1919] Walker: Structure of Orthopteroid Insects 307 



pleural membrane; (3) neither the seventh nor the eighth 

 sterna are prolonged into a sub-genital plate; (4) the cerci 

 are segmented; (5) the paraprocts are feebly chitinized, The 

 last feature has no special significance; the others indicate the 

 primitive nature of both groups, but are otherwise negative 

 in value, when taken alone. 



No relationship to the Dermaptera or Orthoptera is even 

 hinted at in the terminal abdominal segments of the females 

 of Embiids. 



Plecoptera. 



As Crampton and others have pointed out, this order is in 

 some respects the most primitive of existing Pterygote insects, 

 particularly in the cervical and thoracic sclerites, wing venation 

 and cerci. In the abdominal segments, of which ten are well 

 developed, there is a tendency in many forms towards a con- 

 siderable degree of chitinization of the pleural membrane. 

 The ninth and tenth segments may be quite ring-like, even 

 in the adult, while in the nymph all the segments may be 

 annular. Crampton's suggestion that this annular form of 

 segment may be a primitive one seems to me untenable. It is 

 too exceptional among the Tracheata, and even within the 

 Plecoptera there are all grades of chitinization of the pleural 

 membrane. It is moreover, explained by the non-functional 

 character of the abdominal spiracles in the nymph, in which 

 respiration is performed by the tracheal gills, these structures 

 sometimes (e. g., Pteronarcys) persisting in the adult in a 

 reduced form. 



The abdominal spiracles are all pleural in position, the 

 eighth sternum is frequently more or less prolonged to form a 

 subgenital plate, sometimes overlapping the ninth, or even the 

 tenth, sternum. In some forms (e. g., Megarcys signata 

 Hagen)^^ it is bilobed or bifid at apex, these lobes being slightly 

 suggestive of vestigial ventral valvular. In most species of 

 Pteronarcys there is no backward extension of the eighth 

 sternum as a whole, but it bears a pair of slender processes, at 

 or near the hind margin, which are probably true representatives 

 of these valvules, being very similar to these structures as met 



23 Klapalek, Fr., Coll. Zool. Selys., Fasc. IV, p. 12, Fig. G (1912). Smith, Lucy 

 Wright, Trans. Am. Ent. Soc, Vol. XLIII, pp. 433-489, Pis. XXIX-XXXIV (1917). 



