PATTERNS IN THE INTEGUMENT OF INSECTS
85
The problems posed by these microcycles are being investigated in
Calpodes larvae. Photomontages of electron micrographs of the cuticle
have been made at all stages during the fifth stadium for counts of the
number and frequency of lamellae. Up to 400 lamellae may be deposited.
There is an abrupt change in the frequency at which lamellae are
deposited at about 66 hr after ecdysis to the fifth instar. Before this
time, about one lamella is completed every 3 hr, afterwards there is one
about every 10 min. The time of the shift approximately corresponds to
the initiation of several other events, intermolt wax secretion and the
permanent activation of the prothoracic gland by the brain. Endocuticle
deposition (Locke et al., 1965) like intermolt wax secretion (Locke,
1965b) is probably hormonally controlled. The microcycles of deposition
may correspond to fluctuating levels of a general controlling factor, or
more probably are part of an intrinsic rhythm within the epidermis,
whose rate depends upon the level of the controlling factor. Neville
(1963a) thinks of the layered chitin deposition in locusts in terms of a
model with a trigger, the frequency of activity of the trigger being a
function of age. In adult locusts, the period between successive lamellae
is initially less than 1 hr, increasing to 8 hr at 11 days after emergence.
The rhythm of deposition cannot be understood without referring to the
orientation of the microfibers composing the lamellae.
The lamellae have a characteristic pattern of fibers. In sections normal
to the surface, each lamella, seems to be composed of fibers radiating
out in a fan shaped pattern (Fig. 6). The pattern is like a series of C's,
the amplitude of the curve varying with the plane of section. Bouligand
(1965) has shown how this appearance is an optical illusion created by
overlapping planes of fibers. The C pattern results from fibers in planes
or laminae parallel to the surface. The fibers in each lamina are parallel
to one another but the orientation changes slightly in the same direction
from one lamina to the next. Each 180° turn gives rise to a lamella,
each 360° turn to two lamellae differing from one another only if the
component fibers are polarized. A lamella as such only exists with
respect to the plane from which it is viewed, and the C pattern only
appears in sections which are slightly oblique. The pattern is symmetrical
except for the direction of rotation of the planes of successive laminae.
It remains to be seen whether the direction of rotation is constant. The
model is particularly satisfactory in accounting for helical pore canals
which presumably follow gaps in successive layers in a helical fashion.
Bouligand's model also makes it easy to comprehend transitions in
structure between lamellar cuticle and cuticle with layers oriented
primarily in one direction. There are two problems in lamellar structure.
How is the orientation of fibers in each lamina determined, and what
85
The problems posed by these microcycles are being investigated in
Calpodes larvae. Photomontages of electron micrographs of the cuticle
have been made at all stages during the fifth stadium for counts of the
number and frequency of lamellae. Up to 400 lamellae may be deposited.
There is an abrupt change in the frequency at which lamellae are
deposited at about 66 hr after ecdysis to the fifth instar. Before this
time, about one lamella is completed every 3 hr, afterwards there is one
about every 10 min. The time of the shift approximately corresponds to
the initiation of several other events, intermolt wax secretion and the
permanent activation of the prothoracic gland by the brain. Endocuticle
deposition (Locke et al., 1965) like intermolt wax secretion (Locke,
1965b) is probably hormonally controlled. The microcycles of deposition
may correspond to fluctuating levels of a general controlling factor, or
more probably are part of an intrinsic rhythm within the epidermis,
whose rate depends upon the level of the controlling factor. Neville
(1963a) thinks of the layered chitin deposition in locusts in terms of a
model with a trigger, the frequency of activity of the trigger being a
function of age. In adult locusts, the period between successive lamellae
is initially less than 1 hr, increasing to 8 hr at 11 days after emergence.
The rhythm of deposition cannot be understood without referring to the
orientation of the microfibers composing the lamellae.
The lamellae have a characteristic pattern of fibers. In sections normal
to the surface, each lamella, seems to be composed of fibers radiating
out in a fan shaped pattern (Fig. 6). The pattern is like a series of C's,
the amplitude of the curve varying with the plane of section. Bouligand
(1965) has shown how this appearance is an optical illusion created by
overlapping planes of fibers. The C pattern results from fibers in planes
or laminae parallel to the surface. The fibers in each lamina are parallel
to one another but the orientation changes slightly in the same direction
from one lamina to the next. Each 180° turn gives rise to a lamella,
each 360° turn to two lamellae differing from one another only if the
component fibers are polarized. A lamella as such only exists with
respect to the plane from which it is viewed, and the C pattern only
appears in sections which are slightly oblique. The pattern is symmetrical
except for the direction of rotation of the planes of successive laminae.
It remains to be seen whether the direction of rotation is constant. The
model is particularly satisfactory in accounting for helical pore canals
which presumably follow gaps in successive layers in a helical fashion.
Bouligand's model also makes it easy to comprehend transitions in
structure between lamellar cuticle and cuticle with layers oriented
primarily in one direction. There are two problems in lamellar structure.
How is the orientation of fibers in each lamina determined, and what