SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 401 



There are two distinct ways in which the sucker of the 

 proboscis comes into operation, in drinking and in feeding on 

 solid substances ; in the former act the oral sucker is applied 

 to the surface of the fluid, which necessarily passes by capillary 

 attraction into the pseudo-tracheae. The mouth-opening de- 

 scribed by Reaumur is alternately opened and closed. When 

 it is open, a current passes directly into the mouth ; but when 

 it is closed, fluids may flow into the prestomum from the 

 pseudo-tracheal channels, driven into it by their elastic walls, 

 which exhibit quasi-peristaltic movements as the air passes into 

 and out of the air-channels. When, however, the insect feeds 

 on solids, the oral sucker is applied by its edges to the food, 

 and the pseudo-tracheae distribute the salivary secretion of the 

 lingual glands over the anterior surface of the sucker ; this 

 dissolves the food, assisted, perhaps, by the disintegrating 

 action of the stomal teeth, which, as Suffolk [58, p. 339] 

 remarked, leave lines upon the food material ; and the fluid, 

 with small particles in suspension, is drawn into the oral 

 cavity. Under these circumstances, as I formerly suggested 

 [62], the pseudo-tracheae probably conduct it into the pre- 

 stomum and mouth and act as strainers. 



That the little pouches of thin integument in the forks of the 

 rings, which Anthony regarded as suckers, are the main agents 

 of suction, as he appears to suggest, is an untenable hypothesis ; 

 but that they may assist in the passage of fluid into the pseudo- 

 tracheae is, I think, exceedingly probable. Flies undoubtedly 

 collect moisture from damp foliage and smooth surfaces which 

 have only a film of moisture on them, and it appears to me pro- 

 bable that these pouches may be alternately emptied and filled 

 by the alterations in the tension of the oral lobes which occur 

 during the ingestion of food. The disposition of the sacs is such 

 that it is easy to see that they may alternately open on the sur- 

 face, and into the pseudo-tracheal channels. Moreover, capillary 

 attraction may assist in filling the pseudo-tracheae through the 

 minute orifices of these pouches. 



The mouth frequently contains air, whether drawn into its 

 cavity by suction or expelled from the alimentary canal, and 



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