142 NATURAL HISTORY OF AQUATIC INSECTS CH. 



one species, while other closely allied species are not 

 so modified, are described by Fritz Miiller 1 as due to 

 a process of falsification 2 set up by the struggle for 

 existence among free living immature forms. It is to 

 be regretted that our ignorance of the early stages of 

 nearly all the species of Chironomus prevents this 

 interesting question from being fully worked out at 

 present. 



Certain facts in the development of the pupal re- 

 spiratory appendages point to a particular explanation 

 of the two forms, so widely different, which they 

 assume in nearly allied genera. When we watch the 

 developing thoracic appendages of the fly in such 

 Dipterous larvae as Chironomus, Culex or Simulium, 

 we find that they form a dorsal and a ventral series 

 each consisting of three pairs (see diagram). 



D 2 and D 3 will ultimately form the wings and 

 haltcres (poisers) ; V 1 , V 2 and V 3 the legs of the fly. 



1 Facts for Darwin, Chapter xl. 



2 The term falsification is not self-explanatory, and it is open 

 to objection as introducing quite irrelevant associations. The 

 obliteration of ancestral by adaptive characters is the thing 

 meant. 



