THE MUSCLES; THE FAT-BODY AND CŒLOM.
_SPECIAL REFERENCES._
VIALLANES. Histologie et Développement des Insectes. Ann. Sci.
Nat., Zool., Tom. XIV. (1882).
KÜHNE in Stricker’s Histology, Vol. I., chap. v.
PLATEAU. Various Memoirs in Bull. Acad. Roy. de Belgique (1865,
1866, 1883, 1884). [Relative and Absolute Muscular Force.]
LEYDIG. Zum feineren Bau der Arthropoden. Müller’s Archiv., 1855.
WEISMANN. Ueber zwei Typen contractilen Gewebes, &c. Zeits. für
ration. Medicin. Bd. XV. (1862).
_Structure of Insect Muscles._
The muscles of the Cockroach, when quite fresh, appear semi-transparent
and colourless. If subjected to pressure or strain they are found to be
extremely tender. Alcohol hardens and contracts them, while it renders
them opaque and brittle.
The minute structure of the voluntary or striped muscular fibres of
Vertebrates is described in common text-books.[84] Each fibre is
invested by a transparent elastic sheath, the sarcolemma, and the space
within the sarcolemma is subdivided by transverse membranes into a
series of compartments. The compartments are nearly filled by as many
contractile discs, broad, doubly refractive plates, which are further
divisible into prismatic columns, the sarcous elements, each being as
long as the contractile disc. Successive sarcous elements, continued
from one compartment to another, form the primitive fibrils of the
muscle. In cross-section the fibrils appear as polygonal areas bounded
by bright lines. Outside the fibres, but within the sarcolemma, are
nuclei, imbedded in the protoplasm, or living and formative element of
the tissue.
[84] See, for example, Klein’s Elements of Histology, chap. ix.
The muscular fibres of Insects present some important differences from
the fibres just described. The nuclei are often found in the centre,
and not on the surface of the fibres in both Insects and Crustacea. In
both classes the fibrils are frequently subdivided into longitudinal
strands, which have not been distinguished in Vertebrate muscles
(Viallanes). The sarcolemma is often undeveloped. Lastly, Insects, like
other Arthropoda, exhibit the remarkable peculiarity that not only
their voluntary muscles, but all, or nearly all, the muscles of the
body, even those of the digestive tube, are striated.[85]
[85] The exceptions relate chiefly to the alary muscles of the
pericardial septum. Lowne (Blow-fly, p. 5, and pl. v.) states that
some of the thoracic muscles of that Insect are not striated.
_General Arrangement of Insect Muscles._
The arrangement of the muscles in an Insect varies greatly according
to situation and mode of action. Some of the abdominal muscles consist
solely of straight parallel bundles, while the muscles of the limbs
usually converge to tendinous insertions. In certain larvæ, where
the segments show hardly any differentiation, the muscles form a
sheet which covers the whole body, and is regularly segmented in
correspondence with the exo-skeleton. As the movements of the body and
limbs become more varied and more energetic, the muscles become grouped
in a more complicated fashion, and the legs and wings of a flying
Insect may be set in motion by a muscular apparatus almost as elaborate
as that of a bird.
_Muscles of the Cockroach._
The following short notes on the muscles of the Cockroach, aided by
reference to the figures, will render the more noteworthy features
intelligible. A very lengthy description, far beyond our space or
the reader’s patience, would be required to explain in detail the
musculature of the head, limbs, and other specialised regions.
STERNAL MUSCLES OF ABDOMEN.--The _longitudinal sternal muscles_
(fig. 34) form a nearly continuous transversely segmented sheet,
covering the ventral surface between the fore-edge of the second
abdominal sternum and the fore-edge of the seventh. These muscles, in
conjunction with the longitudinal tergal muscles, tend to telescope the
segments.
[Illustration: Fig. 34.--Muscles of Ventral Wall, with the Nerve-cord.
× 5.]
