Biology Reference
In-Depth Information
1. INTRODUCTION
Insect molting and metamorphosis are primarily under the control of
two hormone classes; ecdysteroids, which cause molting, and juvenile hor-
mones (JHs), which modulate ecdysteroid action. In most insects, ecdysone
(E) is synthesized by the prothoracic glands (PGs) and converted systemically
to the active ecdysteroid, 20-hydroxyecdysone (20E). JHs are synthesized by
the corpora allata (CA). The chemical identity of JH varies among different
insect groups; in most insects, the main JH is Juvenile Hormone III (JH-III),
but additional forms are present in Lepidoptera (JH-0, JH-I, 4-methyl JH-I,
and JH-II) and Diptera (JH-III bisepoxide; see review by
Goodman &
Granger, 2005
).
In hemimetabolous insects, metamorphosis occurs during a single molt;
ecdysteroid secretion repeatedly elicits larval molts as long as JH is present in
the hemolymph. In the last larval stage, however, JH declines after ecdysis
and adult development is initiated by ecdysteroid in the absence of JH
(
Hiruma, 2003; Riddiford, 1985
). In holometabolous insects, by contrast,
metamorphosis is a more complicated process that takes two successive molts
to complete. In these insects, as in Hemimetabola, repeated larval molting is
induced by ecdysteroid in the presence of JH, and metamorphosis begins
after several larval ecdyses. When a holometabolous larva enters the last
instar larval stadium, the level of JH in the hemolymph gradually declines
and becomes undetectable (as in
Manduca sexta
—
Baker, Tsai, Reuter, &
Schooley, 1987
) or reaches trace levels (as in
Bombyx mori
—
Niimi &
Sakurai, 1997
), then a small amount of ecdysteroid in the absence of
JH induces larval-pupal commitment of the epidermis. Pupal commitment
is an irreversible event, representing the first crucial step of metamorphosis;
it precedes any externally observable change in the insect's form, being
followed only later by the actual formation of pupal cuticle, an event
that is induced by the secretion of a larger amount of ecdysteroid
(
Muramatsu, Kinjoh, Shinoda, & Hiruma, 2008; Riddiford, 1976, 1985
;
Fig. 3.1
B). Metamorphosis continues after the formation of the pupa, which
only then begins to transform into the imago. Adult development is caused
by ecdysteroid in the absence of JH during the pupal stage. The decline of
the JH level in the last larval stadium is one of the most important events
required for initiation and progress of metamorphosis; the continued pres-
ence of JH prevents metamorphosis so that JH preserves the
status quo
. Thus,
it is the regulation of JH level in the hemolymph that determines whether
and when metamorphosis begins and the manner of its continuation.
