OF MUSCULAR MOTION. 449 



projecting pedal warts, then likewise attaches itself, and thus the body 

 pushes itself forward by gradual contraction of all the segments, which 

 begins behind. The alternating attachment of the anterior and pos- 

 terior ends is repeated as long as the maggot is in motion. We find 

 this mode of progression in all the apodal larvae of the Diptera, and 

 they accomplish it better upon uneven, rough surfaces ; upon a smooth 

 surface their progress is imperfect, and then frequently the short pedal 

 warts refuse their office. Some of the larvae of the Diptera, whose 

 pedal warts, as well as occasionally the entire body, are covered Avith 

 short horny spines or bristles, can perform this creeping motion more 

 quickly and securely, but the mechanism is just the same. As an 

 instance of the first kind of creeping we may cite the larva of the blue- 

 flesh fly (Musca vomitoria, PI. I. f. 25.) ; and, as the representative of 

 the second, the rat-tailed fly, the larva of Eristalis tenax (PI. I. f. 32.). 



The motion of larvae with thoracic feet and one or two pairs of anal 

 prolegs (the geometer caterpillars, PL I. f. 35.) next follows. In them 

 the step-like advance is more distinctly performed by means of the at- 

 tachment of the first and last abdominal segments. The whole of the 

 middle of the body bows itself into an acute arch, so that thereby the 

 anterior and posterior feet are brought closely together; the posterior 

 feet then remain affixed, but the anterior ones are so far pushed for- 

 ward with the extended body until it lies parallel to the surface ; they 

 then also affix themselves, and draw the posterior ones after them by 

 the arching of the body. 



The motion of the caterpillars furnished with thoracic, ventral, and 

 anal legs is indeed the same, but it so far differs that all the segments 

 possessing legs participate in the attachment. This attachment is now 

 no longer from behind forwards, but, after the pairs of thoracic feet have 

 advanced and affixed themselves, the ventral feet follow in succession, 

 until the last pair, or the anal prolegs, move forward whilst the ventral 

 feet still further advance. Thus the whole body is producing a con- 

 stant undulating motion as the raising and attachment of the conse- 

 cutive series advance in regular progression. 



Lastly, the walk of six-legged larvae very much resembles that of the 

 perfect insect. In both, one of the anterior legs, generally the right 

 one, makes a step which is followed almost simultaneously by the left 

 anterior leg and right intermediate one. Whilst now the right anterior 

 leg is making another step the left intermediate and right posterior 

 ones make the same movement, and thus support it ; whereas the left 



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