[Illustration: Fig. 35.--Muscles of Dorsal Wall, with the Heart and
Pericardial Tendons. × 5.]
The _oblique sternal muscles_ (fig. 34), which are very short, connect
the adjacent edges of the sterna (2–3, 3–4, 4–5, 5–6, 6–7). They
extend inwards nearly to the middle line, but, like the longitudinal
sternal muscles, they are not developed beneath the nerve-cord.
Acting together, the oblique sternal muscles would antagonise the
longitudinal, but it is probable that they are chiefly used to effect
lateral flexion of the abdomen, and that only the muscles of one side
of the abdomen contract at once.
The _tergo-sternal_ (or expiratory) muscles (figs. 35 and 36) form
vertical pairs passing from the outer part of each abdominal sternum
to the corresponding tergum. Their action is to approximate the dorsal
and ventral walls, and thus to reduce the capacity of the abdomen. The
first tergo-sternal muscle has its ventral insertion into the stem
of the postfurca, and takes an oblique course to the first abdominal
tergum.
TERGAL MUSCLES OF ABDOMEN.--The _longitudinal tergal_ muscles extend
from the fore part of each abdominal tergum, including the first,
to the same part of the tergum next behind. They are interrupted by
longitudinal spaces, so that the muscular sheet is less continuous than
on the ventral surface, and has a fenestrated appearance. The direction
of the fibres is slightly oblique.
_Oblique tergal_ muscles, resembling the oblique muscles of the sterna,
are also present.
In the thorax the general arrangement of the muscles is greatly
modified by the altered form of the dorsal and ventral plates, and by
the attachment of powerful limbs.
STERNAL MUSCLES OF THORAX.--Two tubular apodemes, lying one behind the
other, project into the thorax from the ventral surface (p. 59 and
fig. 27). To the foremost of these are attached three paired muscles
and one median muscle. The median muscle passes to the second tubular
apodeme. The anterior pair pass forwards and outwards to the base of
the prothoracic leg; the next pair directly outwards to the base of the
middle leg; while the posterior pair pass outwards and backwards to the
arms of the medifurca. From the second tubular apodeme, in front of the
metasternum, four pairs of muscles spring. Those of the anterior pass
forwards and outwards to the coxa of the fore limb; the second pair
directly outwards to the base of the metathoracic legs; the third pair
backwards and outwards to the arms of the postfurca; the fourth pair
backwards to the second abdominal sternum.
[Illustration: Fig. 36.--Muscles of lateral wall, &c. × 5.]
The muscles attached to the medi- and postfurca (other than those
connecting them with the tubular apodemes) are:--(1) A pair passing
from the posterior edge of the arms of the medifurca to the stem of the
postfurca; (2) a pair which diverge from the stem of the postfurca and
proceed to the fore part of the second abdominal sternum; (3) a pair
passing from the posterior edge of the arms of the postfurca, these are
directed inwards and backwards, and are inserted into the hinder part
of the second abdominal sternum; (4) a pair already mentioned, which
correspond in position and action to the tergo-sternal muscles, and
spring from the stem of the postfurca, passing upwards and outwards to
the sides of the first abdominal tergum.
[Illustration: Fig. 37.--Muscles of left mesothoracic leg, seen
from behind. The muscles are--Adductor and abductor of the coxa;
extensor and flexor of femoral joint; flexor and extensor of tibial
joint; flexor of tarsus; and a retractor tarsi, which swings the
tarsus backwards, so that it points away from the head. It is opposed
by another muscle, which moves the tarsus forwards. Both muscles
parallelise the tarsus to the axis of the body, but in opposite
directions.]
The muscles attached to the arms of each furca pass to other structures
in or near the middle line of the body. The pull of such muscles must
alter the slope of the two steps in the ventral floor of the thorax
(p. 58, and fig. 3, p. 12). When the furca is drawn forwards, the
step is rendered vertical or even inclined forward, the sterna being
approximated; while, on the other hand, a backward pull brings the step
into a horizontal position, and separates the sterna.
