chap. xx. J EXTRA LEGS: POSITION DA A. 491 



the normal limb from the two supernumeraries, and the Position 

 may either have been P or A. 



Focnius tarsatorius (Ichneumon): tibia of left posterior leg bears a pair of 

 supernumerary limbs. This is rather a remarkable case by reason of the great 

 similarity in the modes of origin of the three limbs, whence it is difficult to 

 determine positively which is the normal one. The tibia divides into three parts 

 which lie in a horizontal plane and are separate from each other for about $ of the 

 length of the tibia. Of these the anterior is a good deal more slender than the other 

 two which are similar and about of normal size. The middle of the three is shewn 

 by its spurs to be a right limb. Each bears a complete tarsus. The ventral surface 

 of the most anterior tibia is horizontal while those of the other two are not quite so, 

 but converge downwards at a very obtuse angle. From this fact, and from the 

 equality in size between them, it seems probable that the two posterior limbs are 

 the supernumerary pair. The Position is therefore very nearly P or perhaps A. 

 This specimen was described by Mr Harrington in Can. Ent., 1890, p. 124, 

 who was so kind as to lend it to me. 

 | Agestrata dehaanii (Lamellicorn) : the coxa of the right anterior leg has two 

 articulations, one anterior and tbe other posterior. With the anterior there articulates 

 a single trochanter, bearing a normal right leg. The posterior articulation bears a 

 large structure which is composed of two trochanters united together. This double 

 trochanter bears two legs and is placed in such a way that the two do not lie in the 

 same horizontal plane ; but the posterior extra leg is in the same horizontal plane 

 as the normal leg while the anterior extra leg is wedged out towards the ventral 

 surface, between the normal leg and the posterior extra leg. The posterior extra 

 leg is a normally shaped right leg having its structurally anterior surface forwards 

 as normally. The anterior extra leg is fashioned as a left leg and the surface of it 

 which is structually anterior faces backwards towards the other extra leg. These 

 two are therefore a complementary pair, having their structurally anterior surfaces 

 adjacent : all three legs are normal and similar in form, size and colour. [Specimen 

 kindly lent by Mr E. W. Jan son.] 



(4) Positions DA A and DA. 



753. Cerambyx scopolii (Longicorn.) : pair of extra legs arising 

 from the coxa of the right anterior leg. As this is a remarkably 

 simple and perfect case it will be well to describe it in some 

 detail, as it will serve to illustrate the arrangement of such cases 

 in general. 



A normal leg of such a beetle as Cerambyx consists of coxa, tro- 

 chanter, femur, tibia and four tarsal joints. To a proper understanding 

 of the mode of occurrence of the extra legs in this case it is essential 

 that the forms of these parts and their mode of movement with regard 

 to the body and to each other should be accurately known. 



Of the large, irregularly pear-shaped coxa only the hemispherical face 

 is seen from the exterior. It is chiefly enclosed by embracing out- 

 growths from the sternum, forming a socket in which it can be rotated 

 like a ball. Upon its broad, exposed surface it is itself hollowed out 

 to form a socket for the ball of the trochanter. For our purposes it is 

 necessary to find some means of distinguishing the anterior face of the 

 coxa from the posterior face. The structure which at once enables us 

 to do this is the process (Fig. 160, p), which goes down from the coxa 

 to embrace the neck of the ball of trochanter and lock it into its socket. 

 Now in the case of an anterior leg, this process is posterior to the 

 trochanter (but in a middle or hind leg it is anterior to the trochanter). 

 The next point to be considered is the position of the femur. The 



