342 PHYLUM ARTHROPODA. 
As to the origin of wings, it may be mentioned that in many cases 
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 larve 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 
/ x motions are complex. They can 
Fic. 183.-Young may-fly soar rapidly, but their lightness 
ercphemsnds ice Eaton. often makes horizontal steering 
sno peatngahonta them difficult. The wind often helps 
as well as hinders them; thus 
the insects which fly 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, r10 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, 
oe 
