47 



labella. These margins of the pseudotracheae consist of the lobes, or teeth, to 

 be seen at each side in pi. 4, fig. 7. The ring-like structure of pseudotracheae 

 has the same function as the similar structure in the tracheae of insects, or in the 

 wind-pipe of vertebrates, that is the function of holding tubes open, and preventing 

 their sides falling in upon one another. The same peculiar ring-like structure 

 is als9 found, with like functions, in the salivary ducts of diptera. If the labellae 

 are now alternately inflated, a little more and then a little less, the width of 

 the zigzag openings of the pseudotracheae is alternately increased and diminished, 

 and the teeth along the margins of the cleft naturally scratch on the surface 

 on which the fly has pressed his labellae. The same scratching effect is produced 

 when the labellae are simply moved a little back and forth upon any surface. 

 The labellae are, then, files of which the teeth are the serrate edges of the 

 pseudotracheae. During the scratching process the pseudotracheae are, in Musca, 

 moistened with saliva, with which they wash the surfaces of the substance to be 

 scratched or dissolved away. Lowne 23 says of the pseudotracheae (p. 47-48) 

 "these form a fine strainer through which the insect is enabled to filter the fluid 

 from the solid portion of the substances on which it feeds." In Bombylius 

 which offers excellent facilities for studying the pseudotracheae, because they, 

 altho constructed on the same principles as, are more firm and consistent 

 than, in Musca, I have found that, after feeding the fly with a mixture 

 of sugar and gum-arabic, colored with carmine, and then plunging it 

 suddenly into strong alcohol to fix the colored solution in its mouth- 

 parts, that, later, when I cut sections of its labellae, the pseudotracheal teeth, 

 instead of having their ordinary position (see pi. 2, fig. 5, a; in section, fig. C. 

 a), were turned outward from the pseudotracheal channel, into the position seen 

 in pi. 2, figs. 5 and 6, b, and that the colored solution of gum-arabic had not 

 entered the pseudotracheae. Boinbylius also rubs the labellae over the surface 

 of the substance on which it is feeding, in a way similar to that which Musca 

 does. It can be seen that the more purely liquid food any species of diptera 

 eat, for a constant diet, the less necessary and the less developed are the 

 pseudotracheae; Culex, for example, eating a purely liquid food, has no pseudo- 

 tracheae. From the above facts, I am led to think that the primary function, 

 at least, of the pseudotracheae of diptera is to file away substances on which 

 they feed. That the pseudotracheae may have other functions, I do not wish to 

 affirm or deny. 



The pharyngeal sucking organs of the four different genera of diptera which 

 I have studied have been so fully compared in the comparison of the fulcra in 



