NO. 6 



INSECT ABDOMEN — SNODGRASS 



13 



branous fold from the rest of the plate, forming a distinct sternal 

 apofome (fig. 24 D, Apt). Apodemal processes to give more effective 

 action to protractor and dilator muscles are commonly developed from 

 the anterior and lateral margins of the abdominal sterna. 



Notwithstanding the apparent unity of structure in the abdominal 

 sterna, it is probable, as already stated, that the ventral plates of the 

 ])regenital segments in most adult insects are triple structures, each 

 including in its composition the area of the true sternal sclerotization 

 of its segment, and the areas of the limb bases of the corresponding 



Fic. 4. — \'entral plates of al)dunien of Ncsoinachilis niaoriciis. 



I, II, VI, VIII, ventral plates of segments one, two, six, and eight in male. 

 LB, limb basis, or basal plate of appendage ; n^s, muscles of retractile vesicle ; 

 Stn, primitive sternal sclerite; Sty. stylus; /'.v. eversii)le and retractile vesicle. 



segmental appendages. A comparatively generalized condition 'is to be 

 seen in larvae of Ephemerida, in which lateral lobes of the abdomen 

 supporting the gills (fig. 34 A. ]^, LB), though forming a part of the 

 ventral wall of each segment, are distinct from the areas of the primary 

 sterna {Stn), and occu])y the primitive position of limb bases on the 

 sides of the segments between the tergal and sternal sclerotizations. 

 A similar but less ])riniitive condition is that occurring in some of the 

 Thysanura, as in the Machilidae (fig. 4), where each of the definitive 

 sterna in the pregenital region of the abdomen consists of a small, 

 median, true sternal sclerite (Stn) and of two, large, lateral stylus- 

 bearing plates {LB, LB) clearly representing the limb bases. 



