Packard.] 



288 



orders, and some forms of Crustacea. After Savlgny had shown that 

 the mouth-parts of Insects and Crustacea were jointed appendages 

 like those attached to the thorax, and therefore repetitions of an ideal 

 jointed limb or appendage, Audouin proved that in the ideal arthro- 

 mere, of which the bodies of all articulata are a congeries, arranged 

 in a longitudinal series, the periphery should be distinguished into an 

 upper, Qergite Duthiers) lower {sternite Duthier) and pleural part ; that 

 in the thorax the legs were thrust out between the pleurite and 

 sternite, and the wings grew out between the pleurite and tergite. 

 The arthro-pleural region is therefore the limb-bearing region of 

 the body, and the different parts of the ideal ring are developed 

 in a degree subordinate to the uses of the limbs and wings. Thus 

 in the walkers, such as the Carabidffi, the pleural and tergal regions 

 are most developed; while in those insects such as the Dragon- 

 flies, which are constantly on the wing, and rarely walk, the pleural 

 re"-ion is enormously developed, and the tergites and sternites attain 

 to their minimum development. The muscles used in flight are 

 greatly increased in size over the atrophied muscles brought into requi- 

 sition by the act of walking. In the Hymenoptera, however, which 

 are both walkers and fliers, the three portions of the ring are most 

 equally developed. 



These parts of the arthromere are simplest in the abdomen ; and 

 become more diSerentiated in the thorax, where the numerous pieces 

 composing them have been classified and named mostly by Audouin, 

 McLeay, and Lacaze-Duthiers. Scarcely an attempt has been made 

 to trace these parts in the rings of the head by those who have pro- 

 posed theories of the number of arthromeres in the head of insects. 



As we can understand the structure of the thorax better after study- 

 ing the abdomen, so we can only homologize the different head pieces 

 after a careful study of the thorax of insects, and the cephalothorax 

 of Crustacea ; which thus afford us a standard of comparison. 



Since the arthropleural is the limb-bearing region in the thorax, it 

 must follow that this region is largely developed in the head, to the 

 bulk of which the sensory and appended digestive organs bear so large 

 a proportion, and as all the parts of the head are subordinated in 

 their development to that of the appendages of which they form the 

 support, it must follow logically that the larger portion of the body of 

 the head is pleural, and that the tergal, and especially the sternal, 

 parts are either very slightly developed, or wholly obsolescent. Such 

 we find to be the fact. As to the number of rings composing the 

 head, it is evident that it is correlated with the number of appendag'^is 

 they are to support. Hence, as in the thorax there are three rings, 

 bearing three pairs of appendages or legs, it follows that in the head 

 where there are seven pairs of appendages, there must be seven rings. 



