METAZOA INSECTA. 397 



In the Carboniferous rocks of Commentry an insect, 

 Homaloneura bonneri Brong., (PL 957, a restoration) has 

 been discovered which Brongniart considers an ancestor 

 of our Ephemeridae. It has a large, elongated, and plainly 

 segmented body, a sessile abdomen terminated by long 

 caudal setae and two pairs of wings of equal size. Be- 

 sides these appendages of the mesothorax and meta- 

 thorax, the prothorax has a pair of scale -like appendages 

 which according to Brongniart may represent prothoracic 

 wings. This investigator suggests that sometime there 

 may be found in the ancient strata an insect with six 

 wings or rather six expansions which served only as par- 

 achutes, and that later the expansions of the mesothorax 

 and metathorax developed into useful organs of flight while 

 those of the prothorax were atrophied. 



The living Ephemeridae in a very early larval stage 

 have an open tracheal system like that of Campodea, with 

 thoracic spiracles. 1 This is one proof of their descent 

 from ancestral forms possessing an open tracheal system. 

 In changing their habitat from land to water they have 

 adapted themselves to their new surroundings, and in so 

 doing have gradually converted the open tracheal system 

 into a closed one. At the same time external gills of 

 varying form and structure have developed. In fact, so 

 many complex modifications have arisen in these insects, 

 in response to the changed environment, that it is difficult 

 to place them among the most generalized orders unless 

 we bear constantly in mind the general features of the 

 group. These are, in the larvae, a long body (PL 958, 

 fig. i, Chloeon dimidiatum Lubbock) divided into freely 

 movable regions ; a large thorax with slight consolidation 

 of the prothorax, mesothorax, and metathorax ; simple 

 antennae ; and three pairs of similar legs. 



An extremely slow development is often attended with 

 numerous sheddings of the skin, so that no sharp line of 

 demarcation can be drawn between larva and pupa. 



1 Packard, after Dewitz, Text-book of Ent., 1898, p. 460. 



