METAZOA INSECTA. 399 



An advanced pupal stage of Chloeon dipterum Linn., 1 

 is seen in PI. 960. The wing pad on the left side and 

 the gills on the right side have been cut away. In addi- 

 tion to these gills the pupa has caudal gills which also 

 serve as good swimming organs. 



The adult (PI. 958, fig. 6, C. dipterum Lubbock; No. 

 959) is specialized by reduction, so that the mouth parts 

 are wholly incapable of taking food. The insect in this 

 stage lives only a few hours or at most a few days ; hence 

 the name Ephemeroptera (signifying short-lived insect and 

 wing) given to this order. When the function of repro- 

 .duction is performed the insect dies. The loss of efficient 

 mouth parts is correlated with a loss of the second pair of 

 wings (PI. 958, figs. 5, 6; No. 959) and with a peculiar 

 modification of the compound eyes. These organs are 

 divided in Chloeon so that there appear to be a pair of 

 stalked eyes (which, however, are not united at their 

 bases) and a pair of sessile eyes (fig. 7). Besides these 

 eyes there are three ocelli (fig. 7). 



Peculiar modifications of structure are found among the 

 more specialized Ephemeridae. An extreme case of 

 specialization in the pupal stage is seen in Prosopistoma 

 foliaceum Fourcroy (PI. 961, fig. i, x 12). Here a 

 portion of the mesothorax and metathorax with the ante- 

 rior wing pads has formed a carapace that covers the dor- 

 sal part of the body with the exception of the four 

 posterior abdominal segments and the caudal swimming 

 organs. This carapace conceals from view the respira- 

 tory chamber with its five pairs of gi Is supplied with 

 tracheae. The water passes into the respiratory chamber 

 at two openings, seen on either side in the ventral view 

 (fig. 2,0) and flows out at the dorsal opening (fig. i, 0). 

 In the ventral view (fig. 2) the last segment with its 



1 Cloeon dipterum Linn., a synonym for Chloeon dipterum Lubbock; 

 see Eaton, Trans. Linn. Soc. London, (2), Zool., Ill, 1888, pp. 

 183, 188. 



