86
MICHAEL LOCKE
determines the frequency of microcycles of rotation of successive laminae
to make lamellae of different dimensions?
In Calpodes, some of the lamellae laid down in the first 66 hr are
0.5 μ wide and take 6 hr for each 180° rotation. In mid instar, after
the critical period for the operation of the brain on the prothoracic gland,
typical values would be 0.1 μ. wide taking 10 min for their deposition.
If we take 30 Â for the diameter of a chitin-protein microfiber (Rudall,
1963), and follow Bouligand's hypothesis, the two categories of lamellae
above could have up to 170 laminae changing in orientation by about
1°, and 33 laminae with a 6° shift. The rate of deposition is close to
one lamina every 2 min and one every 20 sec respectively. We can see
that the rate of change of orientation in time shifts markedly from y^°
per min to 18° per min after the critical period for the operation of the
brain. These estimates make the assumption that the proportion of filler
to fibrous component is constant in the two categories of lamellae considered, but they serve to illustrate the type of change taking place and
the task facing an epidermal cell in creating this three dimensional
pattern.
The first question concerns the degree to which the epidermis influences
the orientation of fibers. It could be that the orientation of a plane
depends upon the molecular configuration presented by the plane already
in existence. This seems unlikely in view of the widely different dimensions of lamellae, which presumably contain different numbers of fibers.
Some features of the ultrastructure favor a fairly direct participation on
the part of the cell in fiber orientation. The fibers in the cuticle of the
shafts of bristles are for the most part axially oriented, and the microvilli concerned in depositing them are elongated in cross section in the
same direction. When lamellate cuticle is being deposited, on the other
hand, the microvilli are symmetrical. This would support the idea that
the fibers are oriented during deposition by the shape or movement of
the microvilli. However, some of the ultrastructural evidence supports
the idea that the lamellar pattern can arise away from a plasma membrane surface. The flange at the base of the proleg spines extends into
the cells secreting it. Oriented fibers appear towards its center as well
as immediately above the cell surface. This might be explained by supposing that some newly oriented fibrous structures are not seen as
electron microscope images until they are stabilized and this may occur
at a distance from the surface.
The second problem concerns the rate of rotation of the direction of
orientation. Whatever the relation may be between the epidermal cells
and orientation, the change in rate of lamellar deposition after the
critical period implies that there is control over both orientation and
rate of synthesis. The two may be related. From molting to the critical
MICHAEL LOCKE
determines the frequency of microcycles of rotation of successive laminae
to make lamellae of different dimensions?
In Calpodes, some of the lamellae laid down in the first 66 hr are
0.5 μ wide and take 6 hr for each 180° rotation. In mid instar, after
the critical period for the operation of the brain on the prothoracic gland,
typical values would be 0.1 μ. wide taking 10 min for their deposition.
If we take 30 Â for the diameter of a chitin-protein microfiber (Rudall,
1963), and follow Bouligand's hypothesis, the two categories of lamellae
above could have up to 170 laminae changing in orientation by about
1°, and 33 laminae with a 6° shift. The rate of deposition is close to
one lamina every 2 min and one every 20 sec respectively. We can see
that the rate of change of orientation in time shifts markedly from y^°
per min to 18° per min after the critical period for the operation of the
brain. These estimates make the assumption that the proportion of filler
to fibrous component is constant in the two categories of lamellae considered, but they serve to illustrate the type of change taking place and
the task facing an epidermal cell in creating this three dimensional
pattern.
The first question concerns the degree to which the epidermis influences
the orientation of fibers. It could be that the orientation of a plane
depends upon the molecular configuration presented by the plane already
in existence. This seems unlikely in view of the widely different dimensions of lamellae, which presumably contain different numbers of fibers.
Some features of the ultrastructure favor a fairly direct participation on
the part of the cell in fiber orientation. The fibers in the cuticle of the
shafts of bristles are for the most part axially oriented, and the microvilli concerned in depositing them are elongated in cross section in the
same direction. When lamellate cuticle is being deposited, on the other
hand, the microvilli are symmetrical. This would support the idea that
the fibers are oriented during deposition by the shape or movement of
the microvilli. However, some of the ultrastructural evidence supports
the idea that the lamellar pattern can arise away from a plasma membrane surface. The flange at the base of the proleg spines extends into
the cells secreting it. Oriented fibers appear towards its center as well
as immediately above the cell surface. This might be explained by supposing that some newly oriented fibrous structures are not seen as
electron microscope images until they are stabilized and this may occur
at a distance from the surface.
The second problem concerns the rate of rotation of the direction of
orientation. Whatever the relation may be between the epidermal cells
and orientation, the change in rate of lamellar deposition after the
critical period implies that there is control over both orientation and
rate of synthesis. The two may be related. From molting to the critical