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THE AMERICAN NATURALIST [VOL.XLVIII 



always in the same stock (miniature white), and always 

 with the same evidence of weakness and low viability. 

 Fig. 1 is a camera drawing of a typical specimen, made 

 by Miss E. M. Wallace. In morphological characters the 

 fly differs from the normal in 

 V^K/x^T'TTT^x being entirely destitute of wings 

 £yj[ [yj ) ) an d in possessing greatly re- 



• ^^^&* x <J_J_j^' duced balancers. Likewise in 

 Droaophiia ^mpeillhua^^ ° F ph ys i°l°&i ca l characters it devi- 

 ates strikingly from the normal. 

 This is best shown by means of a comparison between 

 apterous flies and normal flies from which the wings 

 have been removed. The latter are not appreciably 

 inconvenienced by their loss of wings; they show char- 

 acteristic vigor in their active running and jumping 

 movements, they easily right themselves if overturned, 

 or extricate themselves if entangled in food or cotton, and 

 they are long lived and breed as prolifically as do winged 

 individuals. In fact they show no ill effects except the in- 

 ability to fly. The true apterous individuals, on the other 

 hand, show marked abnormalities in all these respects. In- 

 stead of being vigorous and active they are weak and usually 

 sluggish ; if overturned they have great difficulty in right- 

 ing themselves ; or, if entangled in food or cotton, they are 

 usually unable to extricate themselves and consequently 

 perish. Moreover, they are always short lived, even when 

 kept under the best possible conditions and prevented from 

 becoming entangled in food or cotton. And lastly they 

 exhibit a most marked inability to breed, as noted in the 

 experiments. This characteristic, as has been mentioned 

 above, is so marked that the apterous flies were at first 

 thought to be sterile. I am convinced now, however, that 

 the difficulty is not one of sterility at all, but is due to a 

 physical weakness which makes it extremely difficult for 

 the flies to copulate, and for the females, even when fertil- 

 ized, to produce and lay eggs. Cytological examination 

 has shown that the males produce spermatozoa in an ap- 

 parently normal manner, yet prolonged observation of 



