PLECOPTERA 



327 



The pterosligma. — In most members of this order a specialized pterostigma has 

 not been developed; but the term pterostigma is commonly applied to the cell 

 beyond the end of the subcosta and between the costa and vein Ri, even though 

 it is of the same color and texture as the remainder of the wing. 



The basal anal cell. — A very constant feature of the anal area of the wings of 

 Plecoptera is the presence of a cross- vein near the base of the wing, which extends 

 from the first anal vein to the second. The cell that is closed by this cross- vein is 

 termed the basal anal cell (Fig. 376^, ba). 



The females drop their eggs in a mass in water. I have taken 

 females of Perla and of Pteronarcys at lights, each with a mass of 

 eggs hanging from the abdomen. 



The metamorphosis is incomplete. The immature forms are all 

 aquatic. These naiads are common on the lower surface of stones in 

 rapids. They can be found easily by lifting stones from such situations 

 and turning them over quickly, when the na- 

 iads will be found clinging to the stones 

 with their fiat bodies closely appressed to 

 them and their legs, antenna, and cerci ra- 

 diating on the surface of the stone , but they 

 are apt to run away quickly. 



The naiads of stone-flies live only in 

 well-aerated water; they are not found in 

 stagnant water or in foul streams. They are 

 said to feed on other aquatic insects, includ- 

 ing smaller individuals of their own species; 

 but according to the observations of Dr. 

 P. W. Claassen they are largely vegetable 

 feeders. 



The body is depressed (Fig. 375); the 

 antenna are long, so too are the cerci Most 

 species possess tracheal gills, situated usually 

 on the ventral side of the thorax just be- 

 hind the base of each leg ; but tracheal gills 

 are found in some species either on the un- 

 der side of the head, on the basal abdom- 

 inal segments, or at the tip of the abdo- 

 men. A large nimiber of the smaller species are destitute of 

 tracheal gills; in these the air supply is absorbed through the thin 

 cuticula of the ventral surface. The colors of naiads are often brighter 

 than those of adults. 



When full-grown the naiads leave the water and transform on some 

 near-by object. The empty exuviae are often found clinging to stones 

 or logs projecting from water or on the banks of streams. 



According to a recent classification of this order, that of Tillyard 

 ('21), it includes seven families; but only four of these families are 

 represented in our fauna. A monograph of the North American 

 species of the order is in preparation by Professor J. G. Needham 

 and Professor P. W. Claassen; this is nearly completed and probably 

 will be published soon. The four families of our fauna can be separat- 

 ed by the following table. 



Fig. 375. — Naiad of a 

 stone-fly, Acroneura. 



