38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



that they have little resemblance to the simple organs of the Ephem- 

 eroptera. Distally each organ splits into a median tube containing 

 the exit duct and a strong outer lobe. In some species the two penes 

 are united in a single organ with three terminal lobes, the median 

 one giving exit to the two ducts, or to only a single duct if one duct 

 is reduced and nonfunctional. The Dermaptera thus give an example 

 of the modification potential of paired penes. 



The division of each penis in the Dermaptera into a mesal lobe 

 containing the duct and into a clasperlike lateral lobe is suggestive 

 of the division of each primary phallic lobe of the higher insects into 

 a mesomere and a paramere. In the latter, however, the ducts never 

 enter the mesomeres but unite with a common duct formed between 

 their bases. 



The Thysanura in the adult stage have a single median penis aris- 

 ing between the bases of the ninth segment stylus-bearing plates ; 

 into the base of this the two genital ducts open by a common orifice. 

 The penis, however, is developed from two small primary lobes that 

 become concave on their opposed surfaces and unite to form the 

 tubular organ of the adult. The penis lobes have no connection with 

 the stylus-bearing plates of the ninth segment, and thus would ap- 

 pear to belong to the tenth segment. Heymons (1899) has noted that 

 in fact the embryonic ducts of Lepisma end in the tenth abdominal 

 segment. 



In the Orthoptera and the higher insect orders, a new type of 

 genital apparatus appears. It likewise begins as a pair of simple 

 lobes, but the lobes do not give exit to the genital ducts. The paired 

 ducts open into a single duct that grows inward between the lobes 

 and becomes the unpaired ejaculatory duct of the adult. It is perhaps 

 possible that the two lobes in this case are the paired penes of the 

 lower insects from which the ducts have withdrawn, but there is no 

 direct evidence for such. A common idea has been that the primary 

 genital lobes are rudiments of appendages serially homologous with 

 the embryonic limb vestiges of the pregenital abdominal segments 

 and the thoracic legs. They arise, however, close together on the 

 venter of the ninth segment in nymphal or late larval stages after 

 the disappearance of the embryonic limb vestiges. Their position 

 on the ninth segment is always behind the area of the sternal plate. 

 Possibly, therefore, they belong to the tenth segment as do the penes 

 of the lower insects, and have been moved forward into the "inter- 

 segmental" membrane of the ninth segment. In any case, the future 

 history of these primary genital lobes has no counterpart in the lower 

 insects. 



