NO. 6 INSECT ABDOMEN — SNODGRASS IQ 



appear to form small suspensional sclerites of the first valvulae, known 

 as the valvifers, which always lack styli. Gonapophyses of the eighth 

 segment are known to be present in male insects only in some species 

 of Machilis, but the eighth segment of the male is frequently more or 

 less modified when associated with the ninth in the copulatory 

 mechanism. 



The ninth segment. — The second genital segment (fig. 6, IX) usu- 

 ally has less of the typical form than does the first. It is the somite of 

 the second gonapophyses (^Gon), or genital processes of the ninth 

 gonopods, which form the second valvulae of the ovipositor in the 

 female, and the usual parameres in the male. The sternum of the 

 ninth segment is generally reduced or rudimentary in the female, but 

 the bases of the gonopods are commonly retained, either in the form 

 of lobes, or as blade-like pieces of the ovipositor, the third valvulae. 

 In the male the ninth segment retains a generalized structure in the 

 Thysanura (fig. 33, B, C), but in the pterygote insects it is subject 

 to many modifications and takes on a great variety of forms. The 

 bases of the gonopods in male Pterygota either remain as distinct 

 lobes of the segment, or they unite with each other, or with the 

 sternum, or with both the sternum and the tergum. The styli, if re- 

 tained, usually take the form of movable clasping organs. Various im- 

 movable lobes also may be developed from the ninth segment of the 

 male, and sometimes from the eighth, which serve as accessory organs 

 in copulation. 



The intromittent organ of the male arises in the Thysanura behind 

 the region of the ninth sternum and between the bases of the gono- 

 pods (fig. 33 B, C, Pen) ; but in insects having the gonopod bases 

 united with the sternum, it arises posterior to, or usually above, the 

 limb base area of the composite sternum. The membranous area 

 from which the organ arises is, furthermore, generally more or less 

 inflected between the ninth and tenth segments, forming a genital 

 chamber above the ninth sternum, and the ninth sternum is often 

 extended posteriorly as the male subgenital plate, or hypandrium. 

 The intromittent organ has the form of a simple, tubular penis in the 

 Thysanura, but in most pterygote insects it is a more complex struc- 

 ture, called the aedeagus, formed of the penis and parameres, or 

 of the parameres alone. The external genitalia will be fully described 

 in Part II of the present paper. 



THE POSTGENITAL SEGMENTS 



Beyond the second genital segment there are never more than three 

 segments represented in the hexapod abdomen (fig. 6, X, XI, XII), 



