NO. 14 INSECT ABDOMEN SNODGRASS 87 



grooved on its inner surface, the two apposed processes thus form- 

 ing a channel through which the spermatic fluid is conveyed from the 

 ejaculatory duct. The four processes are well developed in Leuctra 

 siblcyi (fig. 29 M, /, g), in which collectively they appear as a large 

 arm projecting anteriorly above the end of the abdomen from the 

 up-turned sternum of the tenth segment (L). The longer median 

 in-ocesses (M, g) are flattened and slightly concave on their apposed 

 faces, and are movably attached at their bases in notches of the tenth 

 sternal plates (XS). The slenderer, tapering lateral processes (/) 

 are direct continuations of the sternal plates, but, owing to a partial 

 membranization of the latter, they can be turned outward by sternal 

 muscles inserted laterad of their bases. The genital opening of 

 L. sibleyi appears to be on a small phallic papilla (L, M, Phi) in the 

 membranous area between the ninth and tenth abdominal sterna at 

 a point where the discharged sperm might enter the channel between 

 the sternal processes of the tenth segment. The median exit duct is 

 shown by Klapalek and by Mertens to be very short in species of 

 Leuctra (fig. 28 A), with a median stalked " vesicula seminalis " aris- 

 ing from it at the junction with the vasa deferentia. 



The external genital armature of male Plecoptera consists of secon- 

 dary outgrowths or lobes of the posterior abdominal segments. 

 Though some of these structures are movable by a special adaptation 

 of the segmental muscles, it is clear that none of them represents the 

 styli of the apterygote insects and Orthoptera, or the stylus derivatives 

 in the form of independently musculated claspers, or harpagones, of 

 the Ephemerida and higher insects, which so commonly are the most 

 important features of the periphallic genitalia. In the evolution of 

 the Plecoptera, therefore, the usual accessory genital structures, in- 

 cluding the ovipositor of the female, have been discarded, and in the 

 male there has been developed a profusion of secondary structures. 



The principal accessory genital developments of male stoneflies 

 pertain to the tenth and eleventh abdominal segments, but lesser struc- 

 tures occur frequently on the ninth segment, and sometimes on the 

 eighth. Among the species here illustrated, examples of tergal proc- 

 esses on the eighth and ninth segments are seen in Leuctra sibleyi 

 (fig. 29 L, a) and in Pteronarcys proteus (], b). In Acroneuria the 

 ninth sternum bears a smooth round or oval elevation near its pos- 

 terior margin (B, D, E, c) known as the percussion disc, or hammer, 

 with which the male is said to make a drumming sound by tapping on 

 the surface beneath him (see Macnamara, 1926). Most of the 

 Nemouridae have a small elongate ventral lobe flexibly attached to 

 the base of the ninth sternum (I, K, L, M, d). The posterior margin 



