Vol. 1.] Woodworth. — Wiug Veins of Insects. 29 



aids the flight chiefly by lifting the body, and to a large extent 

 the wing simply feathers forward. 



The oblique or direct stroke is accomplished by shifting the 

 dorsal articulation of the wing backward, or the ventral 

 bearing-point forward, or, in the case of the damsel-flies, b}^ 

 the movement of the whole dorsal region of the meso- and 

 metathorax into a position almost at right angles to that orig- 

 inally held. The stroke is from above and behind forward and 

 downAvard, or vice versa. 



In the majority of insects, especially in those in wliich 

 change to an oblique direction has not been carried to an 

 extreme, the course of the wing is probably that described by 

 Marey. Unfortunately, I can not speak with certainty in 

 regard to either of the insects (Odonata, blowfly) in which 

 the contrary motion has been claimed. In regard to them it 

 may be said that they are both evidently very highly special- 

 ized; both possess the habit of assuming a peculiar position, 

 not one of rest, just before beginning to fly. The damsel- 

 fly shows this particularly well. In the blowfly it is not as 

 evident as in some of the other Dipterous families, such as 

 Syrphida?, Conopid?e, and many others. It is not impossible, 

 therefore, that these insects may pass over the course that 

 they have been described as following, though it involves a 

 remarkable change in the nature of the wing motion as com- 

 pared with that of other insects. 



The presence of air sacs in great abundance in the bodies 

 of all flying insects has often been discussed, and they have 

 often been compared to the swimming bladder in fishes, the 

 explanation offered being that they enable the insect to change 

 its specific gravity. The uselessness of such a contrivance to 

 aid in flight is so evident that it is strange that text-books 

 have so long copied the suggestion. The only other explana- 

 tion of this function that has been offered is that of Packard, 

 who suggests that they are a means by which a larger supply 

 of oxygen is provided for the muscles during their very vio- 

 lent action in flight. This does not appear to be a sufticient 

 explanation, even though the air sacs niay aid respiration 

 materially in the manner suggested. It is especially the dis- 

 tribution of the air sacs that counts against the idea that this 

 is the sole cause of their existei'ice. 



A possible use of the air sacs that may explain their arrange- 



