NO. I 



INSECT THORAX SNODGRASS 



eventually became wings should be those of that body region already- 

 fixed as the locomotor center. Flight, however, being an entirely new 

 mode of progression, the development of the wings and the perfection 

 of their mechanism meant a further and much greater alteration in 

 the structure of the wing- bearing segments than that which was 

 evolved to accommodate the legs. In a study of the thorax, therefore, 

 we should proceed on the assumption that its elemental structure is 

 to be found in insects that never possessed wings, and that the special 



Mth 



Pre 



Fig. I. — Young embryo of an insect, showing its four body regions and their 

 appendages. (Embryo of Naucoris, Heymons, 1899.) 



The primitive head, or procephalon {Pre), bearing eyes, mouth {Mth), and 

 antennae {Ant) ; the jaw region {Gc) of three segments, bearing mandibles, first 

 maxillae, and second maxillae; thorax {Th) of three segments, bearing the legs; 

 abdomen {Ah) of at most twelve segments, each but the last with a pair of 

 rudimentary appendages in some insects. 



thoracic structures of winged insects are characters tliat have been 

 superposed on those primarily adapted to progression on three pairs 

 of legs. 



Facts and theories should run parallel ; in entomolog)' it seems they 

 often diverge. Some theories, however, have served as useful step- 

 ping stones, though they themselves have later been swept away by 

 the current ; others are illusions of the imagination and land us in 

 mid-stream. There have been many speculations concerning the num- 

 ber of segments in the insect thorax — some anatomists have claimed 

 that there are five and even ten segments represented in its construe- 



