476 MERISTIC VARIATION. [part L 



of the extra parts, and secondly the symmetry or relation of form 

 subsisting on the one hand between the extra parts themselves, 

 and on the other between the extra parts and the normal parts. 



In few cases of extra appendages arising from the body 

 itself have these essentials been adequately ascertained. 



For brevity I shall describe the phenomena as seen in extra 

 legs. The same description will apply generally to the antennae. 

 Recorded cases of extra palpi are very few, but probably are not 

 materially different. 



Structure of Paired Extra Legs. 



The parts composing extra legs do not as a rule greatly differ 

 from those of the normal legs which bear them. Though in 

 many instances extra legs are partially deformed, they are 

 more often fairly good copies of the true leg. Not rarely the 

 extra parts are more slender or a little shorter than the normal 

 appendage, but in form and texture they are real appendages, 

 presenting as a rule the hairs, spurs, &c. characteristic of the 

 species to which they belong. 



The next point is especially important. The parts found in 

 extra legs are those parts which are in the normal leg peripheral 

 to the point from which the extra legs arise, and, as a rule no 

 more. Though in extra legs parts may be deficient or malformed, 

 structures which in the normal leg are central to the point of 

 origin of the extra legs are not repeated in them 1 . For instance, 

 if the extra legs spring from the trochanter they do not contain 

 parts of the coxa, if from the second tarsal joint, the first tarsal 

 joint is not represented in them, and so on. 



Extra legs may arise from any joint of the normal leg, and 

 are not much commoner in the peripheral parts than in the 

 central ones, but there is a slight preponderance of cases be- 

 ginning from the apex of the tibia. It is rather remarkable that 

 cases of extensive repetition are not much less rare than others, 

 the contrary being for the most part true of the limbs of vert- 

 ebrates. 



It does not appear that extra legs arise more commonly 

 from either of the three normal pairs in particular. 



Supernumerary legs of double structure are sometimes found 

 as two limbs separate from each other nearly or quite from the 

 point of origin, but in the majority of cases their central parts 

 may be so compounded together that they seem to form but one 

 limb, and the essentially double character of the limb is not then 

 conspicuous except in the periphery. For example it frequently 

 happens that the femora of two extra legs are so compounded 

 together that they seem to have only a single femur in common, 



1 Particular attention is therefore called to one case of extra antennas, which 

 did actually contain parts normally central to the point of origin. (See No. 804.) 



