266 



CHAPTERS ON EVOLUTION. 



physiology or use of a wing is, of course, to serve as an organ of 

 flight. But the use or function of an organ may be, and often is, a 

 secondary and adaptive matter, and may be very far from revealing 

 the original condition of the structure in question. Authority in 

 matters 'entomological, assures us that the wings, as appendages of 

 the insect's body, are in reality parts of the animal's breathing 

 system. They contain branches of the breathing tubes, and expan- 

 sions of the blood-vessels likewise. " Hence," says Packard, " the 

 aeration of the blood is carried on in the wings, and thus they serve 

 the double purpose of lungs and organs of flight." But we must note 

 that many insects are absolutely wingless. The lice, spring-tails, and 

 fleas, and even the plant lice (Fig. 177) and neuter ants (Fig. 178, c), 



belonging to winged 

 groups, are destitute of 

 these organs. No doubt 

 the wingless condition in 

 the latter cases is to be 

 explained on the theory 

 of disuse causing the 

 disappearance of these 

 organs. But the most 

 primitive insects are with- 

 out wings, and we may, 

 therefore, reasonably con- 

 clude that wings are not 

 original belongings but 

 late developments of the 

 race. Furthermore, many 

 insects of relatively high 

 rank, such as the crickets, 

 grasshoppers, &c., quit the 

 egg without wings, and 

 this although they are ex- 

 tremely active in every 

 respect. A wingless state 



FIG. 179. AQUATIC INSECT LARVAE, SHOWING THE is on all grounds, includ- 



BREATHING GILLS. .1 ' -j 



A, Larva of Ephemera or Day-fly ; B, Larva of Che bioculata. mg the evidence Ot de- 



velopment, to be regarded 



as the original condition of the insect class. We have seen the 

 intimate connection which exists between the wings and the 

 breathing of insects. Of the two functions, breathing is, of course, 

 much more primary and essential to life than flight. Hence we 

 may well conclude that as many insects, especially the most 

 primitive, breathe and live without wings, whilst others develop 

 wings and utilise them for breathing as well as for flight, the 



