PLECOPTERA OF NORTH AMERICA 29 



genital plate. The lateral stylets, directed backward ana 

 spread apart with the change of position serve as a guide 

 to the large soft fleshy penis that issues from the apex of 

 the 9th segment beneath and rises and curves forward 

 following the curvature of the ventral surface of the 

 supra-anal process, passing between the stylets, its tip 

 protected by the long membranous flap appended to the 

 tip of that process. The penis is a large erectile cylindric 

 organ a third as large in diameter as is the abdomen. It 

 is soft and white except for a sub-apical ring of thin pale 

 brown chitin. This ring is thinner on the ventral side, 

 where arise a pair of soft, slender, delicate, divergent, 

 white palpiform sensory organs, each of which has a 

 length equal to the diameter of the penis. The subanal 

 plates do not appear to be involved in the copulatory ap- 

 paratus of this type. 



Female. — Only the sternite of the 8th abdominal seg- 

 ment and the portion of the 9th segment adjacent thereto 

 are modified as accessory organs of reproduction in this 

 sex. The oviduct terminates midventrally between these 

 two segments and the 8th sternite is generally prolonged 

 posteriorly into a subgenital plate that overlaps the aper- 

 ture. The form and size of this plate, and the degree of 

 its demarcation from the remainder of the sternite fur- 

 nish useful characters for the discrimination of species. 



Since many species are known only from the female, 

 which sex in its structure offers fewer clues to relation- 

 ships, it may well occur that some of the species will not 

 have been placed next to the ones with which they are 

 most closely allied. 



Tails or cerci. — Excepting in the family Nemouridae, 

 the tails are long, many-jointed and tapering, the seg- 

 ments reaching their maximum number in the larger 

 species of Acroneuria, where the basal segment is very 

 large, and several of the close-ringed following segments 

 are ill-defined. Beyond the base the segments regularly 

 increase in length to rearward, the terminal segments 

 being from three to six times as long as wide. The pubes- 

 cence covering the segments varies much in length and 

 density and is in some genera interspersed with apical 

 verticils of long bristles, that become conspicuous in Per- 

 linella and its allies. 



The reduced, mostly one-segmented, tails of the Ne- 

 mouridae, often remarkably modified as accessory organs 

 of reproduction, are shown for all our species on plates 34 

 to 46. 



