44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 09 



embryonic abdominal appendages, as in the larvae of Hymenoptera, 

 is to be regarded merely as a case of retarded development in organs 

 that are used in their secondary function only in the adult stage of the 

 insect. The genital structures in such cases are developed from " im- 

 aginal disks ", which are rudiments of primitive limbs. Evidence that 

 the " law " against rehabilitation of a lost structure does not apply in 

 ontogeny seems to be clearly demonstrated by the Acarina, in which, 

 it is said, the fourth pair of legs, present in the early embryo, are sup- 

 pressed long before hatching, and are fully restored in a late nymphal 

 stage. In some of the lower insects the genital processes, as described 

 by Wheeler (1893) in Conoccphalus, are derived directly from the 

 persistent rudiments of the abdominal appendages. 



The characteristic features of the pterygote ovipositor and the prob- 

 able correspondence between the parts of the pterygote and thysanuran 

 ovipositors are briefly summarized in the following paragraphs : 



1. The pterygote ovipositor consists of two or three pairs of proc- 

 esses (the first, second, and third valvulae), and of two pairs of basal 

 plates (the first and second valvifers) belonging to the eighth and 

 ninth abdominal segments, and sometimes includes intervalvular 

 sclerites of the ninth segment. 



2. The first and second valvulae are attached to the anterior ventral 

 angles of the first and second valvifers, respectively ; they correspond 

 with the first and second gonapophyses of the Thysanura, and in some 

 insects each has a short muscle inserted on its base, which arises in 

 the corresponding valvifer. The shaft of the ovipositor in most insects 

 is formed of the first and second valvulae alone, but in most of the 

 Orthoptera the third valvulae enter into the composition of the shaft. 



3. The first valvifers belong to the eighth segment of the abdomen, 

 and are provided with dorsal muscles arising on the eighth tergum. 

 Typically they are located in the pleural areas of the eighth segment 

 between the tergum and the sternum, but usually they are more or less 

 displaced posteriorly, and may be more closely associated with the 

 ninth tergum, or with the second valvifers. The first valvifers cor- 

 respond with the stylus-bearing plates of the eighth segment of Thy- 

 sanura, and are therefore the bases of the first gonopods ; in pterygote 

 insects they always lack styli. 



4. The second valvifers belong to the ninth abdominal segment, 

 and are provided with muscles arising on the ninth tergum. They are 

 always located between the lower edges of the tergum and the narrow 

 median venter of the ninth segment. The second valvifers, together 

 with the third valvulae, correspond with the stylus-bearing plates of 

 the ninth segment of Thysanura, and are therefore derived from the 



