CHAP, xx.] SECONDARY SYMMETRY: PRELIMINARY. 477 



and careless observers have often thus declared them to be two 

 legs with one femur. Similarly the two tibia? or the two tarsi 

 may be more or less compounded. In the case of Silpha nigrita 

 (No. 7G9), the two extra legs which arose from a femur were 

 compounded throughout their length, having a compound tibia 

 and tarsus (see Fig. 167). Even in cases when the two extra 

 legs appear to arise separately it will generally be found that 

 they articulate with a double compound piece of tissue which 

 is supernumerary and is fitted into the joint from which they 

 appear to arise. This is especially common in cases of two extra 

 tarsi, which seem to spring directly from a normal tibia. As a 

 matter of fact in all such cases these extra tarsi articulate with 

 a supernumerary piece of tissue, as it were let into, and com- 

 pounded with, the apex of the normal tibia. These bodies are 

 themselves double structures, composed of parts of two tibiae. 

 In determining the morphology of the limbs they are of great 

 importance, but unfortunately they are not generally mentioned 

 by those who describe such formations. But though extra parts 

 are generally present in the leg centrally to the point from which 

 the extra legs actually diverge, it should be expressly stated 

 that if this point is in the periphery of the leg, the central 

 joints are normal : if for example, there are two extra tarsi, 

 there may be parts of two extra tibial apices, but the base of 

 the tibia, the femur, &c. are single and normal. 



Symmetry of Paired Extra Legs. 



To appreciate what follows it is necessary to have a distinct 

 conception of the normal structure of an insect's leg, and to 

 understand the use of the terms applied to the morphological 

 surfaces. 



If the leg of a beetle, say a Carabus, is extended and set at right 

 angles to the body, the four surfaces which it presents are respectively 

 dorsal, ventral, anterior and posterior. In the femur, tibia and tarsu< 

 the dorsal is the extensor, and the ventral is the flexor surface. The 

 anterior surface is seen from in front and the posterior from behind. 

 (The terms 'internal' and 'external' are to be avoided as they de- 

 note different surfaces in the different pairs.) Difficulty as to the use 

 of terms arises from the fact that as the beetle walks or is set in 

 collections, the legs are not at right angles to the body but are rotated 

 on the coxae, so that the plantar surface of the first pair of legs is 

 turned forwards, but the plantar surfaces of the second and third ]>.-m- 

 are turned backwards 1 . 



1 Attention is directed to the fact that in a beetle there is a complement :u > 

 relation not only between the legs of the right and left sides but also imperfectly 

 between the legs of the first pair and those of the second and third pairs which air 

 in some respects images of the first leg of their own side. For iiistnnc. , in < '< r.imlii.c 

 (see Fig. 160) the trochanter of the fore leg is kept in place by a process of the coxa 

 which goes down behind it, but the corresponding process in the -eeoml :md tliii.l 

 legs is in front of each trochanter. Again in Melolontha etc. the tibial serrations of 



