i 4 THE YOUNG NATURALIST. [January 



vigour necessarily tend to become organs of locomotion. Those 

 brancheal leaflets which are attached to the mesothorax and metathorax 

 are generally the largest and would tend to become still larger relatively 

 in those animals which used them most frequently as swimming organs. 

 That this is not purely hypothetical is shewn by the remarkable Poly- 

 nema nutans, which according to Sir John Lubbock, although a per- 

 fectly winged insect, never leaves the water, but uses its wings as 

 swimming organs. Having traced the ascent from a marine worm to 

 a land tracheal breathing form, from this to an aquatic form of higher 

 specialization with respiratory organs like those of the aquatic larvae 

 of existing insects ; having also seen how those respiratory organs in 

 becoming more useful as respiratory organs have also become organs 

 of locomotion ; seeing too that increased use would lead to increased 

 development of them as organs of locomotion ; it is not difficult to re- 

 alize that paired swimming organs which enabled the animal to move 

 swiftly either forwards or upwards in the water would also enable it to 

 move through air, and though no doubt at first, powers of flight 

 through the air would be limited, yet it must be remembered that at 

 this period of the earth's history there were no other flying animals 

 and thus many of the sources of danger to which insects are now liable 

 did not then exist, and it is not unfair to assume that the at first feeble 

 powers of flight might become rapidly greater. 



It will perhaps not now seem so incredable as at first it sounds, 

 when we say that the wing of an insect is simply a gill. To me it is 

 clear though perhaps I have not made it so to you, that we can trace 

 how the gill-leaflet of an aquatic worm-like animal has become under 

 the ever present laws of heredity, variation and natural selection, an 

 organ by which the beautiful operation of insect flight is performed. 

 An examination of the structure of the wing of any insect confirms 

 this view, for on dissection we find that an insect's wing consists of 

 two distinct layers supported and strengthened by nervures inside 

 which are minute spiral vessels communicating with the trachese of 

 the trunk. In other words the wing is an out-pushing of the integu- 

 ments of the body. It may be thought that a butterfly expands its 

 wings by means of muscles, this however is not the case, the expansion 

 is effected by pumping air from the trachese into the spiral vessels of 

 the nervures which are thus forced outwards, simultaneously stretch- 

 ing and expanding the membranes of the wing. This still further sup- 

 ports the explanation of the origin of wings which has been attempted 



