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/lit25760/952/
944 
INSECTA. 
diverge a little from each other, and include 
between them the insertions of the first set of 
diagonal muscles, and at the posterior part 
of the third segment again approach each 
other, and form the third ganglion (3). 
Fig. 406. 
They then again diverge, and continuing their 
course into the next segment, pass on each side 
of the insertion of a second set of muscles, and 
approaching at the hinder part of the fourth 
segment form the fourth ganglion, (4,) from 
which they continue their course side by side, 
and in the next segment form the fifth and last 
ganglion, (5,) that enters into the composition 
of the thoracic portion of the nervous cord in 
the perfect insect. From the fifth ganglion the 
cords are continued in a direct line, into the 
sixth, seventh, and succeeding segments, form¬ 
ing in each a double ganglion, to the eleventh, 
where they form the terminal ganglion (11,12). 
This is considerably larger than any of the pre¬ 
ceding, being in reality composed of two dis¬ 
tinct pairs, which originally were separated 
from each other by intervening cords, and be¬ 
longed to the eleventh and twelfth segments, 
but which seem to approach and become closely 
approximated to each other during the earlier 
period of the larva state, as suggested by Dr. 
Grant, and supported by the fact that these 
ganglia are found more or less approximated 
together in different individuals, the terminal 
ganglion in some being distinctly formed of two 
pairs, scarcely united, and in others so com¬ 
pletely coalesced as hardly to be distinguished. 
In other Lepidoptera, as in Odonestis potatoria 
and Lasiocampa neustria, and as represented 
also by Lyonet, in Cossus ligniperda,* the 
ganglia continue distinct, and are separated by 
a very short portion of the cords. But in the 
Timarcha, the eleventh and twelfth ganglia 
have completely coalesced, and it is remark¬ 
able that they are also united even in the 
rudimentary form of the nervous system in the 
aculeate Hymenoptera, as in the larva of the 
bee, and even in its still more rudimentary state 
in the larva of Ichneumon Atropos, in both 
which there are originally thirteen distinct pairs 
of ganglia, including the supra-cesophageal ones, 
although Burmeister has imagined that the 
apodal larvae of Hymenoptera have a nervous 
system without ganglia,f similar to what he has 
observed and figured in the larvæ of some Dip- 
tera. It was shown by Swammerdam, | that in 
the Lamellicornes, as in Oryctes nasicornis, 
the cords are united laterally, and do not extend 
beyond the fourth segment, from whence the 
nerves radiate into the abdomen. In Dyticus, 
according to Burmeister, § the two approxi¬ 
mated cords are very short, and the pairs of 
ganglia are contiguous to each other, and he 
has found a similar form of the nervous system 
in the Hog-beetles, Calandra sommeri, in which 
there are twelve pairs of closely approximated 
subœsophageal ganglia. The supra-cesophageal 
pair in this species are distinct, as in Timarcha, 
but each pair of the subœsophageal has co¬ 
alesced into a single mass, as in that insect, and 
the whole do not extend beyond the fifth seg¬ 
ment, from whence the nerves radiate into the 
abdomen, as in the Lamellicornes. These are 
the conditions which the double cord presents 
in the different classes. In describing the 
nerves that proceed from it, we shall divide them 
into those of the head, the thorax, the abdomen, 
and the organic functions. 
* Plate ix. 
t Manual, (translat.) p. 279. 
X Biblia Natura, Tab. xxviii. fig. 1. 
§ Op. cit.
        

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