326 NATURAL HISTORY OF AQUATIC INSECTS CH. 



.&c.) except that the external wall of the respiratory 

 chamber is formed in the Ephemera; by a backward 

 extension of the midthorax, instead of by a downward 

 extension of the carapace, as in the Decapods. To 

 this peculiar group belong a singular American form, 

 Baetisca, and an Old World genus, Prosopistoma, 

 found in the Rhone, Garonne, &c., and described by 

 Dr. Em. Joly and Vayssiere. 



The eggs of Ephemerida; are often furnished with 

 singular appendages which in some cases prevent their 

 being swept away by currents of water. In Ephemera 

 vulgata ^ the eggs are described by Dr. H. Grenacher- 

 as possessing striated caps of reddish-brown colour 

 which invest both poles of the egg.^ A mushroom- 

 shaped stalk serves as a base and springs directly 

 from the end of the egg. This is of firmer consistence 

 than the striated part, whose numerous and close- 

 set fibres radiate regularly from it. The fibres had 

 previously been described by Leuckart •* as bundles 

 of spermatozoa, but Grenachcr points out that 

 they are to be found in all stages of development 

 within the larval oviduct, and not merely after fer- 

 tilisation, as Leuckart's interpretation requires. No 

 definite information as to the function of these striated 

 egg-caps has been obtained. According to Leuckart, 

 the eggs of some species of Ephemeridae bear them 

 at one end only, others at both ends. 



Grenacher has also described what appear to be 



1 Grenacher's words are : larva? belonging to Ephemera s. str. 

 •- Zeits.f. wiss. ZooL, Bd. XYIII. (1868), pp. 95-98. 

 •■' A similar striated cap is seen in the ovarian egg of the Gnat. 

 1 Miillers Arch. 1855, p. 200. 



'A 



