Origin of Wings 351 



either wholly or partly in the water, being probably 

 invaders from the shore. The immense majority of 

 insects are terrestrial or aerial, and the aquatic forms 

 appear to have been modified from their land relations. 

 Such evidence is admitted by zoologists as conclusively 

 showing the native element of any class of animals ; 

 mammals are universally regarded as primarily terres- 

 trial, though seals and whales are marine, crustaceans 

 as aquatic, though some crabs and wood-lice live on 

 land. It may be admitted readily that life began in 

 the water, and that to the waters we must go for the 

 remote progenitors of insects. But the class as we 

 know it now is composed of typically land-animals, 

 and we have every reason to believe that its immediate 

 ancestors were air-breathers (170). 



But while denying that the aquatic grub of a 

 Mayfly represents the primitive insect it is still 

 possible to regard it as indicating the precursor of 

 luitjged insects. That is to say it may be held that 

 some of the primitive wingless air-breathing insects 

 took to an aquatic life and developed tracheal gills, 

 which, on transference to the upper air, became changed 

 into wings. But the fact that the majority of winged 

 insects are terrestrial and aerial, and that similar 

 adaptations to aquatic life are found in the larva 

 of different orders — presumably therefore independ- 

 ently and secondarily acquired — tells even against this 

 view. The origin of insect wings must at present 

 be considered a mystery. In all the orders as yet 

 reviewed in this chapter wings arise as outgrowths 

 from the tergites of the second and third thoracic 

 segments, beginning as simple expansions of the 

 tergal plate. Assuming that wings arose thus in 

 primitive air-breathing insects it has been suggested 

 that originally, before they became large enough for 

 flight, they may have served for respiration in a damp 



