60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



Among the mayflies as well as in the Protura and Dermaptera the 

 penes show a tendency to comhine into a single organ (penis con- 

 junctus) as they do also in some Crustacea and Diplopoda. 



The usual median intromittent organ, or phallus, of adult insects 

 is evidently a penis coinuiunis, since neither in its structure nor its 

 ontogeny is there any evidence that the organ has been produced by 

 the union of a pair of rudiments containing the outlets or ends of the 

 lateral genital ducts. It is formed as a tubular outgrowth of the body 

 wall around the mouth of the ejaculatory duct, or by the union of 

 phallic lobes that enclose the gonopore. The theory suggested by 

 Crampton (1920), and formerly accepted by the writer ( 193 1, p. 91), 

 that the phallus has been evolved in part or entirely from mesal lobes 

 of the appendages of the ninth abdominal segment, appears to be 

 quite without anatomical or ontogenetic foundation. The genital ap- 

 pendages are variously and often highly developed, but they take no 

 part in the formation of the phallus. 



The phallic organ of insects is subject to endless variations in 

 form, and it is not to be supposed that the numerous accessory struc- 

 tures encountered, or even the more general modifications are neces- 

 sarily homologous developments in dififerent groups of insects, for it 

 must unquestionably be true that, with apparently unlimited variation, 

 similar forms have been many times produced quite independently. 

 Furthermore, as will later be shown, it is very probable that the 

 median penis itself has been evolved in different ways in several 

 insect groups. The musculature of the phallus, even among closely 

 related insects, is so highly variable and so evidently adaptive in its 

 nature, that it cannot be used for determining anatomical relation- 

 ships or homologies in the phallic structures. 



The intimate association of the phallus with the ninth abdominal 

 segment has involved this segment so closely in the genital function 

 that usually it is the segment of the genital region most affected by 

 structural modifications adaptive to the mechanism of copulation and 

 intromission. The ninth segment, in fact, may be designated the 

 genital segment of male insects. The neighboring segments, however, 

 are often variously modified also as parts of the genital complex, 

 and in the higher Diptera all the segments beyond the fifth form a 

 distinct genital section of the abdomen. A point that should be empha- 

 sized in the study of the male genitalia of insects, particularly in 

 Orthoptera, is the fact that the structure of the phallic organ and of 

 the genital segment is not necessarily, in its principal aspect, an adapta- 

 tion to the functions of copulation and intromission, but may be very 

 largely concerned with the formation of spermatophores and the trans- 

 ference of the latter to the female. 



