342 



PHYLUM ARTHROPODA. 



As to the origin of wings, it may be mentioned that in many case; 

 they are of some use in respiration as well as in locomotion, and the 

 theory seems plausible that wings were originally respiratory outgrowths, 

 which by and by became useful for aerial locomotion. New organs 

 seem often to have arisen by the predominance of some new function 

 in organs which had some prior significance. Moreover, we can fancy 

 that an increase in respiratory efficiency brought about by the out- 

 growths in question would quicken the whole life, and would tend to 

 raise insects into the air, just as terrestrial insects can be made to frisk 

 and jump when placed in a vessel with relatively more oxygen than 

 there is in the atmosphere. Finally, we 

 must note that the aquatic larvae of some 

 insects, e.g. may-flies, have a series of 

 respiratory outgrowths from the sides of 

 the abdomen, the so - called " tracheal 

 gills," which in origin and appearance are 

 like young wings (Fig. 183). 



Insects excel in locomotion. 

 "They walk, run, and jump with 

 the quadrupeds ; they fly with the 

 birds ; they glide with the serpents, 

 and they swim with the fish." They 

 beat the elastic air with their wings, 

 and though there cannot be so 

 much complexity of movement as 

 in birds where the individual 

 feathers move, the insect wing is 

 no rigid plate, and its up-and-down 

 motions are complex. They can 

 FIG. 183. Young may-fly soar rapidly, but their lightness 

 orephemerid.-AfterEaton. often makes horizori tal steering 



onlf T 1 ^ ~A ^*VA~ U 1 

 CUlt. 1 he Wind Often helps 



as well as hinders them; thus 

 in and out of the windows of 



express trains are probably in part sucked along. Marey 

 calculates the approximate number of wing strokes per 

 second at 330 for the fly, 240 for the humble-bee, 190 for 

 the hive-bee, no for the wasp, 28 for the dragon-fly, 9 for a 

 butterfly. For short distances a bee can outfly a pigeon. 



Skin. As in other Arthropods, the epidermis (or hypo- 

 dermis) of Insects forms a firm cuticle of chitin, which in 

 the exigencies of growth has sometimes to be moulted. 

 This cuticle is often finely marked, so that the animal seems 

 iridescent; and there are many different kinds of scales, 



Showing tracheal gills, and wings 

 appearing in front of them. 



the insects which fly 



