ITS OUTER SKELETON. 



33 



same time the chitinous lining of the tracheal tubes is cast, 

 while that of the alimentary canal is broken up and passed 



through the body. 



Fig. 12. Cast skin of older nymph ("pupa"). X 2|. 



Prolonged boiling in caustic potash, though it dissolves the 

 viscera, does not disintegrate the exoskeleton. This shows that 

 the segments of the integument are not separate chitinous 

 rings, but thickenings of a continuous chitinous investment. 

 Nevertheless, their constancy in position and their conformity 

 in structure often enable us to trace homologies between different 

 segments and different species as certainly as between corres- 

 ponding elements of the osseous vertebrate skeleton. 



Parts of a Somite. 



Audouin's laborious researches into the exoskeleton of Insects* 

 resulted in a nomenclature which has been generally adopted. 

 He divides each somite (segment) into eight pieces, grouped in 

 pairs viz., terga (dorsal plates), sterna (ventral plates), epimera 

 (adjacent to the terga), and epi*terna (adjacent to the sterna). 



While admitting the usefulness of these terms, we must warn 

 the reader not to suppose that this subdivision is either normal 

 or primitive. The eight-parted segment exists in no single 



* Audouin. Kech. anat. sur le thorax des Insectes, &c. (Ann. Sci. Nat., Tom I., 

 p. 97. 1824.) 



D 



