THE ORGANS OF CIRCULATION AND RESPIRATION. 161 



when the arrangement of the abdominal segments, and 

 especially when the disposition of the attached muscles are 

 almost identical. Thus, for example, the respiratory movements 

 of a Cockroach are different from those of other Orthoptera, but 

 resemble those of Hemiptera Heteroptera. 



2. The respiratory activity of resting Insects is localised in 

 the abdomen. Y. Gfraber has expressed this fact in a picturesque 

 form, by saying that in Insects the chest is placed at the hinder 

 end of the body. 



3. In most cases the thoracic segments do not share in the 

 respiratory movements of an Insect at rest. Among the singular 

 exceptions to this rule is the Cockroach (P. orientalis), in which 

 the terga of the meso- and meta- thoracic segments perform 

 movements exactly opposite in direction to those of the 

 abdomen. (See fig. 89, Ms. th, Mt. th.) 



Fig. 89. Profile of Cockroach (P. orientalis}. The black surface represents the 

 expiratory contour, while the inspiratory is indicated by a thin line. The 

 arrows show the direction of the expiratory movement. Ms. th., mesothorax ; 

 Mt. th., metathorax. Reduced from a magic-lantern projection. 



4. Leaving out of account all details and all exceptions, the 

 respiratory movements of Insects may be said to consist of 

 alternate contraction and recovery of the figure of the abdomen 

 in two dimensions viz., vertical and transverse. During ex- 

 piration the diameters in question are reduced, while during 

 respiration they revert to their previous amounts. The trans- 

 verse expiratory contraction is often slight, and may be imper- 

 ceptible. On the other hand, the vertical expiratory contraction 

 is never absent, and usually marked. In the Cockroach (P. 



/ 



orientalis) it amounts to one-eighth of the depth of the abdomen 

 (between segments 2 and 3). 



M 



