P A T T E R N S I N T H E I N T E G U M E N T O F I N S E C T S
53
shown in Fig. 17B. When the cuticulin has achieved its definitive surface
pattern and has been reinforced below by the homogeneous layer of the
protein epicuticle, the outmost layer of the cuticulum becomes more
dense, often less regular, and separated by a clear space of about 60 Â
from the rest of the cuticulin (Fig. 21). This layer reacts with lead salts
and is believed to be, or become, the surface wax. The remaining two
layers are now difficult to resolve, as they are packed together with the
other components of the epicuticle which have about the same density.
The dimensions of the final structure are shown in Fig. 17C. Thus,
cuticulin resembles a plasma membrane in having some lipid components and in being composed of thin laminae arranged side by side
with approximately the same dimensions.
FIG. 17. The arrangement of dense laminae and the approximate dimensions in the
cuticulin layer at various stages in its formation 1, 2, and 3 correspond to the
numbered layers in the electron micrographs. A: as the cuticulin layer first appears
in thicker sections (Figs. 18 and 19); B : as it appears in very thin sections when
it is forming and in at least parts of tracheoles all the time (Fig. 20) ; and C : as it
appears when the dense layer of the epicuticle has been deposited and after ecdysis
(Fig. 21).
b. The Formation and Growth of Cuticulin. If the appropriate component molecules are available, we may envisage three possible mechanisms for the generation of membrane structure: (1) growth by accretion
to a preexisting membrane, either by diffuse intussusception or at specific
growing sites, (2) assembly upon templates which create order but play
53
shown in Fig. 17B. When the cuticulin has achieved its definitive surface
pattern and has been reinforced below by the homogeneous layer of the
protein epicuticle, the outmost layer of the cuticulum becomes more
dense, often less regular, and separated by a clear space of about 60 Â
from the rest of the cuticulin (Fig. 21). This layer reacts with lead salts
and is believed to be, or become, the surface wax. The remaining two
layers are now difficult to resolve, as they are packed together with the
other components of the epicuticle which have about the same density.
The dimensions of the final structure are shown in Fig. 17C. Thus,
cuticulin resembles a plasma membrane in having some lipid components and in being composed of thin laminae arranged side by side
with approximately the same dimensions.
FIG. 17. The arrangement of dense laminae and the approximate dimensions in the
cuticulin layer at various stages in its formation 1, 2, and 3 correspond to the
numbered layers in the electron micrographs. A: as the cuticulin layer first appears
in thicker sections (Figs. 18 and 19); B : as it appears in very thin sections when
it is forming and in at least parts of tracheoles all the time (Fig. 20) ; and C : as it
appears when the dense layer of the epicuticle has been deposited and after ecdysis
(Fig. 21).
b. The Formation and Growth of Cuticulin. If the appropriate component molecules are available, we may envisage three possible mechanisms for the generation of membrane structure: (1) growth by accretion
to a preexisting membrane, either by diffuse intussusception or at specific
growing sites, (2) assembly upon templates which create order but play