TERGAL MUSCLES OF THORAX.--The _longitudinal tergal_ muscles are much
reduced in width when compared with those of the abdomen. Sets of
obliquely placed muscles, which may be called the _lateral thoracic_
muscles, arise from near the middle of each tergum, and converge to
tendinous insertions on the fore edge of each succeeding tergum, close
to the lateral wall of the body.
The principal muscles of the legs are figured and named, and their
action can readily be inferred from the names assigned to them.
_Insect Mechanics._
The mechanics of Insect movements require exposition and illustration
far beyond what is possible in a book like this. Even the elaborate
dissections of Lyonnet and Straus-Dürckheim are not a sufficient basis
for a thorough treatment of the subject, and until we possess many
careful dissections, made by anatomists who are bent upon mastering
the action of the parts, our views must needs be vague and of doubtful
value. Zoologists of great eminence have been led into erroneous
statements when they have attempted to characterise shortly a complex
animal mechanism which they did not think it worth while to analyse
completely.[86]
[86] For example, Prof. Huxley, in his Anatomy of Invertebrated
Animals (p. 254), says that “as the hard skeleton [of Arthropods] is
hollow, and the muscles are inside it, it follows that the body, or
a limb, is bent towards that side of its axis, which is opposite to
that on which a contracting muscle is situated.” The flexor muscles
of the tail of the Crayfish, which, according to the above rule,
should be extensors, the muscles of the mandibles of an Insect, and
the flexors and extensors of Crustacean pincers are among the many
conspicuous exceptions to this rule.
The action of flight and the muscles attached to the wings are best
studied in Insects of powerful flight. The female Cockroach cannot
fly at all, and the male is by no means a good flier. Both sexes are,
however, admirably fitted for running.
In running, two sets, each consisting of three legs, move
simultaneously. A set includes a fore and hind limb of the same side
and the opposite middle leg. Numbering them from before backwards, and
distinguishing the right and left sides by their initial letters, we
can represent the legs which work together as--
┏━━━━━━━━━┷━━━━━━━━━┓
R_{1} L_{2} R_{3}
┏━━━━━━━━━┷━━━━━━━━━┓
L_{1} R_{2} L_{3}
The different legs have different modes of action. The fore-leg may be
compared to a grappling-iron; it is extended, seizes the ground with
its claws, and drags the body towards its point of attachment. The
middle leg is chiefly used to support and steady the body, but has some
pushing power. The hind leg, the largest of the three, is effective in
shoving, and chiefly propels the body.
_Muscular Force of Insects._
The force exerted by Insects has long been remarked with surprise, and
it is a fact familiar, not only to naturalists, but to all observant
persons, that, making allowance for their small size, Insects are the
most powerful of common animals. Popular books of natural history
give striking and sometimes exaggerated accounts of the prodigious
strength put forth by captive Insects in their efforts to escape. Thus
we are told that the flea can draw 70 or 80 times its own weight.[87]
The Cockchafer is said to be six times as strong as a horse, making
allowance for size. A caterpillar of the Goat Moth, imprisoned beneath
a bell-glass, weighing half a pound, which was loaded with a book
weighing four pounds, nevertheless raised the glass and made its escape.
[87] Haller. This and other examples are taken from Rennie’s Insect
Transformations.
This interesting subject has been investigated by Plateau,[88] who
devised the following experiment. The Insect to be tested was confined
within a narrow horizontal channel, which was laid with cloth. A thread
attached to its body was passed over a light pulley, and fastened to a
small pan, into which sand was poured until the Insect could no longer
raise it. Some of the results are given in the following table:--
[88] Bull. Acad. Roy. de Belgique, 2^{e.} Sér., Tom. xx. (1865), and
Tom. xxii. (1866).
