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in all respects, does not at once issue from the larval 

 skin ? Why should a pupa-stage be interposed ? I have 

 not found Swammerdam's answers to these questions. 

 It is not likely that he was able to answer them fully, 

 for minute investigation of the tissues is requisite, and 

 histology was wholly undeveloped in Swammerdam's 

 time. Nowadays the most obvious course is to cut 

 transparent sections through the organs of the 

 Butterfly, after the larval skin is stripped off, and by 

 microscopic examination we soon arrive at one signi- 

 ficant fact. The organs of the Butterfly, though 

 recognisable and externally pretty complete, are 

 merely the outward shapes of what they will after- 

 wards become. The muscles, nerves, air-tubes, and 

 other histological elements are either absent or 

 extremely imperfect. Much internal growth has to 

 be accomplished before the wings are fit for flying, or 

 the legs for running. This is the proper office of the 

 pupal-stage of Lepidoptera, to carry on and complete 

 the formation of new parts, necessary to the flying 

 Insect, which were merely blocked out in the larva. 

 It is quite conceivable that the whole growth, both 

 external and internal, might have been completed 

 while the Insect was still to outward appearance a 

 mere larva. Indeed this is very nearly what happens 

 in certain Dipterous Insects, such as Chironomus. In 

 them all the details of the future imaginal organs are, 

 with some slight exceptions, completed in the larval 

 stage, and the pupal stage, which lasts a very short 

 time, often only two or three days, is employed in 

 giving the parts the firmness which they will require, 

 and in filling the new breathing-organs with air. 



