Biology Reference
In-Depth Information
(
Fig. 1.1
). However, in larvae that have attained the correct target size, star-
vation prevents further growth but does not delay pupariation. Before attain-
ing critical weight, larvae must pass another checkpoint called minimal viable
weight, which is the minimal size at which sufficient nutrient storage has oc-
curred to enable the larva to survive through metamorphosis without further
PHOTO GATE
MASS
(Nutrients)
Mass (nutrients)
CW
JH
clearance
JH
PTTH delay time
PTTH
PTTH
Ecdysone
ECDYSONE
Developmental time
Wandering
TGP
Delayed
Normal size
On time
Reduced size
Starvation
Starvation
Feeding
Feeding
On time
Reduced size
Delayed/
arrested
Nutrient dependent
(Flexible)
Nutrient independent
(Fixed)
Figure 1.1 Hormonal regulation of body size and timing of metamorphosis. According
to classical work the larval accumulation of mass corresponding to the critical weight
results in the breakdown of JH that inhibits PTTH release. Above critical weight, PTTH
is release after a delay period determined by the time of JH clearance and the photo-
period gating of PTTH. Starvation before critical weight results in a developmental delay
of pupariation. If feeding is resumed critical weight is attained and normal body size
achieved. In contrast, larvae starved above critical weight, when the metamorphic
program is activated, pupariate on time but with a reduced size. As critical weight is
independent of nutrition, final size is determined by the amount of growth in the inter-
val between critical weight and cessation of feeding (wandering) called the terminal
growth period (TGP). Photo gate, a PTTH gating mechanism imposed by the photo-
period; CW, critical weight.
