GENERAL STRUCTURE. 



end of each dorsal plate save the first. Microscopic 

 examination reveals that we have here openings, 

 denominated spiracles (sfi, Fig. 8), with strange com- 

 plications, leading into internal tubes, called tracheae 

 (Fig. 6), forming the breathing apparatus, and which 

 divide and sub-divide, after the manner of a fibrous 

 root in the soil, until they are found in countless 

 number in every part. 



All animals require oxygen. In those above the 

 Annulosa (page 7), the blood is carried either into 

 lungs or gills by means of vessels, when it appropri- 

 ates oxygen, which, by the circulation, it distributes. 

 In insects, with a local exception noticed later in 

 the chapter, there is no system of blood vessels, so 

 oxygen, as a part of the air, is taken direct from 

 the before-mentioned spiracles, through the tracheae, 

 into all muscles, glands, and organs of the body, 

 not even excepting the wings. As the abdomen 

 is extended and contracted, as is constantly done by 

 the bee, air is drawn into, and then expelled from, 

 these apertures in the sides, precisely as in our own 

 breathing from the mouth. Should an unlucky fly, 

 through not sufficiently controlling his passions and 

 appetites, tumble into the milk, and be saved from a 

 tragical fate by being lifted on to the table-cover, he 

 immediately commences energetically grooming his 

 body with his legs, not because he is especially anxious 

 about his personal appearance, but because here the 

 milk is closing his spiracles, and actually choking him. 



The tracheae consist of an external and internal 

 membrane, between which run spiral threads, highly 

 elastic in character, that prevent the closing of the 



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