MORPHOLOGY AND MECHANISM OF THE 

 INSECT THORAX 



By R. E. SNODGRASS 



bureau of entomology 



CONTENTS 



Introduction 2 



I. Fundamental structure of an arthropod segment 4 



Primary segmentation 5 



Secondary segmentation 5 



II. Elemental structure of a thoracic segment 8 



Ground-plan of a thoracic segment 9 



Evolution of the thoracic sclerites 14 



The tergum i5 



The sternum 17 



The pleuron 22 



The spiracles , . . 37 



III. Special structure of a wing-bearing segment 41 



Ground-plan of a wing-bearing segment 42 



Structure of a wing-bearing tergum 45 



The postnotal plates 48 



Modifications, of the wing -bearing terga 52 



The pleuron of a winged segment 56 



IV. The wings and the mechanism of flight 58 



General structure of the wings 59 



Development of the wings 39 



The adult wing 60 



The wing muscles and the mechanism of wing motion 63 



V. The legs and their muscles 72 



Structure of an insect's leg 72. 



The coxa 74 



The trochanter 78 



The femur 80 



The tibia 80 



The tarsus 80 



The pretarsus 81 



Muscles and mechanism of the leg 83 



Morphology of the leg 93 



VI. Summary (Evolution of the thorax) 98 



Abbreviations used on the figures 102 



References 104 



Smithsonian Miscellaneous Collections, Vol. 80, No. 1 



I 



