162 THE COCKROACH: 



5. Three principal types of respiratory mechanism occur in 

 Insects, and these admit of further subdivision : 

 (a) Sterna usually stout and very convex, yielding but little. 

 Terga mobile, rising and sinking appreciably. To this 

 class belong all Coleoptera, Hemiptera Heteroptera, and 

 Blattina (Orthoptera). 



Fig. 90. Transverse section of Abdomen, Lamellicorn Beetle. The position 

 of the terga and sterna after an inspiration, is indicated by the thick line ; the 

 dotted line shows their position after an expiration, and the arrow marks the 

 direction of the expiratory movement. 



In the Cockroach (Penplaneta) the sterna are slightly 

 raised during expiration. (See figs. 89 and 91.) 



Fig. 91. Transverse section of Abdomen, Cockroach (P. orientalis)- 



(b) Terga well developed, overlapping the sterna on the sides 



of the body, and usually concealing the pleural membrane, 

 which forms a sunk fold. The terga and sterna approach 

 and recede alternately, the sterna being almost always the 

 more mobile. To this type belong Odonata, Diptera, 

 aculeate Hymenoptera, and Acridiaii Orthoptera. (Fig. 92.) 



(c) The pleural membrane, connecting the terga with the 



sterna, is well developed and exposed on the sides of the 

 body. The terga and sterna approach and recede alter- 

 nately, while the pleural zone simultaneously becomes 

 depressed or returns to its original figure. To this type 

 the writer assigns the Locustidce, the Lepidoptera and the 

 true Neuroptera (excluding Phryganidac). (Fig. 93.) 



