METAMORPHOSIS OF INSECTS. 273 



summarised in a terse sentence from the monumental work 

 of Rolleston and Hatchett Jackson ("Forms of Animal 

 Life," Oxford, 1888). "The mouth parts may be similar 

 in all stages of life, and then are either adapted for biting 

 {Menognatha, i.e., jaws persistent), or for sucking {Menor- 

 hyncha, i.e., proboscis persistent ; or else they are adapted in 

 the larva for biting, in the adult for sucking, the change 

 commencing in the pupa, and rarely affecting the larval stage 

 (Metagnatha, i.e., jaws changed)." 



Internal Metamorphosis. — In Insects with no marked meta- 

 morphosis, or with an incomplete one merely, the organs of 

 the larva develop gradually into those of the adult. But in 

 Insects with complete metamorphosis, there is a marvellous 

 internal reconstruction during the later larval, and especially 

 during the quiescent pupal stage. Most of the larval organs 

 are disrupted, and partially absorbed by amoeboid cells, their 

 debris being used in building new structures. Parts of larval 

 organs which have not been highly specialised form the 

 foundations of new adult structures. Of special importance 

 are certain ingrowths of the larval skin (the epi- or hypo- 

 dermis) which form what are called "imaginal discs," from 

 which arise the wings, legs, and epidermis of the imago or 

 perfect insect. The reconstruction is very thorough ; most 

 of the musculature, much of the tracheal system, part of the 

 mid-gut, etc., are gradually replaced by the corresponding 

 organs of the adult. Yet there is no abruptness ; the absorp- 

 tion and replacement of organs is perfectly gradual. 



General Life of Insects. 



The average insect is active, but between orders {e.g., 

 ants, bees, and wasps versus aphides, coccus-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 

 many cases death follows as the rapid nemesis of reproduction. 

 But though the adult life is often very short, the total life 



's 



