INSECT. 



855 



to these two kinds of organs. We will, therefore, 

 only observe, that much still remains to be done, be- 

 fore we arrive at a perfect knowledge of the structure 

 of the various organs of the mouth throughout the 

 insect tribes. Savigny has effected much in clearing 

 away the obscurity in which the subject had long re- 

 mained ; but there is still an ample field for the em- 

 ployment of a steady eye and hand, in dissecting, and 

 especially in delineating the many modifications of 

 form, to which the parts of the mouth have been so 

 fully shown to be subject. Until this have been done 

 in the most Careful manner, it will not be easy to 

 arrive at a true knowledge of the mode in which 

 these modifications of form are effected, and which 

 alone will enable us satisfactorily to trace the analo- 

 gies of the various parts throughout the various 

 orders. 



B. The Thorax, or scat of the organs of Locomotion. 

 This portion of the body lies between the head and 

 the abdomen, and supports the three pairs of legs ; 

 and the two or four wings with which perfect insects 

 are in almost every case furnished. 



In the early efforts made by scientific entomologists, 

 to determine the various parts of the body of insects, 

 too much attention was paid to an organ when it hap- 

 pened to be fully developed, too little to the mode 

 in which this development was effected. Thus it 

 sometimes happened, that the same organ received 

 various names, according to its extent of develop- 

 ment ; and it was not until a rigid attention was paid 

 to the various forms which the same part exhibited in 

 its various modifications, that a fixed nomenclature 

 could possibly be applied. This has been especially 

 the case with the thoracic segments of insects ; and it 

 is only within a very few years that entomologists 

 have given any extended attention to this part of the 

 subject, or have endeavoured to introduce a fixed 

 series of names. Audouin, Kirby and Spence, Mac 

 Leay, and Burmeister, have especially laboured in 

 this field, in which much still remains to be effected. 

 Linnaeus thus described this part of the body which 

 he called the trunk, " TRUNCUS, inter caputet abdo- 

 men, pedatus tkorace, supra dorso, postia scutello, 

 subtus pcclore sternoque?' In his descriptions, how- 

 ever, he applies the term thorax either to the large 

 shield which covers the first thoracic segment in bee- 

 tles, or to the entire trunk, as in the Hymenoptera. 

 This mode of description was, notwithstanding its 

 evident impropriety, and want of precision, adopted 

 by most entomologists. Illiger endeavoured to cor- 

 rect this nomenclature, by giving to the trunk of Lin- 

 naeus the term thorax, designating its upper part, 

 thorax superior, and its lower surface, thorax inferior. 

 Latreille, and others, divided this part of the body 

 into three distinct sections, the two posterior of which, 

 from bearing the wings, Chabrier united under the name 

 Irunc alifcrc, which Kirby and Spence adopted, 

 naming these two segments alitrunk ; and the first seg- 

 ment, which bears the fore legs (which these authors 

 rather regard as arms or hands), manitrunk. This 

 nomenclature has not, however, been adopted, al- 

 though Strauss-Durckheim adopts this division of the 

 thorax, calling the first segment corselet, and the two 

 others the thorax. Most of these authors, however, 

 regard the three segments following the head, as suffi- 

 ciently separate to require separate names. And the 

 terms pruthorax, mesothorax, and metathorax, origi- 

 nally proposed by Nitzsch, have been applied to them ; 

 indeed Audouin, Mac Leay, and Burmeister, regard 



them as segments of equal rank, but as conjointly 

 uniting to form the thorax ; whilst Kirby and Spence 

 employ these terms for the upper surface only of the 

 thoracic segments, giving to their under surfaces the 

 names of antepectus, medipectus, and postpectus. 

 MacLeay applies the term tergum to the upper 

 surface, and pectus to the lower ; thus we have 

 "tergum of the prothorax, pectus of the mesothorax, 

 &<" Burmeister, with more uniformity and philoso- 

 phic acumen, gives the following series of names : 



Thoracic Segment 



1. Prothorax. 



2. Mesothorax. 

 J. Metathorax. 



Upper Surface. 

 Pronotum (T 1). 

 Mesonotum (T 2). 

 Metanotum (.T 3). 



Under Surface. 

 Prosternum (T 1). 

 Mesostemnm (T2). 

 Metastenmin(T3). 



This being the most simple system of nomenclature 

 hitherto proposed for these thoracic segments, we 

 shall adopt it, leaving it optional for our readers to 

 regard the segments, either as entirely distinct, or as 

 conjointly constituting the thorax, or as divisible into 

 two portions, corresponding with the manitrunk and 

 alitrunk of Kirby and Spence. We will only observe 

 that the blunders of unphilosophical describers of in- 

 sects who contented themselves, in many cases, with 

 no more anatomical knowledge than would enable 

 them to distinguish one species from another, and 

 who have, consequently, employed names without 

 knowing or caring for their precise meaning can be 

 no ground for disputing the existence of organs, still 

 less for asserting, as has inconsiderately been done, that 

 the researches of such men as Lyonnet, Leon Dut'our, 

 Chabrier, Herold, Strauss-Durckheim, Savigny, 

 Audouin, and MacLeay, "tend to illustrate a 

 theory, in itself evidently false, rather than to find 

 out and establish plain and solid truths." 



The complicated machinery requisite for the due 

 performance of the two chief kinds of insect locomo- 

 tion, namely, leg-movements, including creeping, run- 

 ning, swimming, climbing, as well as prehension ; and 

 wing-movements or flying, and likewise the great 

 volume required by the organs of motion themselves, 

 have necessarily produced a great increase in the 

 iize of the three segments of the body forming the 

 thorax, which in the larva state were but equal in 

 size to the remaining segments ; hence we find that 

 the thorax in the perfect insect has become the most 

 robust yet compact, as well as the most complicated 

 in its structure, of all the body segments of an insect. 

 Moreover, the great diversity in the organisation of 

 the wings, and the occasional transfer of wing motion 

 to a single pair of wings, this pair being either 

 the anterior, as in the Diptera, or the posterior, as in 

 the beetles, have necessitated a concurrent modifica- 

 tion in the form of the thoracic segments according 

 thereto, as well as similar modifications resulting 

 from the varying motions of the legs. 



It is further to be noticed, that the many parts of 

 which the thorax is thus composed, were provided by 

 entomologists with separate names, whereby the no- 

 menclature of this part of the body has become very 

 complicated, more especially as the same part has 

 received more than a single name, according to the 

 greater or less degree of its development. This has 

 I been more particularly the case in the " Introduction 

 to Entomology," wherein, although Messrs. Kirby 

 and Spence do not describe much more than twenty 

 of its distinct component parts, about forty different 

 words are used for them in the nomenclature of the 

 thorax. The chapter on Orismology is nevertheless, 

 as Mr. MacLeay observes in the Zoological Journal, 



