504 .MKKISTIC VARIATION. [PART I. 



In order to appreciate the nature of these cases it is necessary 

 that the anatomy of the parts should be understood. 



Tin- apex of the tibia in the simple form, e.g. the second or 

 third leg in ('unilms, bears t \vo large articulated spurs. The two 

 spurs are ventral to the articulation of the tarsus, and one of 

 them is placed at tin- anterior border of the tibial apex while 

 tit. oilier is posterior. In these unmodified legs both spurs are 

 placed at the same level in the limb, so that the bases of both 

 are in the same transverse section (cp. Fig. 166). In the forms 

 presenting the sexual modification, the anterior spur is of some- 

 what small size but occupies the same position relatively to the 

 other parts that, it does in a simple leg. 



The posterior spur however, which is large, does not stand 

 at the same level on the tibial apex, but has, as it were, travelled 

 up the tibia so that it stands at a considerable distance central 

 to the apex, and instead of marking the posterior border of the 

 limb it is placed nearly in the middle of the actual ventral surface. 

 A long channel runs from the posterior spur to the anterior one, 

 and the appearances suggest that the modified form is reached 

 by a deformation of the original apical surface, which is twisted 

 so that the posterior spur is thus drawn up into the secondary 

 position. In the fore leg of a male ('arabus the beginning of 

 such a change can be seen, hut in Pterostichus and especially in 

 ('ninth us it reaches a maximum. The change may be briefly 

 described by saying that a section to include the two spurs must 

 be taken in a plane which is oblique to the long axis of the limb 

 instead of transverse to it. 



As a result of this modification the morphological surfaces 

 "f the anterior tarsus of Calathus &c. have a peculiar disposition 

 relatively to the same surfaces of the tibia when compared with 

 other forms. Commonly the ventral surface of the tarsus is 

 parallel to a line taken through the bases of the spurs, but owing 

 to the rotation of the posterior spur into its secondary position 

 this plane is here oblique to the ventral surface of the tarsus. 

 These points will be at once evident if the front leg of a male 

 Calnthiift is examined. 



It was laid down as a principle generally followed in cases of 

 double extra appendages, that the three terminations, when ex- 

 tend,.,!, stand in the same plane, and the chief feature which 

 distinguishes the two following cases is that the three termina- 

 tions are not in one plane. 



Moreover, though the two supernumerary tarsi are a com- 

 plementary pair, and together with the normal tarsus are arranged 

 as a series of images, yet in order to produce the arrangement of 

 the present cases the planes of reflexion would not be parallel to 

 each other (as in Fig. 154) but inclined in the manner to be 

 described. 



