98 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 80 



Though the leg of a caterpillar, both in its structure, as compared 

 with the leg of a centipede (fig. 32), and in its musculature, as com- 

 pared with that of a proturan (fig. 41), irresistibly suggests that it 

 is a primitive organ, it must yet be noted that, in the larvae of 

 Neuroptera and Trichoptera, the adults of which certainly stand 

 below the Lepidoptera, the legs have a dicondylic knee joint and two 

 terminal claws. The same apparent phylogenetic discrepancy is to 

 be observed in the Coleoptera, where the larvse of most Adephaga 

 have two claws, and those of other grou])s only one claw. It should 

 be recognized, however, as shown by Berlese (1913), that the larvae 

 of dififerent insects do not necessarily represent equivalent ontogenetic 

 stages, nor, therefore, equivalent phylogenetic stages. The larvae of 

 more generalized adults are likely to have acquired many adult char- 

 acters, while those of more highly specialized adults may be of an 

 earlier ontogenetic stage and, consequently, may retain more primitive 

 characters. 



VI. SUMMARY (EVOLUTION OF THE THORAX) 



A brief review of the principal points elaborated in the preceding 

 discussions may l)e presented in the form of an outline of the probable 

 evolution of insects, since the specializations of the thorax and its 

 appendages constitute the most distinctive characters of insects. 



1. In the primitive, segmented, but soft-bodied ancestors of the 

 arthropods, the limits of the jjody segments coincided with the lines 

 of attachment of the principal dorsal and ventral longitudinal muscles. 

 Modern adult arthropods, however, with solid body wall plates, have 

 developed a secondary segmentation, to allow a forward contraction 

 of the body, through the union of the muscle-bearing ridges of the 

 body wall with the segmental plates following, thus converting the 

 membranous posterior parts of the segments into the functional but 

 secondary intersegmental areas. 



2. In the evolution of insects, as shown by ontogeny, and probably 

 of all arthropods, the earliest dififerentiation in the long, segmented 

 body consisted of the union of the anterior two or three segments 

 to form a primitive head, or procephalon, containing the first three 

 nerve centers condensed into a brain. 



3. The distinctive character of insects began with the development 

 of the thorax as the locomotor center of the body, this being accom- 

 panied by the reduction of the ap]^endages on most of the segments 

 following, and the transformation of those on the three segments im- 

 mediately preceding into gnathal organs, the segments of which later 

 unite with the procephalon to form the definitive head. The newly 



