XXVI. FIG. 40. i.
[1513] Ibid. FIG. 42. i.
[1514] PLATE XXVI. FIG. 41. i.
[1515] De Geer iii. _t._ xxvii. _f._ 1. Reaum. iii. _t._ xxxii. _f._
3, 9.
[1516] PLATE XXVI. FIG. 40. i.
[1517] See above, p. 318--.
[1518] _Linn. Trans._ xiii.
[1519] _Mag. der Entomolog._ iv. 5.
[1520] Palpi quatuor, subæquales, cylindrici, _ad basin clypei. Germ._
[1521] See above, p. 18, &c.
[1522] PLATE VI. FIG. 1, 4. i´.
[1523] See above, p. 321. _Linn. Trans._ xii. _t._ xxi. _f._ 3.
[1524] Ibid. _f._ 7.
[1525] PLATE VI. FIG. 4. c. i´.
[1526] PLATE XII. FIG. 9. l´´. This circumstance was very recently
discovered; which will account for this plate not being quite correct
in this respect, the bulb being represented as a distinct joint in
FIG. 6, 10, 26.
[1527] See above, p. 498.
[1528] VOL. I. p. 231, 238.
[1529] Oliv. _Ins._ no. 80. _Macrocephalus t._ i. _f._ 1-4.;
_Anthribus f._ 5-12; and no. 83. _Curculio t._ ii. _Calandra f._ 16.
[1530] Schellenberg _Cimices t._ xiv. _f._ 1. _b._
[1531] De Geer vii. _t._ iv. _f._ 7. _a a._
[1532] VOL. II. p. 421.
[1533] PLATES XII. FIG. 28; and XXV. FIG. 9, 24.
[1534] PLATE XXV. FIG. 4.
[1535] PLATES XII. FIG. 29; and XXV. FIG. 28. _a._
[1536] PLATES XI. XII. XXV. k´´.
[1537] PLATES XI. XII. XXV. l´.
[1538] Ibid. m´.
[1539] PLATES XII. FIG. 28; and XXV. FIG. 13. m´´.
[1540] PLATE XXV. FIG. 2, 5, 21. m´´.
[1541] PLATE XII. FIG. 10. m´´.
[1542] Ibid. FIG. 4. m´´.
[1543] PLATE XXV. FIG. 1. m´´.
[1544] PLATE XI. FIG. 23.
[1545] PLATE XXV. FIG. 24.
[1546] Ibid. FIG. 30.
[1547] Jurine _Hymenopt._ _t._ vii. _f._ 3.
[1548] PLATE XXV. FIG. 28.
[1549] Ibid. FIG. 13.
[1550] Ibid. FIG. 5.
[1551] Ibid. FIG. 1.
[1552] PLATE XI. FIG. 12.
[1553] _Linn. Trans._ xii. _t._ xxi. _f._ 3.
[1554] Latreille says _six_, but only _five_ are discernible; the
three last form a kind of bristle.
[1555] Latr. _Fourmis_, 323.
[1556] Jurine _Hymenopt._ _t._ vi. _f._ 3.
[1557] _Ibid._ _f._ 2.
[1558] _Ibid._ _f._ 1. PLATE XXV. FIG. 7.
[1559] PLATE XXV. FIG. 25, 26.
[1560] PLATE XII. FIG. 16-22.
[1561] Ibid. FIG. 19. _a_.
[1562] _Linn. Trans._ xxii. _t._ xxi. _f._ 3, 4. PLATE XXV. FIG. 11.
[1563] PLATE XI. FIG. 17.
[1564] Kirby _Mon. Ap. Angl._ i. 184. _t._ x. **. d. 1. f. 8.
[1565] PLATE XXV. FIG. 27.
[1566] PLATE XII. FIG. 26.
[1567] PLATE XXV. FIG. 32.
[1568] PLATE XII. FIG. 25.
[1569] PLATE XXV. FIG. 17.
