GROWTH AND TRANSFORMATION 175 



or testes of the adult arise directly from the corresponding 

 structures present in the larva with changes as to size and 

 elongation, as well as separation or fusion of segmental 

 structures such as nen^e-ganglia. On the other hand, the 

 digestive system and associated structures undergo dissolu- 

 tion at the close of the last larval stage and the corresponding 

 organs of the winged adult are developed from special 

 imaginal cells, which may be recognised during larval life, 

 either appearing as small scattered units among the larger 

 normal cells of the larv^al digestive epithelium, or forming 

 aggregated groups at definite regions of the larval food- 

 canal. Such small imaginal groups of cells, relatively few 

 in number, give rise to the greater part of the internal organs 

 of the adult, so that these organs may be regarded as new 

 formations during early pupal life, while the larval structures, 

 now no longer needed, are broken down by the chemical 

 action of special enzymes, the effete products of the disso- 

 lution process being devoured by active, wandering 

 ** amoeboid " cells like white blood-corpuscles. This 

 destructive process is known as *' histolysis," and the re- 

 placement due to the growth of the imaginal discs as 

 " histogenesis." For a detailed account and discussion 

 of these processes reference may be made to the treatise 

 of L. F. Henneguy (1904). The muscles and the air- tubes 

 also undergo, like the digestive system, dissolution and 

 reconstruction more or less profound according as these 

 systems in the imago differ from those in the larva. The 

 tissue of the larval fat-body serves as a reservoir of food- 

 material which is drawn upon for the energy needed in the 

 rapid processes of growth and change that go on at the crisis 

 of transformation in the insect's life-history. These 

 internal changes, the nature of which were in part elucidated 

 by the work of A. Weismann (1864) on the metamorphosis 

 of flies, and have been traced in full detail by subsequent 

 students, are no less surprising than the changes in outward 

 form which led some of the earlier naturalists to regard the 

 formation of the pupa and the subsequent emergence there- 

 from of the winged adult as a veritable new birth. All the 



