28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



of these muscles rhythmically brought the cut edges of the back to- 

 gether. From this observation Wallengren concluded that a compres- 

 sion of the terga produced by the diaphragm and the transverse muscle 

 forces the sterna down, and thus causes inspiration. Tonner (1936) 

 likewise claimed that these muscles are the chief effectors of the in- 

 spiratory movements, but he contended that the transverse muscle 

 must contract first in order to push down the sterna, after which con- 

 traction of the diaphragm can produce only a widening of the pos- 

 terior segments of the abdomen. Inasmuch as the posterior half of 

 the abdominal cavity is shut off by the diaphragm (fig. 7), a contrac- 

 tion of the transverse muscle (tmcl) might be supposed to dilate the 

 segments behind it and thus cause an inflation of the respiratory cham- 

 ber. However, a breathing larva shows no special movement of the 

 abdomen in the region of the transverse muscle or the diaphragm ac- 

 companying inspiration. All the segments uniformly contract and ex- 

 pand, but the respiratory movements are most pronounced in the 

 posterior half of the abdomen. 



Artificial respiration practiced on a freshly dead larva shows that 

 no matter how deeply the sternal surface may be pushed in, it auto- 

 matically bulges out again when the pressure is released. The same 

 movement can be observed in a segmental annulus cleared of the 

 viscera and viewed from one end. If the tergum is held at the top 

 with a fine-pointed forceps, and the sternum is then deeply inflected 

 by artificial pressure, the lateral margins of the tergum are drawn in 

 (fig. 9H) ; on release of the pressure the sides of the tergum at once 

 spring outward and the sternum goes down to the passive position (F) . 

 Observation of a live larva quietly breathing shows that the elevation 

 and depression of the sterna are accompanied by corresponding but 

 smaller movements of the terga and a slight extension and regression 

 of the lateral tergal margins, just as a rubber bulb artificially com- 

 pressed expands by its own elasticity. The only muscles that effect a 

 respiratory movement, therefore, are the lateral tergosternal muscles 

 that flex the laterosternites on the terga. There is no evidence or rea- 

 son for believing that the diaphragm and the transverse muscle play 

 any essential part in respiration. Inspiration is due to the elasticity 

 of the arched tergal plates, expiration results from the inflection of 

 the sterna by the lateral tergosternal muscles. The diaphragm and 

 transverse muscle are known to exist only in anisopterous larvae. 

 Since the diaphragm most probably is functionally related to the pro- 

 traction of the labium, a respiratory function need not be attributed 

 to it in order to give it a reason for being. 



The forcible ejection of water from the rectum for purposes of 



