The structure of the foregut and midgut of nymphs, subimagos and imagos ofCloeon dipterum (Ephemeroptera) and the functions of the gut of adult mayflies

The anatomy of the gut and the fine structure of the foregut and midgut of the nymphal, subimaginal and imaginal stages of C. dipterum are described. A new description is given of the nymphal foregut-midgut junction and its r elationship to peritrophic membrane formation. Although the mouthparts are lost at the end of the nymphal stage, and the structure of the gut undergoe s extensive and often degenerative changes both during and after each of th e moults to subimago and image, the structure of the subimaginal and imagin al gut nevertheless suggests that the gut is still performing some active f unction during both adult stages. This is also suggested by the persistence in subimagos and images of rhythmical waves of midgut contraction, an ener getically expensive process for an animal which has ceased to feed. Althoug h a very occasional uptake of fluids by images has been observed, it has be en shown, by the use of a fluorescent stain, that fluid which disappears fr om the mouth region is most frequently being spread as a very thin film ove r the body surface rather than entering the gut. Frequency of midgut contra ction has been measured in resting subimagos and images of C. dipterum and C. simile, in images of C. dipterum during and after being held in a dry at mosphere, in ovipositing images of C. dipterum, in ejaculating male images of C. simile and Ecdyonurus venosus, and after successive bouts of flying b y E. venosus. Previous claims that the midgut contractions either control o r affect oviposition and sperm ejaculation have not been substantiated. The possibility of a key relationship between the midgut contractions and the need for rapid transport of metabolities is discussed.