478 MKRLSTIC VARIATION. [PART I. 



Extra legs may arise from any one of the morphological 

 surfaces, but mre often their origin is in a position intermediate 

 between them, e.^., antero-ventral, or postero-dorsal. 



The next question i- ihat of the determination of parts which 

 are extra from the parts which are normal. Two extra legs 

 spring fn.m a normal leg. The appearance is often that of a 

 leg single proximally, but triple peripherally. All three limbs 

 are often .Dually developed and at first sight it might well be 

 supposed ihat t'he three collectively represent the single leg of 

 ill-- iiniial. 



In many cases of Meristic Variation I have contended that the 

 facts are only intelligible on the view that there has been such 

 collective representation. But in these Secondary Symmetries 

 this MI pj H .-it mn is [('always] inadmissible. On closer examination 

 it is generally more or less easy to see that the three legs do not 

 arise in the same way, but that one arises as usual while the 

 other tw<> are., as it were, ingrafted upon it. It is thus possible 

 in all but a very few cases to determine the normal leg from 

 the others by tracing the surfaces from apex to base, when it 

 will be found that some surface of the normal is continuous 

 throughout the appendage while those of the extra legs end 

 abruptly at some part of the normal leg. 



Nearly always besides, as has been mentioned, the extra legs 

 are more or less compounded together at their point of origin 

 even if separate peripherally. In a few very exceptional cases 

 it happens that one of the extra appendages is compounded 

 with the nnnnal and not with the other extra appendage. A 

 remarkable rase of this in an antenna may be seen in Melolontha, 

 No. *()(), ami in a leg in Platycents camboides, (q.v.) 



We have now to consider the positions of the paired extra 

 legs in regard to the normal leg and in regard to each other. At 

 first sight their dispositions seem entirely erratic ; but though it is 

 true that scarcely two are quite identical in structure, yet their 

 divers structures mav for the most part be reduced to a system. 



*f V 



This system, though far from including every case, still includes a 

 large proportion and even the remainder do not much depart from 

 it except in very few instances. The comprehension of the general 

 system will also greatly help to make the aberrant cases appreciated 

 with comparatively few words. For simplicity therefore, the con- 

 siderati..n of exceptional cases will be deferred and the principles 

 stated iii a general form. It will be remembered that we are 

 as \et concerned only with doi/h/e extra legs. 



When extra appendages, arising from a normal appendage, are 

 thoroughly relaxed and extended, the following rules will be 



the first legs curve backwards, but those of the other legs curve forwards. This 

 i ircumstance is mentioned lest it mi^'lit In- thought to have boon neglected in what 

 follows, but this complementary relation has nothing to do with that which will be 

 shewn to exist between the extra legs. 