_Table of Relative Muscular Force of Insects (Plateau)._
Weight of body Ratio of weight lifted
in grammes. to weight of body.
Carabus auratus 0·703 17·4
Nebria brevicollis 0·046 25·3
Melolontha vulgaris 0·940 14·3
Anomala Frischii 0·153 24·3
Bombus terrestris 0·381 14·9
Apis mellifica 0·090 23·5
One obvious result is that within the class of Insects the relative
muscular force (as commonly understood) is approximately in the inverse
proportion of the weight--that is, the strength of the Insect is (by
this mode of calculation) most conspicuous in the smaller species.
In a later memoir[89] Plateau gives examples from different Vertebrate
and Invertebrate animals, which lead to the same general conclusion.
[89] Loc. cit. 3^{e.} Sér., Tom. vii. (1884). Authorities for the
various estimates are cited in the original memoir.
_Ratio of weight drawn to weight of body (Plateau)._
Horse ·5 to ·83
Man ·86
Crab 5·37
Insects 14·3 to 23·5
The inference commonly drawn from such data is that the muscles
of small animals possess a force which greatly exceeds that of
large quadrupeds or man, allowance being made for size, and that
the explanation of this superior force is to be looked for in some
peculiarity of composition or texture. Gerstaecker,[90] for example,
suggests that the higher muscular force of Arthropoda may be due
to the tender and yielding nature of their muscles. An explanation
so desperate as this may well lead us to inquire whether we have
understood the facts aright. Plateau’s figures give us the ratio of the
weight drawn or raised to the weight of the animal. This we may, with
him, take as a measure of the _relative muscular force_. In reality, it
is a datum of very little physiological value. By general reasoning of
a quite simple kind it can be shown that, for muscles possessing the
same physical properties, the _relative_ muscular force necessarily
increases very rapidly as the size of the animal decreases. For the
contractile force of muscles of the same kind depends simply upon the
number and thickness of the fibres, _i.e._, upon the sectional area of
the muscles. If the size of the animal and of its muscles be increased
according to any uniform scale, the sectional area of a given muscle
will increase as the square of any linear dimension. But the weight
increases in a higher proportion, according to the increase in length,
breadth, and depth jointly, or as the cube of any linear dimension.[91]
The ratio of contractile force to weight must therefore become rapidly
smaller as the size of the animal increases. Plateau’s second table
(_see above_) actually gives a value for the relative muscular force
of the Bee, in comparison with the Horse, which is only one-fourteenth
of what it ought to turn out, supposing that both animals were of
similar construction, and that the muscular fibres in both were equal
in contractile force per unit of sectional area.[92]
[90] Klassen und Ordnungen des Thierreichs, Bd. V., pp. 61–2.
[91] This change in the relation of weight to strength, according to
the size of the structure, has long been familiar to engineers. (See,
for example, “Comparisons of Similar Structures as to Elasticity,
Strength, and Stability,” by Prof. James Thomson, Trans. Inst.
Engineers, &c., Scotland, 1876.) The application to animal structures
has been made by Herbert Spencer (Principles of Biology, Pt. II.,
ch. i.). The principle can be readily explained by models. Place a
cubical block upon a square column. Double all the dimensions in
a second model, which may be done by fitting together eight cubes
like the first, and four columns, also the same as before except in
length. Each column, though no stronger than before, has now to bear
twice the weight.
[92] Contractile force varies as sectional area of muscle. Let _W_
be weight of Horse; _w_, weight of Bee; _R_, a linear dimension of
Horse; _r_, a linear dimension of Bee. Then,
Contr. force of Horse sect. area of muscles (Horse) _R^2_
--------------------- = ----------------------------- = -----.
Contr. force of Bee sect. area of muscles (Bee) _r^2_
_W_ _R^3_ _R^2_ _W_ _r_
But since --- = -----, ----- = --- × ---.
