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 vrhen the anterior vting is en- 

 larged at the expence of the posterior, the mesothoracic 

 muscles are more developed than those of the metathorax ; 

 and vice versa j 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 hmb, 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 vdll be sufiicient 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 PHYSIOLOGICAIi AND INSTINCTIVE PROPERTIES OF INSECTS. 



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

 tiu-e 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 



