APPENDAGES OF ARTHKOPODA. 247 



§ 190. 



Besides antennse^ dorsal appendages occur in no Traclieata except 

 tlie Insects. They are altogether absent in the Thysanura and Col- 

 lembola. As they are only present on the post-cephalic metameres 

 they are innervated^ like all the ventral appendages^ from the ventral 

 chord. They cannot be said to have any relations to the brauchi^ 

 of the Crustacea, or to be derived from the dorsal parapodia of the 

 Annelides, and they may therefore be fairly regarded as independent 

 structures. 



The dorsal appendages have the form of lamellae or filamentous 

 processes of the metameres, which are sometimes grouped into 

 tufts m the aquatic larva3 of the Ephemerida, Perlida, Phrygauida, 

 etc. These appendages have a respiratory function ; and as tracheae 

 pass into them, they are known as tracheal gills. They are 

 widely distributed over the body, not only on the dorsal, but also on 

 the ventral surface ; and are therefore examples of processes in an 

 indifferent condition, of which the dorsal ones placed at definite 

 points acquire a typical significance. The lamellar widely-distributed 

 forms move in such a way as to be of great importance in changing 

 the water, just like the respiratory appendages of the Phyllopoda ; 

 but they cannot be said to have any locomotor functions. 



The wings must be regarded as homologous with the lamellar 

 tracheal gills, for they do not only agree with them in origin, but 

 also in their connection with the body, and in structure. In being 

 limited to the second and third thoracic segments they point to 

 a reduction in the number of the tracheal gills. It is quite clear 

 that we must suppose that the wings did not arise as such, but 

 were developed from organs which had another function,^ such as 

 the tracheal gills ; I mean to say that such a supposition is neces- 

 sary, for we cannot imagine that the wings functioned as such in the 

 lower stages of their development, and that they conld have been 

 developed by having such a function. 



But if the real cause of the development of these dorsal 

 appendages into wings cannot be found in the locomotor function, it 

 must be sought for in another. And this is that of respiration, 

 which would be well served by increase of surface. Every increase 

 of surface increases the respiratory value of the organ, and so leads 

 towards its future function. It is no objection to this hypothesis to 

 say that ontogenetically the wings are formed and developed later 

 than the tracheal gills of other metameres, for these modified 

 tracheal gills can only function when the unmodified respiratory 

 ones have lost their function. 



In many cases the appendicular character of the wings is in- 

 dicated by segmentation, but this can only be regarded as a 

 secondary adaptation. It obtains in the movable second paii- of 

 wings in the Coleoptera and Eorficulidse, where the first pair have 

 been converted into elytra. 



