ARTICULATED ANIMALS. 
472 
tracheae, extending parallel to each other throughout the entire length of the Dody, 
having, at intervals, points from whence numerous branches extend, and which corre- 
spond with certain external orifices, or stigmata* [or, as they have been termed in a 
previous passage, spiracles] , for the entry of the air. All of them have two antennae, 
and the head distinct. The nervous system of the majority of insects (those with six f 
feet) is generally composed of a brain, formed of two ganglions opposed to each other, 
united at their base, and emitting eight pairs of nerves and two single nerves, and of 
twelve ganglions t, all of which are in the inferior part of the body. The two anterior 
are situated near the union of the head and thorax ; the second and two following are 
appropriated to the three segments of which the thorax is composed, and the remaining 
ganglions belong to the abdomen, each ganglion emitting nerves to the organs of their 
respective segments. The two nervous cords which form, by their reunion, the ganglions, 
are tubular, and composed of two tunics, the exterior of which exhibits tracheae. A 
medullary substance fills the central canal. The fine work of M. Herold upon the 
anatomy of the caterpillar of the Great Garden White Butterfly, examined during its 
growth, and until the period of its transformation into the pupa, proves that the nervous 
system and the digestive organs undergo decided modifications, the nervous cords being 
at first longer and wider apart, which confirms the opinion of De Serres upon the origin 
and developement of the nervous system. We have already, in the general observations 
on the three classes of articulated-legged Articulata, stated the dilferent sentiments of 
physiologists upon the seat of the senses of hearing and smell : we shall therefore 
merely add, that, in respect to the former, the small nervous ganglions situated upon 
the forehead, of which we have spoken, appear to confirm the opinion of those who, 
like Scarpa, place this sense near the base of the antennae. In some Lepidoptera, I 
have detected two small apertures near the eyes, which may perhaps be the auditory 
channels. If, in many insects, especially those with filiform or setaceous and long an- 
tennae, these organs are used as tactors, it appears difiicult for us to account for their 
extraordinary developement in certain families, and more particularly in males, if w'e | 
do not admit that they are actually the seat of the organ of smell. Probably, also, as 
regards the taste, the palpi, in those cases 
the heart, or the abdominal portion of this organ, is divided, inter- 
nally, into eight chambers in the Cockchafer, separated from each 
other by two convergent valves, which permit the blood to be pro- 
pelled forwards, but prevent its returning. The definition given by 
this naturalist of the dorsal vessel, whatever may be the interior com- 
position of this organ, evidently proves that it is not a real heart : 
moreover, his observations do not determine the nature of this fluid, 
nor how it is directed into the other parts of the body, to effect their 
nutrition. [The still more recent observations of Cams, Bowerbank, 
and some others, have made us still better acquainted with the nature 
of this dorsal vessel, and its uses, confirming the views of Herold as to 
the existence of a decided circulation in insects, although it is of a 
nature very dissimilar to that of the higher animals]. 
* The number of the segments of the body of the Myriapoda being 
variable, that of their spiracles is so likewise, and extends sometimes 
to more than twenty. In hexapod insects, it is often eighteen,— nine 
on each side. This is, however, more the case with the larva than the 
perfect insect. Caterpillars, and most other larvae, have a pair of spi- 
racles in the segment which bears the first pair of feet. The second 
and third segments are deficient, because, as I presume, the develope- 
ment of wings upon these segments renders the presence of spiracles 
unnecessary. Each of the fourtli and seven following segments ex- 
hibits a pair ; but in the perfect Beetles, in addition to the tw-o anterior 
spiracles which are hidden in the cavity of the prothorax or corslet, 
and which have not been noticed, two others are to be perceived, situ- 
ated between the base of the elytra and wings, being those of the 
mesothorax; but there are none to the metathorax, unless we consider 
those of the first abdominal segment as supplementary to the thorax, 
relying upon what takes place in the pedunculated Hvmenoptera and 
Diptera, where these two segments, together with the demi-scgmcnt 
where they are very dilated at the tip, take 
to which they belong, form part of the thorax. Thus, in general, all 
hexapod insects have eight pairs of spiracles to the abdomen, the two 
last being often obsolete. In the Locusts and Dragon-flies, the sides 
of the mesothorax exhibit a pair of spiracles (triniah-cs,yL. Serres). 
In these and some other insects with uncovered wings, the two first 
thoracic spiracles are placed above, between the pro- and meso-thorax. 
Except in Libeliula, the true thorax does not exhibit any other spira- 
cles. I say the true thorax, because, in some, the trvo anterior abdo 
minal spiracles are transferred to the thorax. The metathorax of the 
Pentatom® and Scuteller® exhibits, on its under side, a pair of spira- 
cles. In the wingless spectre insects [Phasmidm], the mesothorax 
has none, but the metathorax has two pairs. 
[We thus see that Latreille was perfectly aware that each of the 
three thoracic segments was occasionally provided with spiracles ; 
and yet his theory, which has been noticed above (that the hind part 
of the thorax of the petiolated Hymenoptera and Diptera is abdo- 
minal), is founded upon the supposition that the metathorax cannot, 
of itself, possess spiracles, and that consequently the spiracles which 
we see on the hind part of the thorax of those insects, must be those of 
the anterior abdominal segment, transferred to the thorax. The common 
Earwig, as I have shown in a memoir upon the anatomy of that insect, 
published in the Transactions of the Entomological Society, exhibits 
an instance in which the pro-, meso-, and metathorax, are respectively 
furnished with a pair of spiracles. I have also entered fully into this 
question in the Hymenopterous portion of my introduction to the H 
modern classification of insects, with a view to prove the general uni- 
formity of the structure of tlie Hymenoptera with sessile and petio- 
lated abdomens.] 
■\ Some lamellicorn Beetles, in the perfect state, are excentions. 
