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
35
ultrastructure, the cuticulin, a dense homogeneous layer below it, the
protein epicuticle, and the lamellae of chitin and protein which form the
bulk of the cuticle. These layers also differ from one another in their
origin. The cuticulin arises as a membrane at the surface of the plasma
membrane, the protein epicuticle is secreted from vacuoles arising in
Golgi complexes, while the fibers of the lamellate cuticle seem to be spun
from the tips of the microvilli. Each of these layers may vary in its
properties or alter in time depending upon the degree of stabilization of
the proteins by quinones (sclerotization) or other processes. In tracheae,
there is another sort of cuticle with fibers arranged in a meshwork. There
are also fibrous bundles within some cells which have a role in the
morphogenesis of surface patterns. In addition, there are lipids in various
forms which traverse the cuticle before appearing at the surface of the
cuticulin. Thus, the epidermis has a varied repertoire of syntheses which
it can undertake and products which it can secrete at different times. The
development of many patterns depends upon the type of cuticle and the
time at which it is secreted.
1. The Cuticulin (Fig. 2)
The term cuticulin was introduced by Wigglesworth (1933, 1947) to
describe the material composing most of the epicuticle. It has since been
used in a number of different senses. It is used here to mean the dense
lamina forming the outermost layer of the protein epicuticle (Locke,
1966a). It is only a little thicker than a cell membrane ( =^= 100 Â depending upon the stage, location, and insect). In electron micrographs of sections, it appears as a thin dense line covering the entire surface of insects
from tracheoles (Locke, 1958a) to gland ducts (Eisner et al., 1964), hairs
and scales (Paweletz and Schlote, 1964). It seems only to be absent over
some sense organs (Slifer, 1961) and the gut (Bertram and Bird, 1961). It
is the first layer to be secreted during the formation of new cuticle and is
concerned in the formation of the surface patterns which are often
elaborate and limit intermolt growth.
2. The Dense Homogeneous Layer, the Protein Epicuticle (Fig. 3)
Immediately below the cuticulin in most cuticles is a much wider
layer ( ^ 1 μ) of homogeneous, rather dense cuticle, which is the refractile
line called the epicuticle in sections observed with light microscopy.
Nothing is known of its chemical composition, other than that it is
protein and probably the site of enzymes (phenolases; Lai-Fook, 1966a).
It is proposed to use the term "protein epicuticle" for this layer until
more is known of its nature. This is not intended to imply that the
cuticulin part of the epicuticle may not also contain protein. During
the formation of the cuticle, the protein epicuticle is formed from dense
35
ultrastructure, the cuticulin, a dense homogeneous layer below it, the
protein epicuticle, and the lamellae of chitin and protein which form the
bulk of the cuticle. These layers also differ from one another in their
origin. The cuticulin arises as a membrane at the surface of the plasma
membrane, the protein epicuticle is secreted from vacuoles arising in
Golgi complexes, while the fibers of the lamellate cuticle seem to be spun
from the tips of the microvilli. Each of these layers may vary in its
properties or alter in time depending upon the degree of stabilization of
the proteins by quinones (sclerotization) or other processes. In tracheae,
there is another sort of cuticle with fibers arranged in a meshwork. There
are also fibrous bundles within some cells which have a role in the
morphogenesis of surface patterns. In addition, there are lipids in various
forms which traverse the cuticle before appearing at the surface of the
cuticulin. Thus, the epidermis has a varied repertoire of syntheses which
it can undertake and products which it can secrete at different times. The
development of many patterns depends upon the type of cuticle and the
time at which it is secreted.
1. The Cuticulin (Fig. 2)
The term cuticulin was introduced by Wigglesworth (1933, 1947) to
describe the material composing most of the epicuticle. It has since been
used in a number of different senses. It is used here to mean the dense
lamina forming the outermost layer of the protein epicuticle (Locke,
1966a). It is only a little thicker than a cell membrane ( =^= 100 Â depending upon the stage, location, and insect). In electron micrographs of sections, it appears as a thin dense line covering the entire surface of insects
from tracheoles (Locke, 1958a) to gland ducts (Eisner et al., 1964), hairs
and scales (Paweletz and Schlote, 1964). It seems only to be absent over
some sense organs (Slifer, 1961) and the gut (Bertram and Bird, 1961). It
is the first layer to be secreted during the formation of new cuticle and is
concerned in the formation of the surface patterns which are often
elaborate and limit intermolt growth.
2. The Dense Homogeneous Layer, the Protein Epicuticle (Fig. 3)
Immediately below the cuticulin in most cuticles is a much wider
layer ( ^ 1 μ) of homogeneous, rather dense cuticle, which is the refractile
line called the epicuticle in sections observed with light microscopy.
Nothing is known of its chemical composition, other than that it is
protein and probably the site of enzymes (phenolases; Lai-Fook, 1966a).
It is proposed to use the term "protein epicuticle" for this layer until
more is known of its nature. This is not intended to imply that the
cuticulin part of the epicuticle may not also contain protein. During
the formation of the cuticle, the protein epicuticle is formed from dense