THK MUSCULAR SYSTEM. 257 



175. 



B. MUSCLES OF THE THORAX. 



The muscles of the thorax must he considered under several points 

 of view, which proceeds from the differences of structure displayed in this 

 portion of the body. The muscular system differs in insects with a 

 free prothorax from that of those with an immoveable connate one ; 

 to which we may add the muscles of the limbs, which likewise all lie 

 in the thorax, and a portion of which pass to the wings and the rest to 

 the legs. We have thus four main divisions into which the muscular 

 system of the thorax may be separated : we will therefore commence 

 with the system observed in insects with a free prothorax. 



176. 



MUSCLES OF INSECTS WITH A FREE PROTHORAX. 



The prothorax exhibits on each side four muscles, whereby it is held 

 connected with the meso- and metathorax. 



The largest or superior retractor (retractor prothoracis superior) 

 originates from the centre of the mesonotum with a broad basis, and 

 runs pyramidally to the prophragma or the anterior partition of the 

 mesonotum. 



Opposite to it there lies a smaller lower retractor (retractor pro- 

 thoracis inferior}, which unites the internal furcate process of the 

 pro- and mesosternum. 



The elevator (elevator prothoracis} is a small pyramidal muscle, 

 which originates on each side from the exterior margin of the 

 prophragma, and affixes itself to the corresponding fork of the 

 prosternum. 



The fourth and largest of all, the rotator ( rotator prothoracis ), 

 comes from the posterior margin of the pronotum, passes beneath the 

 prophagma, and affixes itself to the exterior edge of the mesophragma 

 or the anterior portion of the metathorax. 



The mesothorax, which, in the beetles, is the smallest portion 

 of the thorax, has but few muscles which unite it with the meta- 

 thorax. 



One, the holder of the mesonotum, is a flat, thin but broad 

 muscle, which passes from the posterior wall of the prophagma to 



