MECHANICAL ORIGIN OF THE LIMBS 



39 



(d 1 ). The intersegmental membrane of the insect's limb is in a 

 degree a two-armed lever, whose pivot (/) lies in the middle. The 

 internal invagination of the intersegmental fold (B, g-li} affords the 

 necessary support to the muscles acting like the tendon in the verte- 

 brate. (Graber.) 



Graber also calls attention to the fact that this insect limb differs 

 in one important respect from that of land vertebrates. The lever- 

 age system in the last is divided at the end 

 into five parallel divisions or digits. In 

 arthropods, on the contrary, all the joints 

 succeed one another in a linear series. 



In insects, as well as in other arthropods, 

 modifications of the limbs usually take the 

 form of a simple reduction in the number of 

 segments. Thus while the normal number 

 of tarsal joints is five, we have trimerous 

 and dimerous Coleoptera, and in certain 

 Scarabaeidae the anterior tarsi are lost. 



Savigny was the first, in 1816, in his great 

 work, " Theorie des organes de la bouche des 

 Crustaces et des Insectes," to demonstrate 

 that not only were the buccal appendages of 

 biting insects homologous with those of 

 bugs, moths, flies, etc., but that they were 

 homologous with the thoracic legs, and that 

 thus a unity of structure prevails through- 

 out the appendages of the body of all 

 arthropods. Oken also observed that " the 

 maxillae are only repeated feet." 



What was modestly put forth as a theory by the French mor- 

 phologist has been abundantly proved by the embryology of insects 

 of different orders to be a fact. As shown in Fig. 23 the antennae and 

 buccal appendages arise as paired tubercles exactly as the thoracic 

 legs. The abdominal region also bears similar embryonic or tem- 

 porary limbs, all of which in those insects without an ovipositor 

 disappear, except the cercopoda, after birth. 



FIG. 23. Primitive band or 

 germ of a Sphinx moth, with the 

 segments indicated, and their 

 rudimentary apprndai. r es : c, 

 upper lip ; <it, antenna 1 , met, 

 mandibles ; nix, my\ first and 

 second maxilla 1 ; /, I', I", legs; 

 al, abdominal legs. After 

 Kowalevaky. 



