XII 



BACKBONELESS ANIMALS 



253 



series. Like birds, they are on an average active, most 

 have the power of flight, many are gaily coloured, sense- 

 organs and brains are often highly developed. 



Contrasted with Onychophora and Myriopods, they have 

 a more compact body, with fewer but more efficient limbs. 

 They are Arthropods which are usually winged in adult 

 life, breathe air by means of tracheae, and have frequently 

 a metamorphosis in their life-history. To this definition 

 must be added the anatomical facts that the adult body 

 is divided into three regions, (1) a head with three pairs 



FIG. 74. WINGED MALE AND WINGLESS FEMALE OF PNEUMORA, A KIND 



OF GRASSHOPPER. 



(From Darwin.) 



of mouth-appendages (= legs) and a pair of sensitive 

 outgrowths (antennae or feelers) in front of the mouth, 

 (2) a thorax with three pairs of walking legs, and usually 

 two pairs of wings, and (3) an abdomen without appen- 

 dages, except in so far as these may be represented by 

 occasional stings, egg-laying organs, etc. 



The wings are very characteristic. They are flattened 

 sacs of skin, into which air-tubes, blood-spaces, and 

 nerves extend. It is possible that they had originally a 

 respiratory, rather than a locomotor function, and that 

 increased activity induced by bettered respiration made 

 them into flying wings. 



