218 



EVOLUTION AND GENETICS 



body, consequently the ambulatory condition is the most primi- 

 tive stage of the terrestrial appendages. 



Cursorial Adaptation. Cursorial animals must move their 

 limbs rapidly, and for a maximum rate of speed must also be able 

 to move by long strides. Rapidity of movement is in part a 

 physiological adaptation, but it is aided by the structural modi- 

 fications which result in lengthened stride. These include 

 lengthening of the limb and slenderness, so that the greatest 

 reach is attained with minimum bulk. The hmb is a lever of the 

 third order in which the point of articulation to the body is the 

 fulcrum and the insertion of a muscle the point of application of 

 power. By lengthening of the entire limb, particularly its distal 



Fig. 128. — Types of insect legs. A, grasshopper, a jumping leg; B, tiger 

 beetle, a running leg; C, gyrinid beetle, a swimming leg; D, mantis, a rap- 

 torial leg; E, mole cricket, a burrowing leg. (From Sanderson and Jack- 

 son's Elemenlary Enlomology, with the permission of Ginn and Company.) 



segments, the ratio of work arm to power arm is increased, conse- 

 quently greater range of movement is acquired at the expense of 

 power. In cursorial Arthropoda this lengthening and slenderness 

 is general (Fig. 128A). In vertebrates it involves the distal seg- 

 ments of the appendages (Fig. 56), while the proximal muscles 

 remain short and bunched, making for quick and powerful con- 

 tractions. 



Correlation with flight lessens the necessity for cursorial adap- 

 tation among the Arthropoda, but such insects as the tiger beetles 

 (Cicindelidae) are able to run very rapidly for their size. Their 

 legs are long and slender. 



In the vertebrates the first step in cursorial adaptation is change 

 of posture. The animal rises to its toes, becoming digitigrade, in- 

 stead of plantigrade, and thus adds to the free length of the hmb 



