that move the various parts of the body, and of fat, surrounding the
organs and serving as a store of food-material. Very many of the
muscle-fibres and the fat-cells also become disintegrated during the
late larval and pupal stages, and the corresponding tissues of the adult
are new formations derived from special groups of imaginal cells, though
some muscles may persist from the larva to the adult. Similarly the
complex air-tube or tracheal system of the larva is broken down and a
fresh set of tubes is developed, adapted to the altered body-form of
pupa and imago.

The destruction of larval tissue and the development of replacing organs
from special groups of cells, derived of course from the embryo, and
carrying on the continuity of cell-lineage to the adult, are among the
most remarkable facts connected with the life-story of insects. The
process of tissue-destruction is known as 'histolysis'; the rebuilding
process is called 'histogenesis.' Considerable difference of opinion has
existed as to factors causing histolysis, and for a summary of the
conflicting or complementary theories, the reader is referred to the
work of L.F. Henneguy (1904, pp. 677-684). In the histolysis of the
two-winged flies, wandering amoeboid cells--like the white corpuscles or
leucocytes of vertebrate blood--have been observed destroying the larval
tissues that need to be broken down, as they destroy invading
micro-organisms in the body. But students of the internal changes that
accompany transformation in insects of other orders have often been
unable to observe such devouring activity of these 'phagocytes,' and
attribute the dissolution of the larval tissues to internal chemical
changes. The fact that in all insect transformation a part, and in many
a large part, of the larval organs pass over to the pupa and imago,
suggests that only those structures whose work is done are broken down
through the action of internally formed destructive substances, and one