THE THORAX. 75 



uniform, without further composition, as in all insects deficient in wings, 

 Pediculus, the Mallophaga, Pulex, &c.) In such no distinct parts are to 

 be noticed, they are consequently named only according to their position, 

 the upper part being called the BACK (dorsuni), the under the BREAST 

 (pectus) and the SIDES, where they are distinct or prominent, (pleurae}. 

 As portions of the whole superficies they are distinguished by the name of 

 the segment upon which they are situated ; for example, the upper side of 

 the prothorax is called the BACK OF THE PROTHORAX (dorsum protho- 

 races] . But this most simple construction of the thorax passes through 

 a great variety of conformation in the different orders, whereby the 

 undivided segments, here seen, become separated into parts of which 

 sometimes one, and sometimes the other, is most strongly developed; but 

 the three great divisions are always distinctly determined, although in 

 some cases the second and third, and in others all three, are so closely 

 united that they appear to form but one undivided whole. 



The orders in which we observe the first segment to be most freely 

 united to the second are the Coleoptera, Orthoptera, Ncuroptera, and 

 Hemiptera; the other four display a tolerably close union of all the 

 three segments into one entire undivided thorax. Although this more 

 distinct separation of the prothorax is evidently conditional upon a not 

 unimportant transformation of organic relations, we shall nevertheless 

 observe no new parts in these orders, but be able to show an analogical 

 structure in all the rest. The greater freedom of union does not seem 

 to imply a higher grade of organisation, for we observe the same structure 

 in the apparently highest and lowest orders; but in the higher orders 

 each thoracic segment is composed of several parts, which in the lower 

 ones unite into one, although we even then find the indication of such 

 separations. 



When most fully developed, the thorax consists of four corneous 

 plates. The superior, which we call PRONOTUM*(P1. IX. and XII. A, A, A, 

 Prothorax of Kirby and Spence), takes very different figures. In 

 general it is more or less quadrate, but so that the sides seldom form 

 straight lines, but either bow out or undulate. The anterior margin is 

 generally emarginate, the posterior straighter ; the lateral margins fre- 

 quently dentate, and sometimes armed with strong spines or with smaller 

 teeth. The centre of the superficies very generally exhibits a narrow 

 longitudinal impression, which beneath and within projects as a sharp 



' This name is compounded of irpo, anterior, ;uul voros, the h;ick. 