[1570] _Linn. Trans_. xii. _t._ xxiii. _f._ 5. _f._
[1571] _Ibid. t._ xxi. _f._ 8 _g._ 9, 10. _c._
[1572] De Geer iv. 219. _t._ viii. _f._ 20.
[1573] See VOL. II. p. 65, 201--.
[1574] De Geer iv. _t._ vii. _f._ 22.
[1575] PLATE XXV. FIG. 35.
[1576] De Geer vi. _t._ i. _f._ 5.
[1577] PLATE XI. FIG. 21.
[1578] PLATE XII. FIG. 29. _a._
[1579] PLATE VI. m.
[1580] Ibid.
[1581] PLATE XXVII. FIG. 1, 3-5. n´.
[1582] Ibid. FIG. 3.
[1583] Ibid. FIG. 4.
[1584] PLATE XXVII. FIG. 1.
LETTER XXXV.
_EXTERNAL ANATOMY OF INSECTS_,
CONTINUED.
THE TRUNK, AND ITS PARTS AND ORGANS.
As the _head_ of insects is the principal seat of the organs of
_sensation_, so is the _trunk_ of those of _motion_; and in it are
contained the muscles by which they are moved: it may therefore be
regarded as the great centre of motion, and as the main support and
prop of the two other primary sections of the body--the head and
abdomen, between which it is situated--it may be deemed the most
important part of the insect, the key-stone of the whole structure.
In treating upon it, for the greater clearness, I shall consider its
_substance_, _general form_, _proportions_, _composition_, _internal
processes_, and _members_. It will first, however, be necessary to
assign my reasons for the nomenclature of its parts that I have adopted.
Had the entomological world been universally agreed upon this
subject, and there was an established system of Orismology[1585],
I should have proposed no alteration without great reluctance, and
the fullest conviction of the absolute necessity of some change;
but as the standard of language in our science is still unsettled,
and different terms are used by different writers, there seems full
liberty left to me to select those that appear upon the whole most
appropriate; and where proper and significant terms seem wanting, to
invent new ones. M. Latreille, in a late Essay[1586], has proposed
many changes of this kind, and seems to hesitate concerning the
adoption of some of those recently coined in France for the parts of
the trunk[1587]; it may therefore, I think, be permitted me to labour
a little in this hitherto imperfectly cultured field, and to suggest
such improvements as the subject may seem to require or admit.
Linné called the part we are now considering the _trunk_, its
upper-side he usually denominated the _thorax_, and its under-side
the _breast_: he notices also the _scutellum_ and _sternum_[1588].
As the _prothorax_ and _scutellum_ are the only _apparent_ parts of
the back of the trunk in his first Orders (_Coleoptera_, _Hemiptera_
L.), the rest being covered, in noticing these he puts the part for
the whole, calling the _prothorax_ the _thorax_, but which strictly
was not synonymous with what he called by the same name in the other
Orders. Linné's phraseology with regard to the trunk of insects was
adopted by Fabricius and other Entomologists, till Illiger employed
the term _thorax_ to designate the whole of the trunk[1589], calling
the upper part _thorax superior_ and the lower _thorax inferior_.
M. De Blainville, M. Latreille, and other French writers, improved
upon this, naming the upper part the back (_dorsum_), and the lower
the breast (_pectus_); and dividing the trunk, or according to
them _thorax_, into three sections, each bearing a pair of legs.
But I see no sufficient reason for this alteration--the terms
_trunk_, _thorax_, and _breast_, in the common acceptation are well
understood, and lead to no confusion or glaring impropriety; I
shall therefore adhere to the old phraseology, especially as French
Entomologists in popular language still do the same.
