146 THE INTEGUMENTAL SKELETON OE THE IMAGO. 



The Pseudotracheae are cylindrical channels on the oral surface 

 of the disc. These channels are sunk more or less deeply in 

 the disc, according to the degree of its inllatiGn. When the 

 disc is fully inflated they open by zigzag, longitudinal fissures 

 on its oral surface. 



The channels of the pseudotracheae are kept open by elastic 

 chitinous rings, which somewhat resemble the rings in the 

 tracheal vessels, hence the name pseudotracheae. 



The pseudotracheae vary in number from twenty-eight to 

 thirty-two pairs. They may be described as forming three 

 sets. The seven anterior on each side unite and form a single 

 trunk, which opens into the prestomum between the first and 

 second teeth. Ten to twelve intermediate pseudotrachejE run 

 straight from the margin of the disc directly into the prestomum. 

 The remainder unite and form a trunk, which runs nearly 

 parallel to its fellow, and opens into the prestomum at its 

 shallow ventral extremity. 



The Teeth (Fig. 31) are oblong plates of chitin, bifid at their 

 free extremities. There are three rows of teeth on each side of 

 the prestomum. The teeth of the innermost row are free, except 

 at their bases, which are connected with the lateral plate of the 

 discal sclerite. The intermediate teeth are only free one- 

 third, and the outermost one-sixth, of their length. 



The free margin of each tooth near its junction with the 

 discal sclerite is thickened, and is continued as a ridge, which 

 diverges from the base of the tooth on the chitinous wall of the 

 prestomum. 



These ridges are parallel with each other and with the margins 

 of the openings of the pseudotracheae, the rings of which appear 

 to be of the same nature as the ridges supporting the teeth 

 (Kraepelin [70]). 



The teeth of the inner row are much stronger and thicker 

 than those of the outer rows, and they lie in the grooves between 

 the ridges which support the intermediate row of teeth. The 

 arrangement will be best understood by the accompanying 

 figure (Fig. 31), or by a study of the proboscis itself. 



The details of the structure of the pseudotrachea; and of the 