_w_ _r^3_ _r^2_ _w_ _R_
Contr. force of Horse _Wr_
Therefore --------------------- = ----.
Contr. force of Bee _wR_
But, by definition,
Contr. f. of Horse
------------------
Rel. m.f. of Horse _W_ Contr. f. of Horse _w_
------------------ = ╺━━━━━━━━━━━━━╸ = ------------------ × ---
Rel. m.f. of Bee Contr. f. of Bee Contr. f. of Bee _W_
----------------
_w_
_Wr_ _w_ _r_ _r^3_ _w_
= ---- × --- = --- = (---)^{1/3} = (---)^{1/3}.
_wR_ _W_ _R_ _R^3_ _W_
The weight of a horse is about 270,000 grammes, that of a bee
·09 gramme; so that
_w_ ·09
(---)^{1/3} = (-----)^{1/3} = (·000,000,3̅)^{1/3} = ·0015 (nearly) =
_W_ 270,000
Calculated Ratio of Relative Muscular Force of Horse to that of Bee.
·5
The Observed Ratio (Plateau) is ---- = ·02128;
23·5
so that the relative muscular force of the Horse is more than
fourteen times as great in comparison with that of the Bee as it
would be if the muscles of both animals were similar in kind, and the
proportions of the two animals similar in all respects.
A later series of experiments[93] brings out this difference in a
precise form. Plateau has determined by ingenious methods what he
calls the _Absolute Muscular Force_[94] of a number of Invertebrate
animals (Lamellibranch Mollusca, and Crustacea), comparing them with
man and other Vertebrates. His general conclusions may be shortly given
as follows:--The absolute muscular force of the muscles closing the
pincers of Crabs is low in comparison with that of Vertebrate muscles.
The absolute force of the adductor muscles closing a bivalve shell may,
in certain Lamellibranchs, equal that of the most powerful Mammalian
muscles; in others it falls below that of the least powerful muscles of
the frog, which are greatly inferior in contractile force to Mammalian
muscles. We find, therefore, that the low contractile force of Insect
muscles is in harmony, and not in contrast, with common observation of
their physical properties, and that the high _relative_ muscular force,
correctly enough attributed to them, is explicable by considerations
which apply equally well to models or other artificial structures.
[93] Rech. sur la Force Absolue des Muscles des Invertébrés. I^e
Partie. Mollusques Lamellibranches. Bull. Acad. Roy. de Belgique, 3^e
Sér., Tom. VI. (1883).
Do., II^e Partie. Crustacés Décapodes. Ibid., Tom. VII. (1884).
[94] _Statical muscular force_ and _Specific muscular force_ are
synonymous terms in common use. _Contractile force per unit of
sectional area_ gives perhaps the clearest idea of what is meant.
The comparison between the muscular force of Insects and large
animals is sometimes made in another way. For example, in Carpenter’s
Zoology[95] the spring of the Cheese-hopper is described, and we are
told that “the height of the leap is often from twenty to thirty
times the length of the body; exhibiting an energy of motion which is
particularly remarkable in the soft larva of an Insect. A Viper, if
endowed with similar powers, would throw itself nearly a hundred feet
from the ground.” It is here implied that the equation
Height of Insect’s leap Supposed ht. of Viper’s leap (100 ft.)
----------------------- = --------------------------------------
Length of Insect Length of Viper
should hold if the two animals were “endowed with similar powers.”
[95] Vol. II., p. 203. The calculation here quoted is based upon an
observation of Swammerdam, who relates that a Cheese-hopper, 1/4 in.
long, leaped out of a box 6 in. deep.
But it is known that the work done by contraction of muscles of the
same kind is proportional to the volume of the muscles (“Borelli’s
Law”),[96] and in similar animals the muscular volumes are as the
weights. Therefore the equation
Work of Insect Work of Viper
---------------- = ---------------
Weight of Insect Weight of Viper
will more truly represent the imaginary case of equal leaping power.