As to the division of the trunk into segments by M. Latreille and
others, it has been regarded as consisting of _three_ primary ones,
which have been called in the order of their occurrence, reckoning
from the head--_prothorax_, _mesothorax_, _metathorax_. The first
of these segments, however--and the learned Entomologist just named
seems to hint as much[1590]--is usually more distinct from the other
two, than they are from each other. If this idea be correct, the
trunk is properly resolvable into _two_ primary segments, the first
bearing the arms or fore-legs, and the other the proper legs and
the organs of flight. M. Chabrier calls the latter _tronc alifère_,
or wing-trunk;--a happy term, which I have adopted and latinized,
calling it the alitrunk (_alitruncus_): the first segment, because
it bears the fore-legs, I have named manitrunk (_manitruncus_). I
adopt likewise the terms above mentioned, _prothorax_, _mesothorax_,
_metathorax_, to signify the three segments into which the thorax of
Linné, or the _upper_ side of the trunk, is resolvable; and those of
the breast I denominate _antepectus_, _medipectus_, and _postpectus_.
If terms be thought necessary to designate the two intire segments
into which the alitrunk is resolvable, the first may be the meditrunk
(_meditruncus_), and the other the potrunk (_potruncus_).
I. _Substance._--With regard to its _substance_, the trunk in general
is softer than the head, and harder than the abdomen, especially as
to its upper surface; but in some cases, where it is not protected by
the elytra, as in the rove-beetles (_Staphylinus_ L.), the abdomen
appears as hard as the trunk. Though usually not very different from
the elytra in this respect, in _Meloe_, _Lytta_, and other vesicatory
beetles, it is of a firmer consistence.
II. _General Form._--In the _Coleoptera_ Order the only part of
the trunk that is visible on its upper-side is the _prothorax_:
the _mesothorax_, with the exception of the _scutellum_, and the
_metathorax_, being entirely concealed by it and the elytra; so that,
with regard to _shape_, it may nearly be considered as merging in the
_prothorax_. Below it is more visible, and may be stated as more or
less quadrangular; in oblong beetles inclining to a parallelogram,
and in shorter or hemispherical ones to a square. In the majority
it is more convex below than above, except in the case of the
hemispherical or gibbous beetles (_Coccinella_, _Erotylus_, &c.), in
which the under-side is flat and the upper-side very convex. In the
_Diurnal Lepidoptera_ the trunk approaches to a cubical shape, in the
_Nocturnal_ it is more spherical. A similar difference obtains in
the _Hymenoptera_ and _Diptera_: in the bees, wasps and flies, the
trunk approaching to the figure of a sphere; in the ants, _Scoliæ_,
crane-flies, &c. to that of a cube. The upper part of it in many
_Ichneumonidæ_, female ants, &c. is very elevated, forming an arch,
and sloping towards the abdomen. In general it may be observed with
respect to the remaining Orders, that the form of the trunk merges
in that of the whole body, the tendency of which is often to a
three-sided figure.
III. _Proportions._--The trunk is usually longer and larger than
the head and longer than the abdomen, but not wider: but there are
exceptions to both these rules. In _Colliuris_, _Mantis_, &c.,
it is more _slender_; and in _Atta megacephala_ and some neuter
ants, it is _shorter_ than the _head_; in _Atractocerus_, many
_Staphylinidæ_, _Phasma_, the _Libellulina_, the _Lepidoptera_, and
various _Hymenoptera_, it is shorter, and in the _Mantidæ_ more
_slender_ than the _abdomen_. The greatest disproportion between it
and the last part is exhibited by the genus _Evania_, parasitic upon
the _Blattæ_, in which the abdomen appears merely as a minute and
insignificant appendage of the trunk. The vertical diameter of this
part, almost without exception, is greater than that of either head
or abdomen. When we consider that it contains the muscles that move
both the organs of flight and the legs, we see clearly the reason why
the CREATOR gave it greater volume.
