446 



TEXT-BOOK OF ENTOMOLOGY 



folds looking towards the lumen of the tracheae." In Fig. 414, i, 

 are represented portions of several tsenidia showing the fissure, 

 which is sometimes interrupted ; at 2 are seen " the formation of 

 what may be called apertures in a chitinous bridge." Stokes re- 



gards the tsenidia as " in- 

 Avardly directed folds of 

 the membrane." Near the 

 spiracles the tracheal 

 membrane is externally 

 studded with minute pa- 

 pillae, as shown at 3, 

 where are represented 

 three broad and incom- 

 plete taenidia, with the 

 tapering end, or the be- 

 ginning, of another. 

 Stokes adds, "Here they 

 are only broad grooves, 

 with no appearance of 

 the narrow fissure of the 

 completed taenidium. At 



. fio. 11T , pr l nnrrirm rvf 



* 



-^Q internal SUl'faCC of a 



large trachea near the 



external orifice, the taenidia being in an incipient stage, evidently 

 forming more or less of a network, as is usually the case next to the 

 stigma " (compare p. 451, and Fig. 414). 



The tracheae of chilopod myriopods appear to be like those of insects. A 

 number of authors have failed to detect the spiral threads in the Julidae. As 

 to the Arachnida, several observers, including Menge and Bertkau, have denied 

 the existence of the spiral thread in the spiders with the exception of the Atti- 

 dae ; and MacLeod finds them " scarcely visible " in Argyroneta. 



Besides the tracheae, the salivary duct is kept permanently dis- 

 tended by taenidia, which, however, are not spiral. They usually 

 form incomplete rings, as in Stomoxys, arranged as shown in Fig. 

 410. 



The labella (proboscis) of flies are supported by incomplete chiti- 

 nous tubes or " pseudo-tracheae," the ends of which form the scrap- 

 ing teeth, this being, according to Dimmock, their primary function. 

 Dimmock describes them as cylindrical channels opening on the sur- 

 face in zigzag slits. These channels are held open by incomplete 

 rings, one end of which is forked. "These rings are apparently 



t 



FIG. 410. End of salivary duct in base of proboscis of 

 Stomoxys calcitrant : a, incomplete and irregular tenidia ; 

 &, two tflenidia making incomplete rings near the distal end 