But the work = weight raised × height, and the weight raised is in both
cases the weight of the animal itself. Therefore
Wt. × Ht. Wt. × Ht.
--------- (Insect) = --------- (Viper),
Wt. Wt.
and Ht. (Insect) = Ht. (Viper). The Viper’s efficiency as a leaping
animal would, therefore, equal that of a Cheese-hopper if it leaped the
same vertical height. Therefore, if the two animals were “endowed with
similar powers,” the heights to which they could leap would be equal,
and not proportional to their lengths, as is assumed in the passage
quoted.
[96] Haughton’s Animal Mechanics, 2nd ed., p. 43.
Straus-Dürckheim observes that a Flea can leap a foot high, which is
200 times its own length, and this has been considered a stupendous
feat. It is really less remarkable than a schoolboy’s leap of two feet,
for it indicates precisely as great efficiency of muscles and other
leaping apparatus as would be implied in a man’s leap to the same
height, viz., one foot.[97]
[97] In any comparison it is necessary to cite not the height cleared
by the man, but the displacement during the leap of his centre of
gravity.
_The Fat-body._
Adhering to the inner face of the abdominal wall is a cellular mass,
which forms an irregular sheet of dense white appearance. This is the
fat-body. Its component cells are polygonal, and crowded together. When
young they exhibit nuclei and vacuolated protoplasm, but as they get
older the nuclei disappear, the cell-boundaries become indistinct, and
a fluid, loaded with minute refractive granules,[98] takes the place of
the living protoplasm. Rhombohedral or hexagonal crystals, containing
uric acid, form in the cells and become plentiful in old tissue. The
salt (probably urate of soda) is formed by the waste of the proteids of
the body. What becomes of it in the end we do not know for certain, but
conjecture that it escapes by the blood which bathes the perivisceral
cavity, that it is taken up again by the Malpighian tubules, and is
finally discharged into the intestine. The old gorged cells probably
burst from time to time, and the infrequency of small cells among
them renders it probable that rejuvenescence takes place, the burst
cells passing through a resting-stage, accompanied by renewal of their
nuclei, and then repeating the cycle of change.
[98] The granules are not shown in the figure, having been removed in
the preparation of the tissue for microscopic examination.
The segmental tubes forming the Wolffian body of Vertebrates have at
first no outlet, and embryologists have hesitated to regard this phase
of development as the permanent condition of any ancestral form.[99] It
is, therefore, of interest to find in the fat-body of the Cockroach an
example of a solid, mesoblastic, excretory organ, functional throughout
life, but without efferent duct.
[99] Balfour, Embryology, Vol. II., p. 603.
[Illustration: Fig. 38.--Fat-body of Cockroach, cleared with
turpentine. _A_, young tissue, with distinct cell-boundaries and
nuclei, a few cells towards the centre with dead contents; _B_, older
ditto, loaded with urates, the cell-walls much broken down, and the
nuclei gone; _tr_, tracheal tubes. × 250.]
The fat-body is eminently a metabolic tissue, the seat of active
chemical change in the materials brought by the blood. Its respiratory
needs are attested by the abundant air-tubes which spread through it in
all directions.
The considerable bulk of the fat-body in the adult Cockroach points to
the unusual duration of the perfect Insect. It is usually copious in
full-fed larvæ, but becomes used up in the pupa-stage.
Extensions of the fat-body surround the nervous chain, the reproductive
organs and other viscera. Sheets of the same substance lie in the
pericardial sinus on each side of the heart.
_The Cœlom._
The fat-body is in reality, as development shows, the irregular
cellular wall of the cœlom, or perivisceral space. Through this space
courses the blood, flowing in no defined vessels, but bathing all the
walls and viscera. In other words, the fat-body is an aggregation of
little-altered mesoblast-cells, excavated by the cœlom, the rest of the
mesoblast having gone to form the muscular layers of the body-wall and
of the digestive tube.