IV. _Composition._--I lately intimated to you that the trunk, though
resolvable into _three_ segments, in most cases properly consists
of only _two_ primary ones. Whoever examines the perfect insects of
every Order, except the _Aptera_[1591], will find this distinction
strongly pointed out, not only by the different direction of the
first pair of legs from that of the two last, but also in a large
proportion by a deep incisure; and in all it is further manifested
by the anterior segment having a motion distinct from that of the
rest of the trunk, and separating readily from it; and this not only
where it is large, as in insects that have a thoracic shield, but
also in those in which the _prothorax_ is less apparent: whereas the
other two pedigerous segments have little or no distinct motion,
will not readily separate from each other, and in some cases exhibit
no pectoral suture between them. Sometimes, however, these two last
segments are more prominently distinguished: in _Lytta_, _Mylabris_,
and other vesicatory beetles, they are separated below by an
incisure, or rather the first or mid-leg segment, is not nearly so
elevated as that of the hind-legs. In some ants (_Atta_ Latr.), in
the neuters, there is no distinction of segments in the trunk; but
in others (_Formica_ Latr.) it follows the general law, and consists
of three. In the _Arachnida_, with the exception of _Galeodes_, in
which the head is distinct, and the three segments of the trunk may
be traced, these parts together form only a single segment. Induced
by these reasons, I consider the trunk as consisting in general of
two _primary_ segments, the _manitrunk_ and _alitrunk_: the latter
resolvable into two _secondary_ ones.
* _Manitruncus_[1592].--The manitrunk, then, is the anterior section
of the trunk, which bears the arms and contains the muscles that move
them. This part has free motion, or a motion independent of that of
the rest of the trunk. This indeed seems a necessary result of the
direction and uses of the arms. It consists of an upper and lower
part--the _prothorax_ and _antepectus_.
i. _Prothorax_[1593].--The upper part of the manitrunk in the
_Coleoptera_, _Orthoptera_, and _Hemiptera_, is by far the most
conspicuous part of the trunk, but in the other Orders it is less so.
With respect to it, insects may be divided into two classes--those
that have and those that have not a _prothorax_. In the _Coleoptera_
Order it is remarkable both for size and variations in its shape and
sculpture. In the _Orthoptera_, though less various, it is almost
equally conspicuous, especially in _Blatta_. In the _Homopterous_
section of the _Hemiptera_, in many genera it has become extremely
short; while in the _Heteropterous_ section its dimensions are not
much reduced. In the majority of the _Neuroptera_, likewise, it is
comparatively large; in the _Libellulina_ much shorter, and in the
_Trichoptera_ and _Lepidoptera_ nearly evanescent[1594].--In the
_Hymenoptera_ and _Diptera_, with very few exceptions, the thoracic
shield altogether disappears, at least if I am correct in an idea,
which I shall hereafter explain, that the _collar_ usually regarded
as the analogue of the _prothorax_, is really a part of the alitrunk.
In these last Orders, though there is no true _prothorax_, the
manitrunk still remains under the form of an _antepectus_, bearing
the fore-legs, and containing the muscles that move them.
The prothorax of insects may in general be considered with respect
to its _parts_, _margin_, _appendages_, _shape_, _sculpture_,
_clothing_, and _proportions_.
1. The _prothorax_, regarded as a whole, distinct from the
_antepectus_ or fore-breast, consists commonly of _two_ pieces--the
_shield_, or upper part[1595], and the _ora_, or under part[1596].
In the shield you are to observe its _apex_[1597], _base_[1598],
_sides_[1599], _limb_[1600], and _disk_[1601]. The _apex_ is the
part next the _head_; the _base_ that next the _abdomen_; the
_limb_ the _circumference_, and the _disk_ the _central_ part. In
many _Orthoptera_ and Heteropterous _Hemiptera_, the shield appears
further to consist of two pieces, an anterior and posterior one.
The _ora_ is a continuation of the shield below the lateral margin,
turned downwards and inwards towards the fore-breast and the legs,
but separated from the former in most cases by a suture, as in
_Carabus_ L.; and in others merely by an impressed line, as in
_Blaps_ F.; but in _Curculio_ and _Cerambyx_ L., &c. there is no ora,
the shield being without a lateral margin, and forming one piece with
the _antepectus_. The part we are now considering varies in different
genera. Sometimes it is very _narrow_, as in _Scarites_; at others
very _broad_, as in _Buprestis_, _Nepa_, &c. In _Lampyris_, except
_L. italica_, and affinities, it projects posteriorly into a _lobe_
or tooth, which forms a right angle with the rest of the _ora_, and
becomes the lower part of the cavity that receives the head; and in
_Dermestes_ this part is excavated into an anterior and posterior one
which admits the antennæ and arms when folded for repose.
