MECHANICAL ORIGIN OF THE LIMBS 35 



impacts resulting from the primitive endeavors of the ancestral 

 arthropods to raise and to support the body while thus raised, 

 and then to push or drag it forward by means of the soft, partially 

 jointed, lateral limbs which were armed with bristles, hooks, or 

 finally claws. 



On the other hand, by adaptation, or as the result of parasitism and 

 consequent lack of active motion, the original number of segments 

 may by disuse be diminished. Thus in adult wasps and bees, the 

 last three or four abdominal segments may be nearly lost, though the 

 larval number is ten. During metamorphosis the body is made 

 over, and the number, shape, and structure of the segments greatly 

 modified. In the female of the Stylopidse the thorax loses all traces 

 of segments, and is fused with the head, and the abdominal segments 

 are faintly marked, losing their chitin. 



While the maxillee have several joints, the mandibles are 1-jointed, 

 but there are traces of two joints in Campodea, certain beetles, etc. 

 In the antenna there is a great elasticity in respect to the num- 

 ber of joints, which vary from one or two to a hundred or more. It 

 is likewise so in the thoracic legs, where the number of tarsal joints 

 varies from one to five ; also in the cercopoda, the number of joints 

 varying from one or two to twelve or more. 



(I. Mechanical origin of the limbs and of their jointed structure 



We have already hinted at the mode of origin of the limbs of 

 arthropods. Like the body or trunk, the limbs are chitinous dermo- 

 muscular tubes, with a dense solid cuticle, and internal muscles, 

 and were it not for their division at more or less regular intervals 

 into segments, forming distinct sets of levers, set up by the strains 

 in these tubular supports, there would be no power of varied motion. 



Even certain worms, as already stated, have their tentacles and 

 parapodia, or certain appendages of their parapodia, more or less 

 jointed, but there are no indications of claws or of any other hard 

 chitinous armature at the extremity, and the skin is thin and soft. 



In the most simple though not the most primitive arthropods, 

 such as the Tardigrades, whose body is not segmented, there are 

 four pairs of short unjointed legs, ending each in two claws, which 

 have probably arisen in response to the stimulus of pushing or 

 dragging efforts. 



The legs of Peripatus are unjointed, and have a thin cuticle, but 

 end in a pair of claws, which have evidently arisen as a supporting 



