P A T T E R N S I N T H E I N T E G U M E N T OF I N S E C T S
57
favors growth at the edges. As the cuticulin is secreted, the surface area
of the plasma membrane is rapidly occupied. Thereafter, the increased
surface area is taken up by the formation of folds which progressively
enlarge. If growth can take place by intussusception all over we should
expect the gaps between neighboring patches to be closed as the cuticulin
occupies all the flat surface before the formation of folds. This does not
happen. Gaps remain in the cuticulin at the base of folds, until the final
area is achieved as though these gaps are necessary for the increase in
area. Also, these gaps continue to be connected with the plasma membrane by wisps of dense material. Most probably then, the gaps in the
cuticulin are growing points. On the other hand some increase in area
occurs after the surface is covered by cuticulin. The few gaps left at the
time of the formation of the second order patterns are probably insufficient to account for the last phase of expansion in the cuticle over the
wax glands and over scale cells. This expansion may be due to growth
by intussusception or it might be caused by reorganization of the material already there. In this connection, it is perhaps significant that the
last phase of expansion coincides with the separation of the surface wax
layer from the cuticulin.
The mechanism of growth which most easily fits the observations is
the side-to-side accretion of membrane units. This probably does not
happen to all three layers of the cuticulin at once. There are often
partial gaps or disturbances in which only the outer two layers are
irregularly arranged or missing, as though these are the last to fit into
place. It looks as though the inner layer may be the first to form, it
then acts as a surface for the creation of order for the other two layers
which are arranged upon it, rather than as a template.
B. The Gradient Organization of Segmentally Repeating Patterns
Wigglesworth (1940) showed that in Rhodnius, the bristles on a piece
of abdominal integument retained their orientation with respect to the
graft when it had been excised and reimplanted, after being turned
through 90° or 180°. In Lepidoptera, the orientation of scales on implants
with altered position and orientation has been studied by Piepho (1955),
Piepho and Marcus (1957), and Marcus (1962) who found a similar
sort of orienting force in the general epithelium. As there is no individual
or species-specific incompatibility to grafting in Rhodnius or Triatoma,
FIG. 23. The adult cuticle pattern on the abdominal tergites of Rhodnius. χ 150.
FIG. 24. The cuticle pattern on the abdominal tergites of a, Rhodnius adult from a
fifth instar larva burned 10 days after feeding. There is a centripetal displacement
of the pattern not observed when the burn is made earlier or later. See Fig. 36.
X 150.
57
favors growth at the edges. As the cuticulin is secreted, the surface area
of the plasma membrane is rapidly occupied. Thereafter, the increased
surface area is taken up by the formation of folds which progressively
enlarge. If growth can take place by intussusception all over we should
expect the gaps between neighboring patches to be closed as the cuticulin
occupies all the flat surface before the formation of folds. This does not
happen. Gaps remain in the cuticulin at the base of folds, until the final
area is achieved as though these gaps are necessary for the increase in
area. Also, these gaps continue to be connected with the plasma membrane by wisps of dense material. Most probably then, the gaps in the
cuticulin are growing points. On the other hand some increase in area
occurs after the surface is covered by cuticulin. The few gaps left at the
time of the formation of the second order patterns are probably insufficient to account for the last phase of expansion in the cuticle over the
wax glands and over scale cells. This expansion may be due to growth
by intussusception or it might be caused by reorganization of the material already there. In this connection, it is perhaps significant that the
last phase of expansion coincides with the separation of the surface wax
layer from the cuticulin.
The mechanism of growth which most easily fits the observations is
the side-to-side accretion of membrane units. This probably does not
happen to all three layers of the cuticulin at once. There are often
partial gaps or disturbances in which only the outer two layers are
irregularly arranged or missing, as though these are the last to fit into
place. It looks as though the inner layer may be the first to form, it
then acts as a surface for the creation of order for the other two layers
which are arranged upon it, rather than as a template.
B. The Gradient Organization of Segmentally Repeating Patterns
Wigglesworth (1940) showed that in Rhodnius, the bristles on a piece
of abdominal integument retained their orientation with respect to the
graft when it had been excised and reimplanted, after being turned
through 90° or 180°. In Lepidoptera, the orientation of scales on implants
with altered position and orientation has been studied by Piepho (1955),
Piepho and Marcus (1957), and Marcus (1962) who found a similar
sort of orienting force in the general epithelium. As there is no individual
or species-specific incompatibility to grafting in Rhodnius or Triatoma,
FIG. 23. The adult cuticle pattern on the abdominal tergites of Rhodnius. χ 150.
FIG. 24. The cuticle pattern on the abdominal tergites of a, Rhodnius adult from a
fifth instar larva burned 10 days after feeding. There is a centripetal displacement
of the pattern not observed when the burn is made earlier or later. See Fig. 36.
X 150.