44
MICHAEL LOCKE
A. The Formation of Surface Patterns
1. Patterns in the Epicuticle
The elaborate sculpturing of the surface, the formation of hairs, scales,
and microtrichia, and the elegant patterning of tracheae and tracheoles
all take place at an epidermal cell surface at the time of formation of
the cuticulin layer. It is questionable which is the prime mover, the
plasma membrane assuming a shape for the deposition of the cuticulin,
or the cuticulin responding to forces which carry the plasma membrane
along with it. At one extreme, we might suppose that all the forces governing pattern formation reside in the cell and its plasma membrane.
We should then expect to see some complicated cellular architecture or
distribution of organelles preceding, corresponding with, and causing
the surface pattern (see Section II,A,l,b). On the other hand, it could
be that many of the complicated and minute surface patterns are the
result of fairly simple and often uniform physical forces operating on
the cuticulin membrane. They might require no corresponding micropattern within the cell (see Section II,A,l,a). Both hypotheses are true
depending upon the pattern under consideration.
a. Patterns Formed by the Cuticulin Alone. The cuticulin is the first
layer of the new cuticle to arise. It is at first smooth, following the
contours of the epidermal cell surface. Later it increases in area with
the formation of the new pattern (Wigglesworth, 1933, 1954b; Locke,
1958a, 1959a). It appears to form the surface pattern when further
expansion is limited. In Rhodnius larval cuticle, the increase in area results
in a pattern of stellate pleats and in the adult, in a pattern of transverse
ripples. These patterns can be simulated in simple models using expanding rubber latex. Expansion with uniform stress in the plane of the
surface results in stellate patterns which can be converted to the side to
side ripples of the adult by axial compression (Locke, 1959a). This
hypothesis for the origin of surface patterns by expansion and buckling
has been explored in more detail in the formation of the helical and
annular patterns of folds in the tracheae and tracheoles of Rhodnius
(Locke, 1958a). It was supposed that these patterns were the result
of the stresses generated during the expansion and subsequent buckling
of the cuticulin. The newly formed cuticular tubes of the tracheae can
be treated as thin-walled, elastic cylinders with uniform properties
varying only in radius. When they increase in surface area against the
restraint of the sheathing epidermal cells, they can be treated as tubes
increasing in diameter, but buckling as if under the action of axial compression. Symmetrical buckling would be expected in biological material
with a low Young's modulus (Timoshenko, 1936). In symmetrical
MICHAEL LOCKE
A. The Formation of Surface Patterns
1. Patterns in the Epicuticle
The elaborate sculpturing of the surface, the formation of hairs, scales,
and microtrichia, and the elegant patterning of tracheae and tracheoles
all take place at an epidermal cell surface at the time of formation of
the cuticulin layer. It is questionable which is the prime mover, the
plasma membrane assuming a shape for the deposition of the cuticulin,
or the cuticulin responding to forces which carry the plasma membrane
along with it. At one extreme, we might suppose that all the forces governing pattern formation reside in the cell and its plasma membrane.
We should then expect to see some complicated cellular architecture or
distribution of organelles preceding, corresponding with, and causing
the surface pattern (see Section II,A,l,b). On the other hand, it could
be that many of the complicated and minute surface patterns are the
result of fairly simple and often uniform physical forces operating on
the cuticulin membrane. They might require no corresponding micropattern within the cell (see Section II,A,l,a). Both hypotheses are true
depending upon the pattern under consideration.
a. Patterns Formed by the Cuticulin Alone. The cuticulin is the first
layer of the new cuticle to arise. It is at first smooth, following the
contours of the epidermal cell surface. Later it increases in area with
the formation of the new pattern (Wigglesworth, 1933, 1954b; Locke,
1958a, 1959a). It appears to form the surface pattern when further
expansion is limited. In Rhodnius larval cuticle, the increase in area results
in a pattern of stellate pleats and in the adult, in a pattern of transverse
ripples. These patterns can be simulated in simple models using expanding rubber latex. Expansion with uniform stress in the plane of the
surface results in stellate patterns which can be converted to the side to
side ripples of the adult by axial compression (Locke, 1959a). This
hypothesis for the origin of surface patterns by expansion and buckling
has been explored in more detail in the formation of the helical and
annular patterns of folds in the tracheae and tracheoles of Rhodnius
(Locke, 1958a). It was supposed that these patterns were the result
of the stresses generated during the expansion and subsequent buckling
of the cuticulin. The newly formed cuticular tubes of the tracheae can
be treated as thin-walled, elastic cylinders with uniform properties
varying only in radius. When they increase in surface area against the
restraint of the sheathing epidermal cells, they can be treated as tubes
increasing in diameter, but buckling as if under the action of axial compression. Symmetrical buckling would be expected in biological material
with a low Young's modulus (Timoshenko, 1936). In symmetrical