76 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



styliger plate is present in the larva (fig. 25 M), where, though it 

 may diiTer in size and shape from that of the adult, it is already 

 separate from the ninth sternum and gives origin to the stylus muscles. 



The styli are slender appendages varying somewhat in relative 

 length and thickness in diiTerent species. They are commonly jointed, 

 i. e., subdivided into several parts called " segments ", but the joints 

 are often mere constrictions, and the intervening parts are not true 

 segments since they are never musculated. Usually each stylus has 

 three subsegments (fig. 25 G-J, L), but in some cases there are four 

 (A), in others only two (K), and in the genus Caenis the styli are 

 undivided. The larval styli may be simple unjointed processes (M, 

 Sty), or they may be subdivided, but, according to Speith (1933), 

 the number of parts is always less than that in the adult stylus of the 

 same species. The basal part of the stylus in some cases might be 

 confused with the coxopodite, or vice versa (A, I, J), and it is only 

 by an examination of the musculature that the two parts of the appen- 

 dage can be certainly identified : the stylus muscles always take their 

 origin in the coxopodite, whether the muscle-containing part of the 

 latter is a free lobe (I, J) or a part of the styliger plate (A, H). 



The posterior position of the mayfly claspers on the ninth abdomi- 

 nal segment may be difficult to reconcile with the idea that the styli 

 are serial homologues of the laterally placed gills of the more anterior 

 segments in the larva (see Snodgrass, 1931), and yet the evidence is 

 equally strong (or weak) in each case that the organ in question is 

 a derivative of a segmental appendage. Perhaps the truth is that some 

 of our ideas about insect morphology are not to be taken too seri- 

 ously, but in their favor it may be said that, where inconsistencies are 

 not too inconsistent, they establish a fundamental concept on which 

 may be based a uniform terminology. The simple form of the genital 

 appendages in certain larval Ephemeroptera (fig. 25 M) connects 

 these organs with the styli of Thysanura and Orthoptera, and there 

 can be little doubt that the ephemerid styli, on the other hand, repre- 

 sent the movable genital claspers of higher insects, though in the 

 latter the organs seldom have a styluslike form. It is tempting to 

 regard the abdominal stylus in any case as the telopodite of a simpli- 

 fied appendage — the only objection to so doing is the presence of a 

 similar coxal stylus on the thoracic legs of Machilis in addition to 

 the true telopodite. 



The penes of the mayflies vary in shape and relative size in dif- 

 ferent species (fig. 25 B-H, J, K, Pen) and are often provided with 

 accessory processes (C, D). They may be united with each other in 

 varying degrees (D, E, H), but there are always two ejaculatory 



