Imaginal Rudiments in Thorax 123 



branches proceed, and these by further division form 

 a multitude of filaments, which are the tracheal gills, or 

 respiratory organs of the pupa. Is it possible that this 

 was ever wing-like, as the corresponding structures of 

 the meso- and metathorax now are ? Is it possible that 

 the nervures of the prothoracic wing have persisted as 

 branching tubes, while the intervening web has been 

 suppressed ? In some species of Chironomus and in many 

 other Nemocera, we find pupal respiratory trumpets 

 on the prothorax, instead of bunches of respiratory 

 filaments. It has been conjectured that such trumpets 

 represent wing-like rudiments, rolled up into tubes. 

 But another origin for the pupal trumpet is suggested by 

 the row of holes which we find upon its upper border in 

 Dicranota. These favour the view that the trumpet is 

 the basal tube greatly enlarged, and dej^rived of all 

 its branching filaments, or else that the tracheal gill 

 has arisen by the drawing out of the margins of the holes 

 in a pupal re.spiratory trumpet. Wing-like dorsal appen- 

 dages are not alwa3^s restricted to the thoracic segments. 

 In Ephemeridae flattened folds of integument are found 

 on the abdominal segments of the larvae, as in the very 

 common Chloeon dipterum, where they resemble in form 

 and insertion the larger plates which enclose the future 

 wings. The abdominal appendages of Ephemeridae are 

 the tracheal gills of the larvae. By their vigorous 

 flapping movements they continually bring a rush of 

 water against their richly tracheated surfaces, and, as it 

 would seem, promote the respiratory gas-exchange. It 

 is possible, though not proved, that the original function 

 of such appendages was respiratory, and that the con- 

 version of some of them into wings is a secondary 

 development. In certain Carboniferous Ephemeridae 

 all three thoracic segments bear expansions, which have 

 the form and the venation of true wings, and the 



