OCR Text |
Show 228 INSECTA. number, 'and arc disposed in a triangle on the top of the head, In most of the Aptet·a and in the larvre of tho~c that arc winged, they replace the former, and are frequently united in a group; those of the Arachnides seem to indicate that they are fitted for the purposes of vision. The mouth of hexapodous insects is generally composed of six principal parts, four of which arc lateral, are disposed in pairs, and move transversely; the other two, opposed to each other in a contrary dir·ection, occupy the space comprised be. tween the former: one is placed above the superior pair, an~ the other beneath the inferior. In the triturating insects (bro. ycurs), or those which feed on solid matters, the four lateral parts perform the oflicc of jaws, the other two being consi· der·ed as lips; but, as we have already observed, the two supe· rior jaws have been distinguished by the peculiar appellation of mandibles, the othet·s alone bearing that of maxillre or jaws; the latter are also furnished with one or two articulated fila· ments called palpi, a character never exhibited, in this class, by the mandibles. Their extremity is often terminated by two d.ivisions or lobes, the exterior of which, in the Orthop· t.era, IS call d the galea. We have already said that the upper hp was called the labrum. The other, or the labium pro. perly so styled, is formed of two parts; the on , inferior· and solid, is the mentum or chin; the other, which is usually provided with two pal pi, is the ligula( l ). (1) With r~spect to th~s~ see what is stated in the general remarks which pre· cede the ~artlc.ular exposttwn of each class. The inferior lip appears tout to be a me~·e ~oddicat10n of the second jaws of the Crustacea Decapod:t, combined with thetr lt~ula. The changes gradually effected in these pat·ts in the Crustnce~ At·achl~tdes, a1~d Myriapoda, seem to authorize this idea. According to thishy· p~thests; the stx thoracic legs are analogous to the foot-jaws, a fact already recog· mzed wtth regard to the Crustacea of the genus Jlpus. The five first abdominal segments of the Iltxapocla will then represent those, which, in the Crustacea De· capoda, bear the legs properly so called, or the third and four following pairsof the Amphipoda and Isopoda. All the observations that have been published on t~e thorax of Insects, although othel'\vise useful, will necessat·ily be liable to con· tm~al chang~s, when that part of the body is compared in the th1·ee classesof arttculated antmals provided with aJ•ticulated feet. Tn this respect our nomencla· ture is far from being fixed. JNSBUTA. 229 In the Suctoria, or those that live by the suction of fluid aliment, these various organs of manducation present themselves under two kinds of general modifications. In the first, the mandibles and the jaws are replaced by little laminre in the form of setre or la11cets, forming, by their union, a sort of sucker, which is received into a sheath, supplying the place of a labium, and is either cylindrical or conical, and articulated in the form of a rostrum, or fleshy or membranous, inarticulated, and terminated by two lips constituting a jJ1·oboscis. The labrum is triangular and arched, and covers the base of the uckcr. In the second modification, the labrum and mandibles are nearly obliterated, or are extremely small: the labium is no longer free, and is only distinguishable by the presence of two palpi, to which it gives insertion: the jaws have acquired a most extraordinary length, and are transformed into tubular filaments, which, being united at their edges, compose a sort of spiral proboscis called the tongue, but which, to avoid all equivocation, it would be better to call spirignatha; its interior exhibits three canals, the intermediate of which is the duct of the alimentary juices. At the base of each of these filaments is a palpus, usua11y very small, and bnt slightly apparent. The Myriapoda are the only insects in which the mouth presents another mode of ot·ganization-it will he explained in treating of that order. The trunk(l) of insects, or that intermediate portion of their body which bears the legs, is generally designated by (1) This term, here, is synonymous with that of thorax. ln order to avoid confusion, I think it would be better to rest1·ict the application of the former to the Linnotan Aptcra with more than six legs, :.md whet•e those organs arc bo1·nc by particular segments, that is, where the head is distinct fl'om the ti·unk. With respect to. the Crustacea in which these p:u·ts of the body are confoundeJ, the' thor~x ~ught be called thomcicla; and cephalo-thomx in the Arachnides, animals presentmg the same characte1·, but in which the trunk or thorax is more simple and provided with fewer appendages. The Entomost1·acn, in this respect, approach the la~tei·, but as they belong to another class, the term tlwraci~a should still be applted to them; that of thorax would then be exclusively nppropriated to the Llexapoda. |