34 THE COCKROACH : 



larval or adult Arthropod. Lower forms and younger stages 

 take us further from such a type, instead of nearer to it ; and 

 Audouin's theoretical conception is most fully realised in the 

 thorax of an adult Insect with well-developed legs and wings. 



The morphologist would derive all the varieties of Arthropod 

 segments from the very simple and uniform chitinous cuticle 

 found in Annelids and many Insect-larvae. This becomes 

 differentiated by unequal thickening and folding in, and a series 

 of rings connected by flexible membranes is produced. Loco- 

 motive and respiratory activity commonly lead to the definition 

 of terga and sterna, which are similarly attached to each other 

 by flexible membranes. A pair of limbs may next be inserted 

 between the terga and sterna, and the simple segment thus 

 composed occurs so extensively in the less modified regions 

 and in early stages that it may well be considered the typical 

 Arthropod somite. 



Special needs may lead to the division of the sterna into 

 lateral halves, but this is purely an adaptive change. The 

 third thoracic sternum of the male Cockroach, and the second 

 and third of the female are thus divided, as is also the hinder 

 part of the seventh abdominal sternum of the female. 



In an early stage every somite has its tergal region divided 

 into lateral halves, owing to the late completion of the body on 

 this side. Traces of this division may survive even in the 

 imago. There is often a conspicuous dorsal groove in the 

 thoracic terga, and at the time of moult the terga split along 

 an accurately median line (see fig. 12). 



Additional pieces may be developed between the terga and 

 sterna, and these have long been termed pleural.* There may 

 be, for example, single stigmatic plates, as in the abdomen of 

 the Cockroach, pieces to support the thoracic legs, and pieces to 

 support the wings ; but the number and position of these plates 

 depends upon their immediate use, and their homologies become 

 very uncertain when Insects of different orders are compared. 

 In general, the pleural elements of the segment are late in 

 development, variable, and highly adaptive. 



This application of the word to denote parts intermediate between terga and 

 sterna has become general since its adoption by Audouin. It appears also in the 

 older and deservedly obsolete nomenclature of Kirby and Spence. Professor Huxley 

 has unfortunately disturbed the consistent use of this term by giving the name 

 pleura to the free edges of the terga in Crustacea. 



