280 THE MECHANICAL FUNCTIONS. 



Some larvae avail themselves of their jaws in 

 order to fix the head, and drag the rest of the body 

 towards it. In this manner the larvae of the Ce- 

 ramhyx, or Capricorn beetle, advance along the wind- 

 ing passages which they have themselves excavated; 

 liolding by the jaws, and dragging themselves for- 

 wards. These movements are assisted by the re- 

 sistance afforded by short tubercles which project 

 from different parts of the back, and under surface 

 of the body ; so that these insects advance in the 

 passage by an act similar to that by which a 

 chimney-sweeper, exerting the powerful pressure of 

 his elbows, shoulders, and knees, manages to climb 

 up a chimney. 



For the purpose of enabling insects to take stronger 

 hold of the surfaces they pass over, we often ob- 

 serve them furnished with spines, or hooks, which 

 are moved by appropriate muscles, and occupy 

 different situations on the body. Modifications 

 without end occur with regard to these and other 

 external parts subservient, in various degrees, to 

 progressive motion. Every possible gradation is 

 also seen between the short tubercles already men- 

 tioned, and the more regularly formed feet or legs. 

 Those which are regarded as spurious legs, or pro- 

 legs, as they have been termed, occupy an inter- 

 mediate place between these two extremes. They 

 consist of fleshy and retractile tubercles, and are 

 often very numerous ; while the number of the true 

 legs, as they are called, is limited to six. These 

 last are the representatives of the legs of the future 

 perfect insect ; for they are attached to the three 

 segments of the thorax ; and are formed of those 

 portions articulated to each other, corresponding to 



