LABIUM OF ANAX JUNIUS. 287 



structures, and was consequently led astray regarding most of the 

 major functions, i.e., extension and flexion of the labium and the 

 abduction of the lateral lobes. It may further be stated that he 

 applied no modern system of nomenclature to the muscles and 

 other structures described. 



Neither of these authors published adequate figures of the 

 larval structures. All available illustrations of the imaginal labium 

 are concerned solely with the skeletal parts, and the transitional 

 stage has never been figured. 



For these reasons an investigation of the structure of the 

 labium of the larva and of the adult and its transitional condition 

 has seemed worth while. 



MATERIALS AND METHODS. 



The specimens of Anax junius studied were fixed in Bouin's 

 fluid or injected with 95 per cent, alcohol. Both fluids gave good 

 results, though the yellowing of the muscles by the picric acid made 

 it possible to clear and mount dissections without resorting to 

 further staining. In fact, the definition and relations of the 

 muscles in such preparations was often superior to those stained 

 in borax carmine. Specimens were fixed in both the extended 

 and flexed positions. 



While a part of the work was done from cleared and mounted 

 specimens, details of the structure were obtained much more ef- 

 ficiently by the dissection of alcoholic materials under a Greenough 

 binocular microscope. By this method the parts could be pinned 

 down in 70 per cent, alcohol in a translucent paraffin tray for dis- 

 section with needles, and viewed alternately by reflected and trans- 

 mitted light. Care was taken to clear away all fat or coagulated 

 body fluids which might interfere with clear vision. Adult labia 

 were cleared and studied in oil without mounting. 



Most of the drawings were outlined under the camera lucida, 

 though a few were made to measurement. 



THE LABIUM OF THE LARVA. 

 General Structure. 



The labium of a full grown larva is between sixteen and twenty 

 millimeters long when extended (Fig. i). It consists of two main 



