Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 216

Core
mechanism

Zeitgeber Input Output

Black
box

Neurosecretory
cells in CNS

Clock
&
Counter

Photoperiod Receptors

Neurohormones
Neurotransmitters

Photoperiodic calender

Developmental
hormones

Development Diapause

Figure 4.10 Schematic structure of the photoperiodic calendar. Environmental signals
such as photoperiod are perceived by poorly characterized receptors and processed in a
core mechanism composed of a photoperiodic clock and counter. Physiological principles
underlying functionality of a core mechanism remain unresolved in insects. Most of the
current knowledge comes from experiments using a black box approach. Output pathways
include neurosecretory cells that produce neurohormones, neurotransmitters, and mitogens,
which in turn influence biosynthesis and the release of developmental hormones, the presence/
absence or titer of which determine developmental destiny (direct development vs. diapause).
Source : From Koštál (2011) .

insects decide on whether to continue developing or stop growing and fall into a
safer dormant state until a more conducive season arrives. Their decisions are based
on the photoperiodic cues, shortening of the day length, and decline of temperature,
which presage the approach of winter. On identifying these cues, the animal turns
off its development, then turns it back on when it perceives a sufficient change in the
day length and temperature, which presage the approach of spring.
A simplified schema of the mechanics of the photoperiodic calendar in determin-
ing the diapause in insects would look as follows:
Photic cues on the length of the day are received by photoreceptive neurons in
the retina (directly from the brain in some insects). Shortening of days are perceived
and counted in the brain. When the number of the shortening days reaches a set
point, particular neurons in the pars intercerebralis and pars lateralis of the insect
brain start a neuroendocrine cascade. Juvenile hormone (JH) and ecdysteroids act as
downstream signals, inducing diapause with interruption of growth and development
while maintaining a minimal metabolism. The structure of the insect's counter to the
shortening days in its brain, which may be at the heart of the photoperiodic calendar,
represents a black box ( Koštál, 2011 ).
The present state of knowledge on the photoperiodic calendar and the circadian
clock does not allow for conclusions about whether there is a single system that per-
forms both functions of the circadian clock and photoperiodic calendar or whether
there are two separate, but cooperating systems ( Koštál, 2011 ).