Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/919/
INSECTA. 911 
head is found beneath the skin of the second 
segment, does not depend upon its having be¬ 
come confluent or united with a portion of that 
segment, but upon the development of those 
parts which already existed in it in the larva, so 
that the diminution which the second or pro- 
thoracic segment undergoes is simply an atro¬ 
phied condition, which results from the deve¬ 
lopment of the adjoining parts, and not from 
an actual union or coalescence with them ; 
since in every instance in which apart becomes 
confluent with an adjoining one, it loses its dis¬ 
tinctness of form and character, and does not 
remain free as when simply atrophied, or ar¬ 
rested in its developement. But when the head 
of the perfect insect is smaller than that of the 
larva, as in the Lepidoptera, the extent of the 
pro-thoracic segment is not diminished, unless 
encroached upon from behind by the enlarge¬ 
ment of the meso-thorax. 
The thorax is that region of the body 
which immediately follows the head, and bears 
all the organs of locomotion in the perfect 
insect. It is always composed of three very 
distinct segments, first, the pro-thorax, which 
bears the first pair of legs ; second, the meso- 
thorax, which bears the first pair of wings and 
second pair of legs; and third, the meta¬ 
thorax, which bears the second pair of wings 
and third pair of legs. Besides these seg¬ 
ments, which are analogous to the second, 
third, and fourth in the larva state, there is also 
another, the fifth segment of the larva, which 
enters in part into the composition of the thorax 
of the perfect insect, and forms its connexion 
with the abdominal region. We have already 
alluded to this in our account of the changes of 
the larva, (p. 877, 8,) during which we have 
shewn that at least one segment of the body 
always becomes atrophied, and very frequently 
almost disappears, and that this segment is the 
fifth. But we have not there sufficiently ex¬ 
plained that this segment belongs partly to the 
thoracic and to the abdominal regions, on which 
account we propose to designate it the tho- 
racico-abdominal segment, and consequently the 
number of segments of which the abdomen is 
composed will depend upon whether or not we 
include this in that region. For our own parts 
we prefer to consider it as forming a most dis¬ 
tinct part, for reasons which we shall presently 
explain. Now it has been shewn by M. Au- 
douin, in an admirable and elaborate series of 
investigations, that each segment of the thorax 
is normally composed of four sub-segments, 
which sub-segments or annuli are each formed 
of distinct parts, one upper or dorsal, one lower 
or pectoral, and two lateral. The four annuli 
thus formed are easily demonstrable on the 
upper surface of each thoracic segment, but are 
less readily detected on the pectoral or under 
surface, in consequence of the parts having 
there become confluent, in order to afford a 
greater degree of solidity to the skeleton ; and 
in consequence also of the diminished extent of 
the pectoral as compared with the dorsal sur¬ 
face, which, as before explained, (page 877,) is 
dependent upon the greater extent of change 
that takes place on the pectoral than on the 
dorsal surface during the metamorphoses of the 
insect. The parts capable of demonstration in 
each segment, according to the views of Au- 
douin, are, on the upper or dorsal surface, the 
prœscutum, scutum, scutellum, and post-scutel- 
lum ; on the inferior or pectoral surface a single 
piece, the sternum, and on the lateral two 
pieces, the episternum and epimeron on each 
side ; in addition to which there are also two 
evanescent pieces, which are of considerable 
size in some species, but scarcely distinguish¬ 
able in others. These are the paraptera, por¬ 
tions of the thorax not articulating with the 
sternum, but with the episternum, anterior to 
each wing, and the trochantin, articulating with 
the epimeron and coxa of the leg, the parap¬ 
tera of the pro-thorax being, according to 
Audouin, absent. Hence the number of 
pieces he describes as forming the external 
thorax are ten for the pro-thorax, twelve for the 
meso-thorax, and a like number for the meta¬ 
thorax, making in all thirty-four pieces. These 
are parts capable of being demonstrated, if we 
regard each sternum as formed of two trans¬ 
verse pieces united, and corresponding to the 
episterna and epimera. But as remarked by 
Mr. Macleay,* each sternum at the maximum 
of development ought to be regarded, like the 
dorsal surface of each segment, as composed 
of four transverse sub-segments united longitu¬ 
dinally, and the sides of the same number. If 
then the four portions on the dorsal surface of 
eacn segment, and the sternum on the under, 
be also divided in the median line, the number 
of pieces in the thorax will amount to seventy- 
two. But this number, as Mr. Macleay has 
well remarked, can never appear together in 
any insect, owing to the great extent to which 
some parts are developed, and the consequent 
atrophy of others. At the same time it must 
be observed, that if we adopt this, which ap¬ 
pears to be the correct theoretical mode of con¬ 
sidering the subject, the number of pieces 
which enter into the composition of the thorax 
is in reality greater than that given by M. 
Audouin, who has not described any parts be¬ 
longing to the pro-thorax as analogues of the 
paraptera of the meso- and meta-thorax, but 
which we think may be found in a pair of those 
little detached plates that exist in the articu¬ 
lating membrane between the head and pro¬ 
thorax in Coleoptera, and which have been 
described by Straus Durckheimf as pièces 
jugulaires, and conceived by him to represent 
the remains of two distinct segments, situated 
originally between the head and pro-thorax, but 
which have disappeared during the transforma¬ 
tions. But we are more inclined to consider 
them as detached portions of the pro-thorax 
than as remains of distinct segments, since we 
are totally unaware that any such disappear¬ 
ance of segments ever takes place between the 
head and pro-thorax; the head or first segment 
of every Coleopterous larva being the proper 
representative of the head of the perfect insect ; 
and the second segment of the larva being in 
* Zoological Journ. vol. i. p. 177. 
t Considerations Gén. &c. p. 75.
        

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