THE THORACIC MECHANISM OF A GRASSHOPPER, 

 AND ITS ANTECEDENTS 



By R. E. SNODGRASS 



Bureau of Entomology 



U. S. Department of Agriculture 



CONTENTS 



PAGE 



Introduction i 



I. General discussion 3 



The thoracic terga 5 



The thoracic pleura 10 



The thoracic sterna 2i' 



II. The thoracic skeleton of Dissostcira 33 



The cervical sclerites 33 



The prothorax 34 



The pterothorax ^7 



III. The thoracic muscles of Dissostcira 51 



Muscles of the neck and prothorax 53 



Muscles of the pterothorax 59 



The mesothoracic muscles 60 



The metathoracic muscles 66 



IV. The legs and their muscles 72 



Structure of the legs 73 



Muscles of the legs 78 



V. The wings and their mechanism 84 



Structure of the wings 84 



The wing mechanism 92 



VI. The spiracles 99 



Abbreviations used on the figures 108 



References I09 



INTRODUCTION 



The principal elements in the motor mechanisms of arthropods are 

 the muscles and the body wall, though the blood often plays an im- 

 portant secondary part as a hydraulic medium. All movements, how- 

 ever, come primarily from muscle contractions. A contracted muscle, 

 when it relaxes, must be actively extended before it can operate again, 

 and therefore muscles generally occur in antagonistic sets. But the 

 muscles of insects are not necessarily opposed by other muscles ; the 

 counter force may be produced by the elasticity of the part of the 

 body wall on which a muscle is attached. For this reason it is often 



Smithsonian Miscellaneous Collections, Vol. 82. No. 2. 



