16 INSECT A. 



the two are firmly consolidated. The metathorax 

 (PI. I., Fig. 3, /^"') is also a wing-bearing segment, 

 and larger than the mesothorax, but is similarly- 

 divided. 



PI. I., Fig. 3, t^ is the scutum, ts^ the scutellum ; PL I., 

 Fig. 7, Jfi the episternum, hs^ the epimerum ; PI. I., Fig. 

 5,^'" the sternum. 



Both teachers and scholars will probably be more or 

 less confused by the presence of secondary sutures and 

 the complex character of the segments. Some entomolo- 

 gists consider that each of the thoracic rings is composed 

 of several segments ; but the majority hold the opinion 

 here given, and strong confirmation of this view is found in 

 the fact that in the youngest stages there are only three 

 simple rings in the thoracic region. The sutures which 

 subdivide the rings of the thorax arise subsec^uently during 

 growth, and must, therefore, be regarded as of secondary 

 origin. In primitive insects segmentation was probably 

 due to the mechanical effect of the motions of a cylindrical 

 body upon a crust-producing skin. This tendency became 

 fixed and hereditary in the type, and now these primitive 

 constrictions appear in the young, and show us the num- 

 ber of rings which may be considered as primitive rings. 

 The concentration of the terminal rings to form the head 

 can be explained as in the Crustacea. ^ The similar con- 

 centration of the thorax may be referred to the reactions 

 due to the use of the wings and legs in balancing and 

 transporting the body in the air and upon the earth. The 

 exercise of these functions would naturally tend to bring 

 those segments which bore both legs and wings into closer 

 connection, to solder them together, and increase their 

 diameter, on account of the necessary increase in size of 

 the muscles used in moving the wings and legs. The 



1 Guide No. VII., p. 41. 



