l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 23 



(1918) there is no trace of a diaphragm in a zygopterous larva. In 

 these larvae the labium is relatively smaller than in an anisopterous 

 larva, and possibly the narrowness of the abdomen compensates for 

 the absence of a diaphragm. 



Evolution of the feeding mechanism. — It is reasonable to suppose 

 that the first step in the evolution of the larval feeding organs con- 

 sisted of an elongation of the prementum and the conversion of the 

 palpi into grasping organs. The palpi were already provided with ab- 

 ductor and adductor muscles, but to increase the grasping power of 

 the palpi the adductor muscles were greatly enlarged, and, instead of 

 remaining confined to the prementum as in other insects, their bases 

 have transgressed the premento-postmental hinge line and transposed 

 their origins to the distal part of the postmentum. A next step must 

 have been the elongation of the postmentum to form a movable stalk, 

 giving the prementum a greater reach in its new role. The ordinary 

 insect postmentum, however, is immovable, since no head muscles are 

 attached on it. Hence, some accessory mechanism had to be developed 

 in order that the labium could swing on the base of the postmentum. 

 The postmentum acquired articular points with the head on the an- 

 terior lip of its base, and to hold it in place, whatever jXDsition the 

 labium took, a long, strong apodeme was developed from the base of 

 the hypopharynx with a terminal crossbar imbedded in the posterior 

 lip of the postmental base. Still the postmentum had no motor ap- 

 paratus. Finally, then, evolution provided a hydraulic mechanism by 

 which blood pressure could be applied against the soft neck membrane 

 behind the base of the postmentum and cause the latter to swing for- 

 ward on its anterior hinge points with the head. Perhaps in the first 

 place blood pressure was engendered by contraction of the anterior 

 abdominal muscles and the thoracic muscles, but to insure greater effi- 

 ciency the anterior part of the abdominal cavity was shut off by a 

 strong, transverse muscular diaphragm. Thus at last the complete 

 mechanism of the modern anisopterous larval labium was perfected. 



Along with the development of the labial mechanism, the head 

 necessarily underwent an adaptive modification. From a primitive 

 hypognathous condition, preserved in the adult, the larval head be- 

 came flattened and prognathous so that the prey captured and brought 

 back to the head by the labium could be readily grasped by the pro- 

 tracted maxillae and delivered to the mandibles. The antennae took 

 a more practical position for the larva by being brought forward close 

 to the clypeus. 



The question now confronts us as to how we are to visualize the 

 evolution of a complex mechanism such as that of the feeding ap- 



