NO. 3 INSECT HEAD SNODGRASS 45 



According to the views of the earlier students of the digestive organs 

 of insects, the proventriculus constituted a gizzard ; its inner chitinous 

 fold, and its sheath of strong muscle fibers, it was pointed out, must 

 serve to break up the larger or harder pieces of the food material not 

 sufficiently crushed by the jaws. Experimental evidence of this 

 function, however, is lacking, and Plateau (1874, 1876) argued that 

 the proventriculus is merely an apparatus for passing the food from 

 the crop into the stomach. More recently, Ramme (1913) has shown 

 that the proventriculus, in Orthoptera and Coleoptera at least, has 

 another important function in that the furrows between its chitinous 

 ridges serve to conduct the digestive secretions of the ventriculus into 

 the crop, where they attack the food material in advance of its en- 

 trance into the stomach. The channels between the proventricular 

 folds, then, rather than the folds themselves, are to be regarded as 

 having the primary functional importance. Otherwise, the proven- 

 triculus serves to conduct the food mass into the ventriculus. In 

 Dytiscus, according to Ramme, the armature of the proventriculus 

 retains the indigestible ]iarts of the food, which are later ejected 

 from the mouth ; but in Orthoptera all the food matter passes through 

 the alimentary canal. 



THE HYPOPHARYNX 



When the gnathal segments are added to the protoce])halun during 

 embryonic growth, their sternal parts lose their identities in the gen- 

 eral postoral ventral wall of the definitive head. On this region there 

 is developed a median lobe between the bases of the mouth parts 

 known as the hypopharyux (fig. 18 D, Hphy). The name is poorly 

 chosen, because the organ in question lies on an exterior surface of the 

 head entirely outside the pharynx, but it is a heritage of earlier days 

 in entomology and is now w^ell established in entomological ter- 

 minology. 



There is a diilerence of opinion among embryologists as to how 

 many of the gnathal sterna contribute to the formation of the hypo- 

 pharynx. According to Heymons (1901), the hypopharynx is formed 

 in insects on the sternal region of the mandibular and first maxillary 

 segments, but in the chilopods it arises on the mandibular segment 

 alone. The fusion of the bases of the second maxillae in insects, and 

 the similar union of both pairs of maxillary appendages in the chilo- 

 pods gives reasons for this view, but, as will be shown ]M-esently, the 

 primitive adductor muscles of all the gnathal appendages have their 

 origin on the hypopharyngeal region in the chilopods and in the 

 aptervgote insects — a condition which indicates that at least some part 



