474 ArxTICULATED ANDIALS. 



ciilar trachea : this, linwever, as well as the choroid, is wanting in various darkling 

 insects; 3rd, of nerves, wliich arise from a hirge trunl-; proceeding immediately from 

 the hrain, wliich there dilates in a reversed conical form, the broad base being towards 

 the cornea, and of which the threads, running through the choroid and inner jilaster of 

 the cornea, terminate separately in each of the facets. There is no crystalline nor 

 vitreous tumour. 



Miiny insects have, in addition to these composite eyes, simple eyes [o'r'/A'], the 

 cornea of which is smooth. Tlicy are generally three in number, and arranged in 

 a triangle upon the crown of the head. In the majority of apterous insects, and 

 the larva? of those which gain \^■ings, the ocelli replace the eyes, and arc often in- 

 serted in a [;'roup ; judging from the eyes of the Arachnida, they are evidently fitted 

 for vision. 



The mouth of Hexapod insects is in general composed of six principal pieces, their 

 form being lateral, arranged in pairs, and rnostlv transversely ; and two others, opposed 

 to each other in a ditection contrary to tliat of the preceding, filling up the space be- 

 tween the former : one is situated above the upper pair, and the other below the lower 

 pair. In the masticating insects, or those which feed upon solid materials, the four 

 lateral pieces perform tlie otfice of jaws (nidchoires), and the two others are considered 

 as lips ; but, as we h;ive already observed, the two u])per jaws have been distinguished 

 by the particular name of mandibles, whilst the two others have alone retained the 

 name of maxilke (mijc/wires) : the latter are also pro\'ided with one or two articulated 

 filaments wdiieh are caWed palpi, — a character which is never possessed, in this class, by 

 the mandddes. The extremity of the maxilke is often terminated by two divisions, or 

 lobes, of wdiich the outer, in the Orthoptera, is termed the galea. AVe have already' 

 said that the upper hp is called the lahnim. The other lip, or the lalj'unn (levre, pro- 

 ])erly so called), is formed of two parts: the one, solid and inferior, is the mentum ; 

 the upper, which often bears two palpi, is the tonguelet (langiieite), [or %«/«].* 



In the suctorial insects, or those which derive their food from fiuid aliments, these 

 difi'erent organs of manducation appear under two general modifications. In the 

 first, the mandd)les and maxiUre are replaced by small, setaceous, lancet-like plates, 

 forming, by their union, a kind of sucker, which is received in a sheath which takes 

 tlic place of tlie kdiium, and is either cylindrical or conical, and articulated, in the 

 form of a beak (roslraiii), or membranous and fleshy, inarticulated, and terminated 

 by two lips {proboscis). The labrum is triangular and arched, covering the base of 

 the sucker. 



In the second of these modifications, the laljrum and mandibles are nearly obsolete, 

 or extremely small. The labium is no longer a detached piece, and is only distin- 

 guished by the pjresence of a pair of palpi, of which it is the sup]iort. The maxilla' 

 have acquired a very great length, and are transformed into two tubular threads, which, 

 uniting by tlie edges, forms a kind of proboscis wdiich is rolled up in a spiral manner, 

 and is named the tongue, but which, to avoid misconception, it would be preferable to 

 term the spirignatha : its interior presents three canals, of which the middle one forms 



* According to wUnc I ln\vo snid in the iiitroduclyry oliscirations 

 upon the Articiilfit.-i ill b't'iierHl. IcciiiKifier the lower lip to he hut a modi, 

 riiniioii of the .second maxilla of the deeapod Crustncea, comhioed 

 Willi tlicir tnoKue (hlIl^,luctte). Tlie ifraduiil ehaiii^es ivliieh take pKee 

 ill liie form of tliese orRiinJi, in tlie Cruataccii, Amchnidn. and IMyria. 

 liiida, oiiturally lead to tliia supposition. In this hypothesis, the si\ 

 Ihoraeic lctf> must he aiialoitous to the foot-jaws of ttie Crahs; and as 

 lias hcen shown, in the erustaccous genus Apus. Moreover, the live 



anterior alidoniinal scf,^mcnts of hexapod ioseets will represent ihc 

 segments whieh bear the true legs in the decapod Crustacea, or ttie 

 third and four succeeding segments of the aniphipod and isopod 

 Crustacea. The various works puhlishcd in respect to the thorax of 

 insects will necessarily require revision when this p.art of the hody is 

 comjiared throughout the three aiinulose classes, its nomenclature 

 heing far from lised in this resjiect. 



