402 



PHYLUM ARTHROPODA 



larval, and especially during the quiescent pupal stage. 

 The more specialised larval organs are disrupted, their 

 debris being used in building new structures. In some 

 cases, such as flies, phagocytes play a very important part 

 in this metamorphosis ; in other cases there is no true 



phagocytosis. Parts of larval 

 organs which have not been 

 highly specialised form the 

 foundations of new adult 

 structures. Of special im- 

 portance are certain ingrowths 



jv\ P of the larval skin (the epi- or 



hypo- dermis) which form 

 w^hat are called *' imaginal 

 discs," i.e. embryonic or ger- 

 minal areas, from which arise 

 the wings, legs, etc., of the 

 adult insect. The reconstruc- 

 tion is very thorough ; most 

 of the musculature, much of 

 the tracheal system, part of the 

 mid-gut, etc., are gradually re- 

 placed by the corresponding 

 organs of the adult. There is 

 first a disruptive process of 

 histolysis, and then a recon- 

 structive process of histo- 

 genesis. Yet in most cases the 

 Fig. 224.— Fly about to emerge disruption and replacement of 

 from pupa-case, one end of organs is very gradual. 



■ Ite Ha'^^r'' ^'''^^'' ''^'~ Ecology.— The average in- 

 ^\ / * ,, „ sect is active, but between 



E., Compound eyes; M.P., mouth- , / ' , , 



parts; £1., first leg ; P.C., pupa- ordcrs {e.g. auts, bccs, and 



case; S., respiratory aperture into ^aspS VeVSUS aphidcS, COCCUS 

 pupa-case. . jt 1 1 \ 1 



insects, and bugs), between 

 nearly related families, between the sexes {e.g. male and 

 female cochineal insect), between caterpillar and pupa, we 

 read the constantly recurrent antithesis between activity 

 and passivity. 



The average length of life is short. Queen-bees of five 

 years, queen-ants aged thirteen, are rare exceptions. In 



