74 ON THE ORIGIN AND [CHAP. 



In the larva of Chloeon (PI. IV., Fig. i), for in- 

 stance, which in other respects so singularly resembles 

 Campodea (PL III., Fig. 5), several of the segments 

 are provided with foliaceous expansions which serve 

 as respiratory organs.. These so-called branchiae arc in 

 constant agitation, and the muscles which move them 

 in several points resemble those of true wings. It 

 is true that in Chloeon the vibration of the branchiae 

 is scarcely, if at all, utilized for the purpose of loco- 

 motion ; the branchiae are, in fact,, placed too far back 

 to act efficiently. The situation of these branchiae 

 differs in different groups ; indeed, it seems probable 

 that originally there were a pair on each segment. 

 In such a case, those branchiae situated near the 

 centre of the body, neither too much in front nor too 

 far back, would serve the most efficiently as propellers : 

 the same causes which determined the position of 

 the legs would also affect the wings. Thus a division 

 of labour would be effected ; the branchiae on the 

 thorax would be devoted to locomotion ; those on 

 the abdomen to respiration. This would tend to 

 increase the development of the thoracic segments, 

 already somewhat enlarged, in order to receive the 

 muscles of the legs. 



That wings may be of use to insects under water 

 is proved by the very interesting case of Polynema 

 natans? which uses its wings for swimming. This, 

 however, is a rare case, and it is possible that the 

 principal use of the wings was, primordially, to 

 enable the mature forms to pass from pond to pond, 

 thus securing fresh habitats and avoiding in-and-in 



1 Linnean Transactions, 1862. 



