472 



ARTICULATED ANIMALS. 



trachea?, 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 antennce, 

 and the head distinct. The nerv^ous system of the majority of insects (thoi^e with six 

 feet) is generally composed of a brain, formed of two ganglions opposed to each other, 

 united at their base, and emitting eiglit pairs of nerves and two single nerves, and of 

 twelve 2;anglionst, cdl 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 traehecC. A 

 medullary substance tills the central canal. Thu fine work of I\I. Herokl 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 pu}ia, proves that the nen'ous 

 system and the cHgestive organs undergo decided modifications, the nen'ous cords being 

 at first longer and wdder apart, which confirms the opinion of De Serres upon the origin 

 and developement of the nervous system. We have ah'cady, in the general observations 

 on the three classes of articulated-legged Articulata, stated the diflerent sentiments of 

 physiologists upon the seat of the sensus of hearing and smell : wc shidl 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 antennpe. In some Lepidoptera, I 

 have detected two small apertures near the eyes, which may perhaps be the auditory 

 channels. If, in manj^ insects, especially those with filiform or setaceous and long an- 

 tenna?, 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 orgao, is diviilcd, inier- 

 unlly, into i:ight chambers in the Cockclitifcr, separated frum each 

 Other by two eonver^etit valves, which permit tlic hlood to be pro- 

 pelled forwards, but prevent its returnin.^. Tlie definition given by 

 this naturalist of the dorsal vessel, wlmtever may be the interior com- 

 position of this organ, evidently proves that it is not a real heart! 

 moreover, his obscrVBtiuna do not determine the nature of this fluid, 

 nor hoiv it is directed into the other parts of the body, to effeet their 

 nutrition. [The still more recent observations of Caraa, Bowerbank, 

 and some othero, have made us still better acquainted with the nature 

 of this dorsiil vessel, and its uses, confirming the views of Herold ag lo 

 Ae existence of a decided circulation in insects, although it is of a 

 D;iturc very dissimilar to ttiat of the hi){hcr oniinalsj. 



• The uumber of the segments of the body of the Ulyriopoda being 

 Tariable, tliat of their spiracles ia so likewise, and extends sometimes 

 to mure than twenty. In hexapod insects, it is often eighteen, — nine 

 on GHch side. This is, however, more the case with tlie larva than tJie 

 perfect insect. Caterpillars, and most other larva;, have a pair of spi- 

 racles in the segment which be.irs tbe first pair of feet. The second 

 and third segments are deficient, because, as 1 presume, the develope- 

 ment of wings upon tliese segments renders the presence of si)iracles 

 onneceasary. Each of the fourth and seven following segments ex- 

 hibits a pair; but in the pL-rfecI Beetles, in addition to the two anterior 

 Bplracles which are hidden in the cavity of the prothorox or corslet, 

 and which have nut been noticed, two otliers are to be perceived, situ- 

 ated between the base of the elytra and wings, being those of the 

 nicfiothnrax; but there are none tu the metathorax, unless we consider 

 those of the first abdominal segment as supplementary to the thorax, 

 relying upon what takes place in the pedunculated I-Ivnienojitera and 

 Dlptera, where these two segments, together with the demi-scijmeiU 



to which they belong, form part of the thorns. Thus, in genera., all 

 hexapod insects have eight pairs of spiraclt's to the abdomen, the two 

 last being often obsolete. In the Locusts and Dragon-flies, the sides 

 of the mcaothorax exhibit a pair of spiracles {Iri'nain's, M. Serres), 

 In these and some other insects with uncovered wings, the two first 

 thoracic spiracles are placed above, between the pro- and meso-lh')rris. 

 Kxecpt in Libellula, tbe true thorax does not e.xhibit any other spira- 

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

 minal spiracles are transferred to the thurax. The melalliorax of the 

 Pentalomie and KcutellerEE exhibits, on its ander side, a pair of spira- 

 cles. In the wingless spectre insects [Phasmid^], the mcsothorax 

 has none, but tlie metatiionix has two pairs. 



[We thus see that Latreillc was perfectly aware that each of the 

 three thoracic segments was occasionally provided witli spiracles; 

 and yet his theory, wliicJi has been noticed above (that the hind part 

 of the thora.x of the pctiolated Hymenoptera and Diptera is abdo- 

 minal), is founded upon the supposition that the metathorax cannot, 

 of itself, possess spiracles, and that conserjuently the spimcles which 

 we -tee on the hind part of tlie thorax of those insects, must be those of 

 the anterior abdominal segment, transferred to tlie thorax. The common 

 Earwig, as I have shown in a memoir upon the analimiy of that insect, 

 published in the Trmiinctioiis n/ l/ie Entomoluf^'irjil Sorieli/, nxhihila 

 an instance in which the pro-, nieso-. 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 cbiHsifieatioii of insects, wiili n vi^w tu |iro\e the general uni- 

 formity of the structure of tlie ll;jin.iii.|ai rii uitli sessile ;iud petii>. 

 lated abdomens,] 



t Some lumellicurii Beetles, in tin; iierlect state, (ire c.vceplion* 



