EPHEMERIDA 



311 



at intervals to wash off the eggs that have issued from the mouths of 

 the oviducts during her flight or else she creeps down into the water — 

 enclosed within a film of air with her wings collapsed so as to overhe 

 the abdomen in the form of an acute narrowly linear bundle, and 

 with her setae closed together — to lay her eggs upon the under side of 

 the stones, disposing of them in rounded patches, in a single layer 

 evenly spread, and in mutual contiguity." (Eaton '83). 



Fig- 355- — Metamorphosis of a May-fly, Ephemera varia: A, adult; B, naiad. 

 (After Needham.) 



The metamorphosis of May-flies is incomplete. The wings are 

 developed externally, as in the Orthoptera; the development of the 

 compound eyes is not retarded; but the immature forms, or naiads, 

 are "sidewise developed" to fit them for aquatic life. In most species 

 the form of the body of the naiads is elongate and furnished with two 

 or three long "caudal set^," that is, cerci and in some a median 

 caudal filament; in these respects the naiads resemble, to a greater 

 or less degree, the adults (Fig. 355); but except in the early instars 

 the abdomen of a naiad is furnished with tracheal gills (Figs. 355 a.nd 

 356.) 



The tracheal gills are usually large and prominent; in most 

 species there are seven pairs, borne by the first seven 

 abdominal segments. They vary greatly in form in the 

 different genera. In some each gill is divided into two 

 long narrow branches, which lie in one plane (Fig. 355); 

 in others the gills consist of a scoop-shaped covering 

 piece beneath which is a more delicate part consisting 

 of many thread-like branches. A detailed account of 

 the various forms of tracheal gills of May -flies is given 

 by Miss Morgan ('13). 



The naiads of May -flies are all aquatic; they are 



very active; and are almost entirely herbivorous, -p. ^ 



feeding largely on the decaying stems and leaves of iad of a May- 

 aquatic plants, the epidermis of moss and of roots, fly. 



