318 PTILOTA ! PHYSIOLOGY AND 



to be performed by the organs to which they are at- 

 tached : thus, the muscles of the mandibles are much more 

 robust than those of the maxillae : in like manner the com- 

 parative size of the various organs influences that of the cor- 

 responding muscles. Thus when the anterior wing is en- 

 larged at the expence of the posterior, the mesothoracic 

 muscles are more developed than those of the metathorax ; 

 and vice versa; in like manner, the muscles of the hind-legs 

 of a leaping insect, as a grasshopper, are more extensive 

 than those of the simple walking legs. Muscles, as regards 

 their action, are either flexors or extensors, levators or de- 

 pressors : by the flexors, which are attached to the inner sur- 

 face of a limb, it is drawn towards the base, and consequently 

 as it were folded up ; extensors, on the contrary, are attached 

 on the exterior part of the cavity of the outer covering of a 

 limb, and act in an opposite direction, so as to extend the 

 limb. In like manner levator muscles cause the elevation of 

 a limb, whilst depressors bend it downwards. 



F. The Generative System. 



It will be sufficient to refer to what has been stated in 

 preceding pages relative to the general nature of the genera- 

 tion of insects, as a description of the numerous and greatly 

 complicated internal organs would occupy far too great a 

 space in a work like the present. 



SECTION IV. 

 THE PHYSIOLOGICAL AND INSTINCTIVE PROPERTIES OV INSECTS. 



Having brought to a close our review of the general struc- 

 ture of insects, as exhibited not only in their preparatory and 

 perfect states, but also in their external and internal organ- 

 ization, it would naturally remain for me to lay before the 

 reader the modes of operation by which each of these various 

 structures is rendered serviceable towards the fulfilment of 



