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The delayed terminal differentiation of late CCAP neurons may reflect
the need to discriminate between larval and pupal ecdysis, both of which
require functional CCAP-EN neurons ( Veverytsa & Allan, 2011 ). By differ-
entiating late CCAP neurons just before they are needed for pupal ecdysis,
this developmental delay may act as a sophisticated switch to ensure that
CCAP neurons can distinguish between larval and pupal ecdysis behavior.
While the late CCAP neurons are sufficient for pupal ecdysis, animals still fail
to inflate their wings, suggesting that the other CCAP neurons have some
functionality during pupal development. For this reason, it seems likely that
the switch from larval to pupal ecdysis is accomplished by integrating the late
CCAPs into the existing CCAP network, as opposed to a true switch where
existing CCAP neurons would surrender all
functionality to the late-
emerging CCAP neurons.
10. JHs AND DEVELOPMENTAL TIMING
In this last section, we will discuss recent findings that explore the rela-
tionship between JHs and developmental timing as well as circadian rhythms.
JHs function in a wide range of biological processes, includingmorphological
processes, behavioral changes, caste determination in social insects,
and diapause and vitellogenesis in adult insects ( Ishikawa et al., 2012;
Saunders, Richard, Applebaum, Ma, & Gilbert, 1990; Sroka & Gilbert,
1974 ). JH blocks the differentiation of imaginal discs, but promotes their
proliferation, consistent with its role in inhibiting metamorphosis and pro-
moting juvenile stages. Multiple lines of evidence from the last two decades
show that JH antagonizes ecdysone production by repressing PTTH secre-
tion in the final instar of holometabolous insects. In the Manduca sexta , during
the last larval instar, JH appears to control the timing of pupal commitment
by regulating the gated release of PTTH ( Berger & Dubrovsky, 2005;
Mizoguchi, 2001; Rountree & Bollenbacher, 1986; Sakurai, Okuda, &
Ohtaki, 1989 ). However, the role of JH in regulating PTTH secretion is
not well defined in Drosophila .
The discovery of the JHs started with a series of elegant experiments car-
ried out by Vincent B. Wigglesworth in the 1930s, who identified a brain-
derived hormone activity that could suppress molting to the adult form
in last (i.e. 5th) stage Rhodnius nymphs ( Wigglesworth, 1934, 1936 ). Essen-
tially, Wigglesworth joined immature nymphs from an earlier stage to 5th
stage nymphs using a tube that allowed the hemolymph of the two animals
to mix, a technique referred to as parabiosis. Using this approach, he tracked
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