NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS 33 



in the thorax of the roach as representative of the primitive structure 

 of the thoracic sterna in insects. 



A search for a generaHzed thoracic sternum among all groups of 

 insects, the writer believes, would reveal nowhere the thing sought for, 

 because it does not exist. A study of the arthropods as a whole, how- 

 ever, suggests that the original area or areas of sclerotization in the 

 ventral region of each segment spread into a continuous plate between 

 the leg bases. The thoracic sterna of insects have been variously modi- 

 fied by the development of apodemal braces where rigidity is demanded, 

 and by secondary divisions or by reductions in the areas of sclerotiza- 

 tion where flexibility is important. This theory recommends itself by 

 the fact that it permits all kinds of specific stiiictures and sclerotic 

 patterns to arise, and does not assume that homologies must exist 

 where none can be established. 



II. THE THORACIC SKELETON OF DISSOSTEIRA 



The Carolina locust, Dissosteira Carolina, is here used as the subject 

 for a special study of the thorax and its mechanisms because it is an 

 insect sufficiently large for work on internal structure and is readily 

 obtained, and because its muscular system is simple and comparatively 

 easy to dissect. The thorax of the Acrididae is by no means general- 

 ized, but for this reason it ofifers a good test for the application of gen- 

 eral principles to the solution of specific problems. The structural 

 features of the thorax in the locust, however, are those common to 

 all insects, and in the musculature there is almost no addition of special 

 muscles such as are found in most of the higher insect orders and to 

 some extent in the other orthopteran families. 



The thorax of the jumping Orthoptera is so distinctly divided into a 

 prothorax and a ptcrotJiom.v that it is scarcely to be regarded as a unit 

 in the organization of the body. The box-like structure of the com- 

 bined mesothorax and metathorax, the oblique slant of the pleurites 

 of these segments, and the firm connection of the first abdominal seg- 

 ment with the metathorax are characters evidently correlated with the 

 development of the hind legs as saltatorial organs. 



THE CERVICAL SCLERITES 



The grasshopper ordinarily keeps its head retracted against the pro- 

 thorax, in which position the insect appears to have no neck, for the 

 ample neck membrane (fig. 20 B, Cv), as well as the back part of the 

 head, is thus concealed within the projecting anterior rim of the 

 pronotum. 



