70 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



The external genitalia of the Protura consist of a large bifid organ 

 in each sex eversible from behind the sternum of the eleventh abdomi- 

 nal segment. In the female the common oviduct opens between the 

 bases of the arms of the genital organ ; in the male the lateral gono- 

 ducts traverse the arms and open separately near their apices (see 

 Berlese, 1910; Prell, 1913 ; Tuxen, 1931). Nothing is known of the 

 development of the proturan genital organ; its structure suggests an 

 origin from a pair of organs. Crampton (1918) says its two arms 

 are " doubtless homologous " with the paired penes of Dermaptera ; 

 but the different segmental relations of the organs in the two cases 

 (with the eleventh abdominal segment in Protura, and the tenth or 

 ninth in Dermaptera) would preclude any question of actual homology. 

 The female has no seminal receptacle, and the mating habits of the 

 proturans have not been recorded. 



DIPLURA AND THVSANURA 



The compound structure of the gonad first appears among the 

 Hexapoda in the Diplura (Entotrophica) and the Thysanura (Ecto- 

 trophica). In Campodca the ovaries and the testes consist each of a 

 single tube, as do the testes of Japyx, but in other members of both 

 orders the gonads are compound, the number of tubules varying from 

 two to seven on each side. In some cases, as in the female of Japyx, 

 the gonadial tubes are segmentally arranged on the duct, suggesting 

 that the compound structure of the gonad originated by the outgrowth 

 of segmental diverticula from an elongate gonadial sac, each diver- 

 ticulum carrying in its apex a part of the original germarial band. 

 The gonoducts discharge through a single median aperture, which in 

 the female is between the eighth and ninth abdominal segments, and 

 in the male between the ninth and tenth. The male gonopore may be 

 on a small integumental papilla in the Diplura ; the Thysanura have 

 a short tubular penis. 



It is from the Thysanura that we derive the best evidence for the 

 generally accepted view that the definitive sternal plates of the insect 

 abdomen are in most cases composite structures formed in each seg- 

 ment by a continuous sclerotization of the areas of the true sternum 

 and the flattened limb bases. In Macliilis the venter of each abdominal 

 segment anterior to the genital region is occupied by a small, anterior, 

 median, triangular sclerite (fig. 24, A, Stn), and two large lateral 

 plates (Cxpd) projecting posteriorly as free lobes and united with 

 each other behind the median sclerite. The median sclerite is pre- 

 sumably the primary sternum of the segment. The lateral plates 

 appear to be the bases (coxopodites) of the segmental appendages; 



