Opinions vary as to the use of this portion of the digestive canal. Graber compares it to the gizzard of birds, and likens the action of the rosette of teeth to the finer radiating teeth of the sea-urchin, and styles it a chopping machine, which works automatically, and allows no solid bits of food to pass in to injure the delicate walls of the stomach (mid-gut).
He also states that the food when taken from the proventriculus is very finely divided, while that found in the œsophagus contains large bits.
Kolbe says that this view has recently been completely abandoned, and that the teeth are used to pass the food backwards into the chylific stomach. “But Goldfuss had denied the triturating action of the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum quorundam Œconomiam, 1843), stating that the contents of the same are already fluid in the gullet, so that the fore-stomach (Kaumagen) does not need to comminute the food” (Kolbe). In the Gryllidæ and Locustidæ, just before the posterior opening of the proventriculus into the stomach the chitinous lining swells into a ring and projects straight back as the inner wall of the cylindrical chylific stomach. The muscular layer forms two sac-like outgrowths or folds, which separate on the circular fold from the chitinous membrane. This apparatus only allows very finely comminuted food to pass into the stomach.
In the Acrydiidæ (Eremobia muricata) at the end of the proventriculus, where it passes into the stomach, is a small circular fold which hangs down like a curtain in the stomach.
The œsophageal valve.—Weismann[[50]] states that the origin of the proventriculus in the embryo of flies (Muscidæ) shows that it should be regarded as an intussusception of the œsophagus. While in the embryo the invaginated portion of the œsophagus is short, after the hatching of the larva it projects backwards into the mid-intestine. Kowalevsky also observed in a young muscid larva, 2.2 mm. in length, that the œsophagus, shaped like a tube, extends back into the expanded portion (proventriculus) and opens into the stomach (Fig. 315, A). In a larva 10 mm. long the funnel is shorter, the end being situated in the proventriculus (Fig. 315, B, pr). In the cavity between the outer (o) and inner wall (i) no food enters, and the use of this whole apparatus seems to be to prevent the larger bits of food from passing into the chylific stomach (Kowalevsky).
Fig. 315.—Œsophageal valve of young muscid larva: m, its opening: t, thickening of the cells; mes, mesoderm.—After Kowalevsky.
Beauregard has found a similar structure in the Meloidæ, and calls it the “cardiac valvule” (Fig. 318, Kl). It was observed by Mingazzini in the larvæ of phytophagic lamellicorn beetles, and Balbiani described it in a myriopod (Cryptops) under the name of the “œsophageal valvule.”
Gehuchten describes a homologous but more complicated structure in a tipulid larva (Ptychoptera contaminata), but differing in containing blood-cavities, as a tubular prolongation of the posterior end of the œsophagus which passes through the proventriculus and opens at various positions in the anterior part of the chylific stomach (Fig. 316).
The three layers composing this funnel are distant from each other and separated by blood-cavities, the whole forming “an immense blood-cavity extended between the epithelial proventricular lining and the muscular coat.”