34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



ent parts of the same piece of chitin. The strong chitinons portions 

 of the insect's joint continuously pass into thinner chitinons parts, 

 which unite them with each other, while in vertebrates the different 

 skeletal pieces are isolated and held together by muscles and ten- 

 dinous structures only. The joints of insects have no capsules like 

 those of vertebrates. 



4. Similarities between the articulations of insects and vertebrates 

 can, therefore, never be true homologies. 



5. The complicated parts of the joint serve definite ends. 



6. The pterale A raises and lowers the wing, working like a single- 

 armed lever. 



7. The pterale B strengthens the joint in a vertical direction. 



8. The pterale C prevents or softens, like a buffer, the shocks at 

 the end of each downstroke. 



9. The processus pterale thoracis IV, the so-called stroma, has a 

 similar function. It also contributes to softening the shocks or jerks 

 of the wing-movement, particularly at the end of the up- and down- 

 strokes. 



10. There is in the wing a device, the flexible zone, which pre- 

 vents the insufficient elasticity of the organ from being injurious. 



11. The flexible zone renders possible a free, pendular vibration 

 of the middle and distal parts of the wing at each reversal of the 

 movement. 



12. While the indirect muscles produce the elevation and depres- 

 sion of the wings, the direct muscles change the shape and position 

 of the wings themselves, and the course they travel during each 

 beat. 



13. The direct muscles are probably also the steerers. 



14. The downstroke of the wing is forward relative to the insect, 

 and directed, in ordinary forward flight, downward and forward 

 also relative to the surrounding air. It produces an elevation of 

 the insect's body, which overcomes gravity but retards the forward 

 movement. 



15. During the horizontal movement of the wings, when de- 

 pressed forward and also during the upstroke, the wings act as 

 gliders, counteract gravity and retard horizontal advance, or vice 

 versa. The fly descending glides forward, or ascending loses its 

 horizontal velocity. 



16. The torsion of the wing attains its maximum at the end of 

 the downstroke, because its anal portion lags behind the anterior 



