Fig. 254.—Under side of body of Pteronarcys regalis, imago. (After Newport.) g, Tracheal gills; o, sternal orifices.
Great difference of opinion has prevailed as to the relations of the branchiae to the stigmata, it having been contended that the falling off of some of the branchiae left the stigmatic orifices. The facts appear to be only consistent with the conclusion that the two are totally independent organs. This subject has been investigated by Palmén,[[325]] who finds that in Perlidae—contrary to what occurs in may-flies—the species are either entirely destitute of gills, or these organs are persistent throughout life. It is not to be inferred from this that the gills in the perennibranchiate Perlidae are as conspicuous as they are in the exceptional Pteronarcys: for it appears that at the final moult the gills usually become very much contracted and concealed by the new integument; in some cases they merely appear as slight prominences in the neighbourhood of the stigmata.
Pictet, Dufour, Newport, and Imhof[[326]] have studied the internal anatomy. The alimentary canal is remarkable for the enormous oesophagus; there is no distinction between this and the crop. A proventriculus is quite absent, and there are no chitinous folds in the position it usually occupies. The true stomach is small, and only commences in the fourth abdominal segment. It has a prolonged lobe on each side in front, and in addition to this eight sacs; thus there are formed ten diverticula, fastened to the posterior part of the oesophagus by ligaments. The terminal portion of the stomach is small, and apparently only distinguished from the short intestine by the point of insertion of the Malpighian tubes; these vary in number from about twenty to sixty. There are two pairs of large salivary glands. In Pteronarcys the caecal diverticula of the stomach are wanting. In some Perlidae the terminal parts of the gut are more complex than in Perla maxima; Newport figures both an ilium and colon very strongly differentiated, and states that these parts differ much in Perla and Pteronarcys. According to him the stomach is embraced by a network of tracheae, and Imhof tells us that he found the stomach to contain only air.
Fig. 255.—Alimentary canal and outline of body of Perla maxima. (After Imhof.) l, Upper lip; mh, buccal cavity; ap, common termination of salivary ducts; o, oesophagus; s, salivary glands; ag, duct of salivary gland; b, anterior diverticula of stomach; lg, their ligaments of attachment; mp, Malpighian tubes; r, rectum; af, anal orifice.
The brain is small, but, according to Imhof, consists of four amalgamated divisions; the infra-oesophageal ganglion is small, and placed very near the brain. There are three thoracic and six abdominal ganglia on the ventral chain. The nerves to the wings are connected with the longitudinal commissures of the ventral chain by peculiar, obliquely-placed, short commissures. The reproductive glands are peculiar, inasmuch as in each sex the pair of principal glands is connected together in the middle. The testes thus form an arch consisting of a large number of sub-spherical or pear-shaped follicles; the vasa deferentia are short in Perla maxima, and there are no vesiculae seminales; the ejaculatory duct is divided into three parts by constrictions. In Pteronarcys and in Perla bicaudata, according to Newport and Dufour, the vasa deferentia are very long and tortuous, and there are elongate vesiculae seminales. The arrangement of the extremely numerous egg-tubes is analogous to that of the follicles of the testes, so that, as Dufour says, there is but a single ovary; connected with the short, unpaired portion of the oviduct, there is a large receptaculum seminis, and near the terminal orifice of the duct there is in P. maxima an eight-lobed accessory gland.
Fig. 256.—The pair of united ovaries of Perla maxima: o, egg-tubes; ov, oviduct; r, receptaculum seminis concealing the orifice of the duct and an accessory gland.
