32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



rudiments which reach their full development as organs of locomo- 

 tion only on the thorax. The appendages of the head became feed- 

 ing organs; those of the abdomen are represented by vestiges in the 

 embryo which disappear unless some of the external genital organs 

 are derived from them. 



The thoracic legs of the insects are 6-segmented; those of most 

 other arthropods usually have seven segments. The segments, be- 

 ginning at the leg base, are named coxa, trochanter, femur, tibia, 

 tarsus, and pretarsus. In a 7-segmented arthropod leg there are two 

 segments in the trochanteral region, the basipodite and the ischiop- 

 odite. A leg segment is best defined as a section of the limb inde- 

 pendently movable by muscles. The tarsus may be a single segment, 

 but it is commonly divided into as many as five small parts which, 

 though they are frequently called "tarsal segments" are really sub- 

 segments or tarsomeres since the only muscle of the tarsus are those 

 of its base arising in the tibia. The pretarsus bears the terminal 

 claws of the leg (often called tarsal claws). The pretarsus, however, 

 is clearly an end segment of the leg corresponding with the 

 crustacean dactylopodite. In some insects it is a simple clawlike seg- 

 ment; in others it becomes 3-clawed by the development of a pair 

 of lateral claws, but generally the median claw is lost and the typical 

 insect foot has only the pair of lateral claws. 



The pretarsus has only a single ventral muscle of several branches 

 arising in the more proximal segments of the leg; these attach on 

 it by a long tendon traversing the tarsus. In this feature the insects 

 resemble the centipedes (in the Crustacea there are both levators 

 and depressors of the dactylopodite). 



The leg segments are connected by short membranous areas that 

 allow them movement on each other. These flexible areas are the true 

 joints of the limbs (from the French word joindre, to join), and 

 this term should not be used for the segments themselves. Move- 

 ments of the segments are controlled by articulations between them 

 which are sclerotic extensions through the joint membranes from 

 the opposing ends of the segments. The movement of the distal seg- 

 ments at a joint depend on the nature of the articulation, some are 

 dicondylic, others monocondylic. 



The so-called prolegs of insect larvae are short, unsclerotized, 

 cylindrical outgrowths of the body. They have no structural re- 

 semblance to the thoracic legs, and there is no proof that they 

 originate from the abdominal leg vestiges usually present in the 

 embryo. The best-known examples are those of the caterpillar, each 

 of which end in a claw-bearing foot pad. Body muscles are attached 



