External Anatomy of Imago.
The imago (Pl. 39, Fig. 28) varies in size, but is on the average about 3.3 cm. in length, and in general shape and appearance resembles the nymph. It is a very swift runner, but a poor flier, is olive green in colour, and is covered with a pubescence of hairs which prevents it from becoming wet when in contact with water. When at rest, the wings are folded flat over the back, the right wing over the left in both cases, so that only the basal portion of the left forewing is visible.
This is similar in every respect to that of the nymph, showing the same sclerites and arrangement, but is attached directly to the thorax, the cervicum being reduced.
The Mouth Parts.
These resemble the nymphal structures, but are reduced in size. The mandibles are less heavily chitinised than those of the nymph. All parts exhibit the full number of sclerites. The two lobes of the hypopharynx have lost their suture and appear as one structure.
The thorax differs only in the distal extremities of its appendages, the legs, in that the meso- and metathorax bear wings, and that the thoracic spiracles are now open. The tibia (Pl. 35, Fig. 19) bears a tibial spur, but the aquatic adaptations of the nymph are now lost and replaced by tarsal pads, and the distitarsus has developed an empodium. The terminal ungues persist, but there are no pulvilli.
The only difference in this region is the development of the external genitalia. The five abdominal gills persist as withered appendages, but are still provided with tracheae from the main longitudinal trunks. Newport (1851) described gills as being present on the thoracic segments of Pteronarcys, and demonstrates that in the adult the sacs with which the tracheae communicate are provided with trachea from the main longitudinal trunk, and states that they are functional. Similarly it is possible that in moist conditions the gills in Stenoperla may be functional.
The cerci in the male (Pl. 39, Fig. 29) bear on the inner sides of their short, basal joints sets of short, stout bristles, and on all the remaining segments longer more slender hairs on the outer. These latter are borne on all segments in the female.
The Male Genitalia.
The external male appendages (Pl. 39, Figs. 30 and 31) consist of a ventral, unpaired, inferior appendage, a pair of superior appendages, and an unpaired aedeagus. The sternum of the ninth segment is large, convex, and projects backwards for a short distance over the inferior appendage and forms the hypandrium or sub-genital plate, which bears on its inner surface the ejaculatory sac. The inferior appendage protects the aedeagus ventrally; whilst the superior appendages are upturned to form a short copulatory hook by means of which the male clasps the female during copulation. The aedeagus is an unpaired structure, pointed at its apex, and some-what shorter than the appendages. It lies between the superior appendages and the anus.
The Female Genitalia.
The female (Pl. 39, Fig. 32) has no external appendages, the two oviducts uniting and opening to the exterior on a slit-like invagination on the eighth segment, which condition must represent an extremely primitive condition. There is no ovipositor.