chap, xx.] EXTRA ANTENX.E : PRELIMINARY. 513 



Paired Supernumerary Antenna 



In dealing with extra antennae there is more difficulty in de- 

 termining the true nature of the parts than there is in the case 

 of extra legs. We have seen that the real duplicity of com- 

 pounded extra parts often appears only in the fact that they 

 have a bilateral symmetry, while in the normal appendage one 

 side is differentiated from the other. Now in very many species 

 of Insects the antenna seems to be a bilaterally symmetrical fila- 

 ment, having joints cylindrical or elliptical in section. When from 

 such an antenna there proceeds an extra filament, itself bilaterally 

 symmetrical, it is almost impossible to determine whether the 

 extra filament is really a single repetition of the normal or 

 whether it is made up of two homologous borders of a pair. 

 (Cp. Nos. 801 and 764.) In speaking of actual cases of 

 duplicity in Arthropodan appendages we shall have to return 

 to this subject. 



Meanwhile evidence will be given as to examples of obvious 

 duplicity in extra antennae. It will be seen that in species having 

 normally a marked differentiation between the anterior 1 and 

 posterior borders of the antennae (Lainellicorns, Lucanidae, &c. ). 

 and the case has been really studied, there is often clear proof 

 not only of the duplicity of the extra parts but also that they 

 are arranged as images, almost as described for legs. 



We shall moreover meet cases where of the paired extra 

 parts one springs free from the normal at a point proximal bo 

 the point of origin of its fellow. Among extra legs there is scarcely 

 any certain example of this phenomenon, Platycerus caraboides No. 

 778 being perhaps the clearest case. But among antennae there 

 are several where no other interpretation seems possible. These 

 cases I have set in a separate section. 



Of the remainder, little can be said with confidence. Probablv 

 if they were carefully examined microscopically it would be found 

 that differentiation between the two sides exists in respect of 

 the distribution of sense-organs or hairs, and that thus the du- 

 plicity and symmetry might be traced. 



After giving the clear cases I have thought it enough bo give 

 a list of those of this doubtful order. As has been said, there is 

 little doubt that with careful study of the specimens many 

 of the cases now included in the list of supposed single extra 

 appendages might be shewn to be cases of extra parts in 

 Secondary Symmetry. 



1 This term is used, as in the case of legs, to denote the border which ia anterior 

 when the appendage is extended horizontally at right angles to the body. The 

 upper surface will then be dorsal, the lower ventral. These terms are thus applied 

 without any intention of affirming that they are morphologically correct. 



b. 33 



