OCR Text |
Show 224 INSECTA. mit air. They all have two antennm and a distinct head. The nervous system of most Insects--the Hexapoda-is gene· rally composed of a brain formed of two opposing ganglions united at base, giving off eight pairs of nerves and two sing!~ ones, and of twelve ganglions(l ), all inferior. The two first are situated near the junction of the head with the thorax and are longitudinally contiguous; the anterior sends nerve~ to the lower lip and adjacent part'S; the second, third and fourth belong to each of the three first segments, or those which form the thorax in the Hexapoda; the remaining gan. glions belong to the abdomen, so that the last or the twelfth corresponds to its seventh ring, and is immediately followed by those which compose the organs of generation; each of these ganglions transmits nerves to the parts of its respective segments. The two last, which are closely approximated, also send some to the terminal annuli of the body. The frontal region exhibits three particular ganglions called frontal by L yo net, from the first of which arises posteriorly a great nerve with enlargements, the longest of all, that he denomi. nates the recurrent. The first ordinary or sub-resophagean of the elytra and that of the wings: they belong to the mesothora:i:. There are none in the metathorax, unless we consider the two of ·the first abdominal seg· ment, as supplementary to the thorax, a consideration founded on what occurs in the Diptcra and Hymenopterous Insects with a pediculated abdomen, where these two stigmata, with the semi.segment in which they are placed, make part of th_e thot~x. Thus, generally speaking, the hexapoda have eight pairs of ubdommal st1gmata, the two last of which, however, are frequently obliterated. _In Acrydium, Truxalis, and Libellula, each side of the mesothornx presentu strgma, or those which Marcel de SctTes calls tremaeres. In these latter Insects, as well as in others with naked wings, or without elytra, the two first thoracic stig· ma.ta arc placed above, between the prothornx and the mesothorax. With the exception of the Libellulre, the thorax proper offers no other distinct stigmata-! say th.orax proper, because, as we have already observed, the two first of the abdo· men, m several, are referable to the posterior extremity of the thorax. The me· ta.thorax of the Pentatomre, and Scutcllerre is provided inferiorly with a pair of strgmata. In the apterous Spectra, there is none in the second segment or mcsot~ lOrax; .but in .the :allowing one, or the meta.thorax, there are two pairs, one ante· rior, wluch bemg sttuatcd ncar the articulation of this segment with the preceding, may be considered as belonging to the latter, and the other smaller, and placed close to that of the first abdominal segment. (1) Several of the Lamellico1·nes in a perfect state form exceptions. INSECTA. 225 ganglion, gives ofT~ according to him, four pairs of nerves, and h of the following ones, two; so that by counting the eight ea~rs of the brain, and the ten spinal bridles, which may also pm . • be considered as so many pmrs of nerves, we shall have m all forty-five pairs, exclusive of the two solitary nerves above. mentioned, or from twelve to fourteen more than are found in the human subject. The two nervous cords which form the ganglions by their union, are tubular and composed of two tunicks, i~ the exterior of which we observe trachere; a medullary substance fills the central canal. The admit·able work of.\f. Herold on the anatomy of the larva of the great Papilio brassicm, L., studied throughout its various degrees of development, and to the period of its transfot·mation into a chrysalis, shows us that the nervous system and that of the digestive organs experience remarkable changes; that in the beginning, the nervous cords are longer and further apart, an observation which strengthens the opinion of one of the greatest zootomists of the age, Doctor Serres, on the origin and development of the nervous system. In our general remarks on points common to the three classes of articulated animals provided with articulated feet, we mentioned the various opinions of physiologists with respect to the· seat of the sense of hearing and of smell. We will merely add, in regard to the former, that the little nervous frontal gang lions of which we have spoken, seem to confirm the opinion of those who, like Scarpa, place it in the origin of the antennre. I have detected two small orifices near the eyes of certain Lepidoptera, which, perhaps, are auditoJ·y canals. If, in several Insects, particularly those furnished with filiform, or long, setaceous antennre, they (the antennre) are organs of touch, it seems to us difficult to account for the extraordinary development they acquire in certain families, and more particularly in the males, if we refuse to admit that they are then the seat of smell. The palpi also, in some cases, as when they are greatly dilated at the extremity, may possibly be the principal organs of smell, part of which sense may also perhaps belong to the ligula. The digestive system consists of a preparatory or buccal VoL. III.-2 D |