Vol. 1.] Woodu-ortJi . — Wi iig Veins of Insects. 13 



habit of vibrating the body, apparently to assist in respira- 

 tion by causing the water bathing the gills to mix with the 

 surrounding water and so become purer and fresher. When 

 an insect possesses gill covers, the same change may be effected 

 by the vibration of these. Any process attached to the body 

 as the gill covers are, would be caused to vibrate with every 

 movement of the segmental muscles and particularly by the 

 contraction of the dorso-ventrals. The gill cover that Ave have 

 been considering moves quite freely. The economy of this 

 means of keeping up the circulation of water is at once evident. 

 The musculature and the first steps toward the specialization 

 of the thorax for flight are thus provided for. 



A difficulty that has presented itself to the minds of many 

 students of the subject of imaginal characters is that of account- 

 ing for the repression of an organ during larval life, or the hold- 

 ing of its development in abeyance until the final molt. The 

 precursor of the wing, according to any theory connecting it 

 with the tracheal gill, must have been functional in the larval 

 insect. Wings would, therefore, seem to offer one of the most 

 difficult of these problems; that is, the entire su|)pression of 

 all the earlier stages. 



A closer study of this subject shows that this is only an 

 apparent difficulty, for there is no real suppression in the 

 larval stages in the case of the wings, since they often, and 

 perhaps generally, appear quite as early as the true gills, and 

 represent at first nearly the condition of gill covers. The 

 change into wings (that is, its development beyond the gill- 

 cover stage) is all that belongs to the penultimate stage. 



Another kind of suppression has really occurred in tbe 

 case of insect wings, whereby the organs have become limited 

 to the thorax. Much of the difficulty in this case disappears, 

 however, when we note that the same problem has been met 

 in the case of gills that retain their original function. I refer 

 to the remarkable case of insects of the genus drnis, where 

 one pair of gill covers has become very like an elytron, is 

 almost or quite gill-less, and protects the naked gills of the 

 following four segments. On the other segments of the abdo- 

 men the gills have entirely disappeared. An exactly similar 

 process, but one resulting in two pairs of gill-less covers and 

 an entire suppression of all gills^in the last molt, would give us 

 the condition of the winged insect. 



