472 



ARTICULATED ANIMALS. 



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 

 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 f, 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 different 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 difficult for us to account for their 

 extraordinary developement in certain families, and more particularly in males, if we 

 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 where they are very dilated at the tip, take 



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, 

 uor how it is directed into the other parts of the body, to effect their 

 nutrition. [The still more recent observations of Carus, 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 

 Tariable, 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 fourth and seven following segments ex- 

 hibits a pair ; but in the perfect Beetles, in addition to the two 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 Hvmcnoptcra and 

 Diptera, where these two segments, together with the demi-segment 



to which they belong, form part of the thorax. Thus, in genera.., 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 (trtmaeres, M. 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 Libellula, the true thorax does not exhibit any other spira- 

 cles. I say the true thorax, because, in some, the two anterior abdo 

 minal spiracles are transferred to the thorax. The metathorax of the 

 Pentatomee and Scutellerx exhibits, on its under side, a pair of spira- 

 cles. In the wingless spectre insects [Phasmidae], 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 <>f 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 

 modern classification of insects, with a view to prove the general uni- 

 formity of the structure of the Hymenoptera with sessile and petio- 

 lated abdomens.] 



t Some lamellicorn Beetles, in the perfect state, are exceptions. 