2. The _margin_ of the _prothorax_ is a ridge, either defining its
sides or whole circumference. In many cases this margin is broad and
dilated, but in others it is merely a thread or bead that separates
the shield from the _ora_. Though generally terminating the upper
surface, it sometimes, as in _Staphylinus_, dips below it. In many
insects, however, as I just observed, the thoracic shield has no
lateral margin whatever.
3. Various and singular are the _appendages_ with which the
_prothorax_ of numerous insects is furnished. Many of these are
_sexual_ distinctions, and have been before described to you[1602];
but there are others common to both sexes, the most remarkable of
which I shall notice.--Some are distinguished by a long _horn_ which
overhangs the head, as _Membracis cultrata_, _ensata_, &c.[1603];
in others it stands upright, as in _Centrotus spinosus_[1604];
_C. Taurus_ has a pair of thoracic horns like those of a bull,
only dorsal[1605]; in _Ledra aurita_ they are flat, and represent
ears[1606]; in some species of _Tingis_ (_T. Echii_, _Pyri_, &c.)
a kind of reticulated hood, resembling lace, is elevated from the
anterior part of the _prothorax_, which receives and shelters the
head[1607]. In _Centrotus globularis_ and _clavatus_ F., especially
the former, the part in question is armed by a most singular and
wonderful apparatus of balls and spines,--in one case standing
erect[1608], and in the other being horizontal[1609],--which gives
these animals a most extraordinary appearance. In many of the species
here quoted the _prothorax_ is producted posteriorly into a long
scutelliform horizontal horn, which more or less covers the wings and
abdomen; a circumstance which also distinguishes the genus _Acrydium_
F. (_Tetrix_ Latr.). This horn seems to have been sometimes regarded
by Linné and Fabricius as a real _scutellum_, and sometimes only as
a process of the prothorax: but that it is merely the latter will be
evident to you, if you examine carefully any insect furnished with
this appendage; for if you remove that part, you will discover the
true scutellum and other parts of the trunk concealed beneath it. A
very remarkable prothoracic appendage is exhibited by some species
of _Mantis_. In general the part we are treating of in this tribe is
very slender; but in _M. strumaria_, _gongyloides_, &c., it appears
dilated to a vast width, and assumes, either partially or generally,
a subrhomboidal form; but if it is more closely examined, it will be
found that the form of the _prothorax_ is really similar to that of
the rest of the tribe, but that this part is furnished on each side,
either on its whole length or anteriorly, with a large membranous
flat subtriangular appendage resembling parchment[1610]. Perhaps
this kind of sail may be useful to the animal in flight. In _Prionus
coriarius_ &c. its sides are armed with teeth, and in many _Lamiæ_,
_Cerambyces_, and other Capricorn beetles, and often in various bugs
(_Pentatoma_ Latr.) with sharp fixed spines. But the _prothorax_
has _moveable_ as well as _fixed_ appendages; of this kind are
those spines (_umbones_), whose base is a spherical boss moving in
an _acetabulum_ of the thoracic shield of the Capricorn subgenus
_Macropus_ Thunb. If I might hazard a conjecture, I should say that
these organs were given to this animal by an all-provident CREATOR,
to enable it to push itself forward, when in the heart of some tree
it emerges from the pupa, that it may escape from its confinement.
