56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



are distinct, though only the second hears a pair of small, sometimes 

 two-segmented gonopods, and the terga of the two segments are 

 united in a single dorsal plate. In other forms the second sternum 

 is either much reduced or obliterated, but the gonopods of the first 

 segment or of both segments may be present. Both genital sterna 

 appear to be retained in the male of Scolopocryptops (fig. 20 B, D, E), 

 though the second {2gS) is very small and is absent in the female (C). 

 A single tergal plate covers the back of the genital region of the male 

 (D, gT). Neither pair of gonopods is represented, but a large median 

 penis (B, D, E, Pen) is protractile from a bursa genitalis (genital 

 chamber) invaginated above the genital sterna. In Lithobius there 

 is a single large genital sternum in each sex, which presumably is 

 that of the first genital, or pregenital, segment (F, G, gS). A pair 

 of well-developed, three-segmented gonopods is present in the female 

 of the species figured (F, H, Gp), but in the male (G) the appendages 

 are rudimentary. The penis is a small conical organ opening ventrally 

 by a median slit (I), and is ordinarily almost entirely concealed in 

 the bursa genitalis (G). The male of Scittigera forceps (J) has two 

 pairs of styluslike gonopods, the first borne by the pregenital sternum 

 (igS), the second by a small membranous fold possibly representing 

 the venter of the second genital segment. In the female (K) only 

 the anterior gonopods are present, but these appendages have the form 

 of long two-segmented claspers borne by the large pregenital sternum 

 (igS). Copulation in the chilopods has not been observed, but the 

 males of some species are known to produce spermatophores, and 

 spermatozoa are found in the seminal receptacles of the female. 



The chilopods resemble the hexapods in the possession of a median 

 penis and in the modification of the appendages in the genital region 

 to form accessory reproductive organs. The fact, however, that quite 

 dififerent somites may be involved in the development of the genital 

 complex eliminates the question of homology between the parts, and 

 the superficial likeness in the external genitalia of the Chilopoda and 

 Hexapoda is thus no evidence of close relationship between these 

 two groups. 



X. HEXAPODA 



The fundamental feature in the structural organization of the 

 Hexapoda that distinguishes the six-legged arthropods from members 

 of all other arthropod groups is the concentration of the locomotor 

 mechanism in the three body segments immediately following the 

 fourth postoral somite. The Hexapoda, therefore, include the Protura 

 and CoUembola as well as the ordinary insects. The number of seg- 



