CHAPTER XIV 



TWINNING (DUPLICITY) IN LIMBS 



As long ago as 1894 Bateson, in his classic work 

 Materials for the Study of Variation^ devoted two chapters 

 to "Supernumerary Appendages in Secondary Sym- 

 metry." A representative series of instances of super- 

 numerary appendages such as antennae, palpi, and legs 

 is described, involving many groups of insects, Crustacea, 

 and vertebrates. These extra appendages may be either 

 entirely separate outgrowths near the normal appendage, 

 or, as is the case in the majority of instances, they may 

 occur as outgrowths from 

 an appendage — such as 

 extra legs growing from 

 normal legs. In both 

 types of cases the sym- 

 metry of the supernum- 

 erary appendage appears 

 to bear a definite relation 

 to that of the neighbor- 

 ing appendage or to the 

 one upon which it grows. 

 It is quite common to find 

 that the supernumerary 

 appendage growing from 

 another appendage is 

 itself a twin appendage 

 in which the two parts are mirror-images of each other. 

 A very pretty case of this relation is seen in Figure 66, 



Fig, 66. — An example of limb 

 duplicity in an insect, Pterostichus 

 miihlfeldii. The component to the 

 right is the normal tarsus. The 

 extra tarsus on the left is duplex 

 and shows mirror-image symmetry. 

 (After Bateson.) 



193 