Another kind of moveable appendages are attached to the thorax of
_Lepidoptera_, usually in the form of a pair of concavo-convex scales
covered externally with a tuft of hairs[1611]. M. Chabrier, who
examined these scales in recent insects, describes them as vesicles,
which appeared to him full of a liquid and of air, and capable of
being alternately inflated and rendered flaccid; he regards them
as accessories to a pair of spiracles, which he looks upon as
vocal[1612], opening into the manitrunk just above the insertion of
the arms. These organs are quite distinct from the _tegulæ_ that
cover the base of the primary wings of insects of this Order[1613],
and are what, borrowing a term from Mouffet[1614], I have called in
the table _patagia_, or tippets. Under this head I may include the
_caruncles_ at the anterior angles of the _prothorax_ of a genus
of beetles with soft elytra, named by Fabricius _Malachius_. When
pressed, says De Geer of these insects, a red inflated soft vesicle,
of an irregular shape, and consisting of three lobes, emerges from
the thorax and from each side of the anterior part of the abdomen,
which re-enters the body when the pressure is removed[1615]. M.
Latreille seems to think that these vesicles have some analogy with
the poisers of _Diptera_ and the pectens of scorpions; and that they
are connected with the respiration[1616].
4. We are next to say something upon the _shape_ of the _prothorax_.
The forms of the thoracic shield, especially in the _Coleoptera_,
are so various, that it would be endless to aim at particularizing
all; but it may be useful to notice a few of the most remarkable. The
prothorax of _Moluris_, a darkling-beetle, approaches the nearest
of that of any insect to a spherical form, from its remarkable
convexity; in the wheel-bug (_Reduvius serratus_) it is compressed,
and longitudinally elevated into a semicircular serrated crest: it
is crested, also, in many _Locustæ_ and _Acridæ_, in some having
two parallel ridges; but, generally speaking, its surface is
more depressed. In _Necrodes_ it is nearly circular, in _Blatta
petiveriana_ semicircular, in _Nilion_ and some _Coccinellidæ_
crescent-shaped, in _Carabus_ obcordate, in _Cantharis_ and _Sagra_
approaching to a square, in _Languria_ to a parallelogram; in many
_Cimicidæ_, _Belostoma_, &c., it is triangular, with the vertex
truncated; it is trapezoidal in _Elater_, in _Ateuchus_ rather
pentagonal, and exhibiting an approach to six angles in some
other beetles[1617]: but the prothorax most singular in form is
that of some species of M. Latreille's genus _Helæus_[1618], as
_H. perforatus_, _Brownii_, &c.: in these its anterior angles are
producted, and curving inwards, lap at the end one over the other,
so as to form a circular orifice for the head, which otherwise would
be quite covered by the shield. Thus the upper portion of the eyes
can see objects above, as well as their lower portion those below. I
might enumerate many other forms, but these are sufficient to give
you some notion of the variations of this part.
5. The _prothorax_ is equally various in its _sculpture_; but since
in the Orismological table almost every instance of it has its place,
I shall here only notice it as far as it is common to the whole
tribes, genera, or subgenera. The _Scarabæidæ_ of Mr. W. S. MacLeay
are distinguished by a small _excavation_ on each side of this part,
which, as has been before remarked[1619], furnishes an elevated base
for an internal process with which the anterior _coxæ_ ginglymate. In
_Onitis_ and _Phanæus_, to these excavations are superadded a pair
impressed in the base of the _prothorax_, just above the scutellum;
in _Carabus_ L. a longitudinal channel divides the thoracic shield
into two equal portions; and many genera of that great tribe have
in addition, at the base on each side, one or two excavations or
short furrows. _Elophorus_ F. has on this part several longitudinal
channels, alternately straight and undulated. Generally speaking,
in _Carabus_ L. the _prothorax_ has no impressed points; but in
one or two subgenera of _Harpalidæ_ (_Chlænia_ &c.) it is thickly
covered with them. In numbers of _Locusta_ Leach, the part we are
considering is what Linné terms _cruciate_, being divided into four
longitudinal portions by three elevated lines, the intermediate one
being straight, and the lateral ones diverging from it both at their
base and apex, so as to form a sinus or angle[1620]. In certain
_Acridæ_ K. (_Locusta_ F.) there are only two of these lines or
ridges, but notched or toothed; and in some of the genus first named
only one[1621]; in _Locusta Dux_ and affinities the _prothorax_ has
several transverse channels or rather folds[1622], with corresponding
ridges on its internal surface.