474 



ARTICULATED ANIMALS. 



cular trachea : this, however, as well as the choroid, is wanting in various darkling 

 insects ; 3rd, of nerves, which arise from a large trunk proceeding immediately from 

 the brain, which 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 plaster of 

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

 vitreous tumour. 



Many insects have, in addition to these composite eyes, simple eyes [ocelli], the 

 cornea of which is smooth. They 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 wings, the ocelli replace the eyes, and are often in- 

 serted in a group : 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 mostly transversely ; and two others, opposed 

 to each other in a direction contrary to that 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 the office of jaws (mdchoires), and the two others are considered 

 as lips ; but, as we have already observed, the two upper jaws have been distinguished 

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

 name of maxilla? (mdchoires) : the latter are also provided with one or two articulated 

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

 the mandibles. The extremity of the maxilla? is often terminated by two divisions, or 

 lobes, of which the outer, in the Orthoptera, is termed the galea. We have already 

 said that the upper lip is called the labrutn. The other lip, or the labium (levre, pro- 

 perly 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 (langvette), [or ligula].* 



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

 different organs of manducation appear under two general modifications. In the 

 first, the mandibles and maxilla? are replaced by small, setaceous, lancet-like plates, 

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

 the place of the labium, and is either cylindrical or conical, and articulated, in the 

 form of a beak (rostrum), 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 labrum and mandibles are nearly obsolete, 

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

 guished by the presence of a pair of palpi, of which it is the support. The maxilla? 

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

 uniting by the edges, forms a kind of proboscis which 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 what I have said in the introductory observations 

 upon the Articulata in general, I consider the lower lip to be but a modi- 

 6cation of the second maxillae of the decapod Crustacea, combined 

 with their tongue (languette). The gradual changes which take place 

 in the form of these organs, in the Crustacea, Arachnida, and Myria- 

 poda, naturally lead to this supposition. In this hypothesis, the six 

 thoracic legs must be analogous to the foot-jaws of the Crabs; and as 

 has been shown, in the crustaceous genus Apus. Moreover, the five 



anterior abdominal segments of hexapod insects will represent ihe 

 segments which bear the true legs in the decapod Crustacea, or the 

 third and four succeeding segments of the amphipod and isopod 

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

 insects will necessarily require revision when this part of the body is 

 compared throughout the three auuulose classes, its nomenclature 

 being far from fixed in this respect. 



