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
83
cellular mechanisms are involved in several of the syntheses; the cuticulin arises at the tips of one form of microvillus, the protein epicuticle
by the discharge of vesicles, intermolt wax arises from a different form
of microvillus containing smooth endoplasmic reticulum, and the lamellate endocuticle from yet another form of microvillus. Thus, coordination
is required to make extensive morphological changes as well as for the
time of use of the appropriate organelles.
A method of approach to this problem of intracellular coordination
may come from studies on the intermolt series of syntheses (Locke et al.,
1965) and from some syntheses which are part of the molt cycle but,
which are nevertheless not controlled as part of the sequence initiated
by the molting hormone (Cottrell, 1962a-e; Fraenkel and Hsaio, 1962).
In the fifth-instar larva of Calpodes, two molting activities, wax secretion (Locke, 1965b), and endocuticle deposition (Locke et al., 1965)
recur during the intermolt period. Although these activities may require
the presence of an active prothoracic gland, they are not part of the
triggered molting sequence but are under the continuous control of
something from the head. This may act through intermediary metabolism
and other tissues, but the effect of decapitation cannot be duplicated by
starvation alone. If the molt and intermolt phases of synthetic activity
are causally independent, one might expect two periods of preparation
on the part of the epidermis. Evidence for this has come from autoradiographic studies of the rates of RNA synthesis at different times in
the stadium. There are two peaks of maximal activity in each cycle,
one preceding the intermolt period at about 30 hr after molting and one
preceding the molt, 156 hr after molting and 36 hr before pupation.
Intermolt wax and endocuticle synthesis are thus dependent both upon
the preparation of the cell and upon something from the head to control
the use of the cellular machinery previously made ready. Similar syntheses taking place at molting are not controlled in this way. The
lamellate cuticle secreted at molting and the immediate postecdysial
wax bloom are probably the terminal events in the sequence initiated
by ecdysone.
Thus, we have similar or even identical activities controlled in entirely
different ways, intrinsically when part of the triggered molting sequence,
extrinsically when occurring in the intermolt. Studies of the extrinsic
controls of intermolt syntheses may give a clue to the mode of action
of the intrinsic controls of the events at molting. The two sorts of control
may only differ in the time taken to cause an effect, and the source
of one of the links in the chain of command. If we think of the intrinsic
control as the primitive condition then the evolution to hormonal control
may be merely the hypertrophy of one segment of the chain of command
in some cells, coupled with an atrophy of the same part of the sequence
83
cellular mechanisms are involved in several of the syntheses; the cuticulin arises at the tips of one form of microvillus, the protein epicuticle
by the discharge of vesicles, intermolt wax arises from a different form
of microvillus containing smooth endoplasmic reticulum, and the lamellate endocuticle from yet another form of microvillus. Thus, coordination
is required to make extensive morphological changes as well as for the
time of use of the appropriate organelles.
A method of approach to this problem of intracellular coordination
may come from studies on the intermolt series of syntheses (Locke et al.,
1965) and from some syntheses which are part of the molt cycle but,
which are nevertheless not controlled as part of the sequence initiated
by the molting hormone (Cottrell, 1962a-e; Fraenkel and Hsaio, 1962).
In the fifth-instar larva of Calpodes, two molting activities, wax secretion (Locke, 1965b), and endocuticle deposition (Locke et al., 1965)
recur during the intermolt period. Although these activities may require
the presence of an active prothoracic gland, they are not part of the
triggered molting sequence but are under the continuous control of
something from the head. This may act through intermediary metabolism
and other tissues, but the effect of decapitation cannot be duplicated by
starvation alone. If the molt and intermolt phases of synthetic activity
are causally independent, one might expect two periods of preparation
on the part of the epidermis. Evidence for this has come from autoradiographic studies of the rates of RNA synthesis at different times in
the stadium. There are two peaks of maximal activity in each cycle,
one preceding the intermolt period at about 30 hr after molting and one
preceding the molt, 156 hr after molting and 36 hr before pupation.
Intermolt wax and endocuticle synthesis are thus dependent both upon
the preparation of the cell and upon something from the head to control
the use of the cellular machinery previously made ready. Similar syntheses taking place at molting are not controlled in this way. The
lamellate cuticle secreted at molting and the immediate postecdysial
wax bloom are probably the terminal events in the sequence initiated
by ecdysone.
Thus, we have similar or even identical activities controlled in entirely
different ways, intrinsically when part of the triggered molting sequence,
extrinsically when occurring in the intermolt. Studies of the extrinsic
controls of intermolt syntheses may give a clue to the mode of action
of the intrinsic controls of the events at molting. The two sorts of control
may only differ in the time taken to cause an effect, and the source
of one of the links in the chain of command. If we think of the intrinsic
control as the primitive condition then the evolution to hormonal control
may be merely the hypertrophy of one segment of the chain of command
in some cells, coupled with an atrophy of the same part of the sequence