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

Abstract

The anatomy of the gut and the ®ne 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 relationship to peritrophic membrane formation. Although the mouthparts are lost at the end of the nymphal stage, and the structure of the gut undergoes extensive and often degenerative changes both during and after each of the moults to subimago and imago, the structure of the subimaginal and imaginal gut nevertheless suggests that the gut is still performing some active function during both adult stages. This is also suggested by the persistence in subimagos and imagos of rhythmical waves of midgut contraction, an energetically expensive process for an animal which has ceased to feed. Although a very occasional uptake of¯uids by imagos has been observed, it has been shown, by the use of a¯uorescent stain, that¯uid which disappears from the mouth region is most frequently being spread as a very thin ®lm over the body surface rather than entering the gut. Frequency of midgut contraction has been measured in resting subimagos and imagos of C. dipterum and C. simile, in imagos of C. dipterum during and after being held in a dry atmosphere, in ovipositing imagos of C. dipterum, in ejaculating male imagos of C. simile and Ecdyonurus venosus, and after successive bouts of¯ying by E. venosus. Previous claims that the midgut contractions either control or 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.