74 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



The minute tenaculum of the third abdominal segment (fig. 30 C) 

 looks like a miniature furcula, and likewise suggests that it has been 

 produced in the same manner by the union of the bases of a pair of 

 appendages. 



THE ABDOMINAL APPENDAGES OF THYSANURA 



It is in the Thysanura that the abdominal appendages best preserve 

 the fundamental structure characteristic of the abdominal appendages 

 of adult Pterygota, as shov^n in the gonopods of the latter group ; but, 

 as will be seen later, it appears that the abdominal appendages have 

 a more primitive form in the larvae of Ephemerida and in the larvae 



smcls 



grncls 



Gon 



A 



Fig. 31. — Diagrams of structure of abdominal appendages of Thysanura and 

 Pterygota. 



A, a typical pregenital apnendage. B, a gonopod, or genital appendage. 



gmcls, muscles of gonapophysis ; Gon, gonapophysis ; LB, limb basis, usually 

 a lobe or plate of body wall ; ri's, retractor muscles of eversible vesicle ; smcls, 

 muscles of stylus ; Stn, sternum ; Sty, stylus ; Vs, eversible and retractile vesicle. 



of certain holometabolous insects. The thysanuran appendages retain 

 most completely their independence in the Machilidae. 



The appendages of the pregenital region of the abdomen are 

 typically developed on each of the pregenital segments except 

 the first in Machilidae (fig. 4, II, VI), and those of each pair are 

 distinct from the small median sternal plate (Stn), though their 

 bases (LB, LB) are ankylosed with the latter, and are united medi- 

 ally with each other. Each appendage consists of a broad basal plate 

 (fig. 31 A, LB), of a stylus (Sty) borne by the distal free margin of 

 the basal plate, and of an eversible sac, or vesicle (Vs), lying mesad of 

 the stylus and retractile into the basal plate (fig. 4, II, Vs). The 

 posterior part of the basal plate projects from the ventral wall of the 



