STRUCTURE AND CLASSIFICATION. 



23 



CHAPTER III. 



THORAX AND ABDOMEN. 



The thorax, or middle region of the body, is, as has been 

 already stated, composed of three rings, termed in their order 

 the pro-, meso-, and metathorax ; meaning fore, middle, and 

 hind thorax. These rings vary greatly in their proportion to 

 each other : it is rare that they are equally well developed, and 

 whenever this is the case it indicates a low type of insect, — that 

 is to say, one very little specialized. Usually they are grown 

 together to some extent, and this tendency continues until we 

 find in the flies and bees the entire thoracic region apparently 

 composed of one solid mass, — the rings being not at all movable 

 upon each other. Insects of this type are highest in the scale 

 and the most speciaHzed. In other orders, notably the Coleop- 

 tera, or beetles, the prothorax only is free and well developed, 

 the two other rings being united together and not movable one 

 upon the other. This forms a sort of intermediate type, and the 

 rank of insects is determined, to some extent, by the development 

 of this region of the body. 



If the thorax of a grasshopper, a bettle, or a wasp be rather 

 closely examined, it will be found that it is made up of a large 

 number of little pieces, divided by impressed lines, and sometimes 

 these pieces are movable one upon the other. They are called 

 sclerites, and a sclerite, whenever that term is used, always 

 means a solid piece bounded by impressed lines, or sutures^ as 

 they are termed. Perhaps ' ' seams ' ' better represents the actual 

 case than "lines," because they mark the places where pieces 

 have been joined together, even if the joint be now a solid one. 

 Though these pieces seem confused at first sight, yet there is a 

 distinct plan in their arrangement, and, while it is not necessary 

 to go much into detail, it is well to know at least the regions 

 of the segments and the terms applied to them. In Fig. 11 

 will be found an illustration of the under side of a beetle, in 

 which all the sclerites of that part of the body are named. Those 

 who wish a more accurate knowledge can compare a series of 



