Periodic Behavior

 

Insects, like most other living organisms, exhibit cycles of activity that are synchronized with periodic events in their environment.  Hatching, molting, foraging for food, courtship, and nest building are all examples of common behaviors that may recur at regular intervals.  When these periods of activity are repeated at about the same time each day, an animal is said to exhibit diel periodicity (a daily activity cycle).

An actograph is a device that entomologists use to record movement (or other activity) of a caged insect.  Data from an actograph is usually displayed by a chart recorder – much like a seismograph registers earthquakes.  Mouse over each highlighted term (below) to see a corresponding pattern of daily actograph data.

• Diurnal insects are active during daylight hours
• Nocturnal insects are active at night
• Crepuscular insects are active at dawn and dusk

Two additional sub-categories exist to differentiate dawn and dusk periods:

• Matinal behavior occurs only near dawn
• Vespertine behavior occurs only near dusk

Entrainment

Environmental cues such as temperature, light intensity, or location of the sun provide the information animals need to synchronize their behavior with the earth’s daily, monthly, or yearly cycles.  The process of establishing and maintaining this synchrony is commonly known as entrainment.  In insects, entrainment of periodic behavior usually occurs in one of two ways:

1. Exogenous entrainment involves direct response to environmental cues that “trigger” the onset and termination of a particular behavior.  In some species of crickets, for example, males begin chirping at nightfall in response to low light intensity and they continue until the light returns at dawn.  These crickets will also chirp during a solar eclipse or any time they are covered by a box that keeps out the light.  They will not chirp at all if they are exposed to constant illumination.  As the name implies, exogenous behavior is regulated by forces “outside” the organism.  An experimenter can control when crickets chirp simply by changing the light intensity in their environment.
2. Endogenous entrainment involves an internal biological clock that “measures” the passage of time and sends “start” and “stop” signals to the nervous system.  These periodic behaviors are often called circadian rhythms.  In a cricket species where chirping behavior is controlled endogenously, the males will always start to chirp around dusk, regardless of light intensity.  They will not be fooled into chirping during a solar eclipse, or when covered by a box, and they will start chirping around dusk even if the lights are always on.  Start and stop signals are generated from “inside” the organism and cannot be arbitrarily changed by an experimentor.

Even with this internal chronometer, an animal’s circadian rhythms must still be synchronized with the environment.  As day lengths change throughout the year, insects have to adjust their activity cycles to compensate.  In fact, a slight adjustment is made each day in response to a cue (usually light or temperature) from the environment.  This signal, which serves as a daily “reset” mechanism for the biological clock, is called a zeitgeber – derived from the German word for “time” (zeit) and the verb “to give” (geben).

In the complete absence of any environmental cues, a biological clock becomes “free-running”.  Its endogenous period is usually between 24.5 and 25.0 hours, so the animal’s behavior tends to occur a little later each day as the internal clock gradually drifts out of phase with the environment.  Humans experience this same phenomenon in sleep/wake cycles when housed under conditions of constant light or constant dark.  We also experience “jet lag” when we travel to a distant time zone.  In both humans and insects, it takes several days for the body to adjust to a new photoperiod regime.

An actograph of an entrained insect shows a constant pattern of diurnal activity under a regimen of 8 hours light and 16 hours dark. When held in constant light (or constant dark), the same insect shows a phase shift in behavior as its biological clock becomes “free-running.”

Recent discoveries in insect genetics and physiology have revealed that there are different biological clocks for different rhythms.  The clock that controls when a fly lays an egg may not be the same as the clock that controls when the fly forages for food.  Biological clocks are emergent properties of individual cells and may be widespread throughout the body.  Although they regulate different functions, they all appear to have a common genetic origin.  In fruit flies (Drosophila spp.), the clock mechanism consists of four regulatory proteins that interact through a negative feedback system.  The clock cycle begins when two of these proteins (named JRK and CYC) bind together in the nucleus where they “activate” genes that produce two other proteins (PER and TIM).  As the levels of PER and TIM accumulate over time they inactivate the genes that produce JRK and CYC.  This, in turn, leads to reduced production of PER and TIM and, eventually, renewed production of JRK and CYC to complete the cycle.  Normally, this system completes one cycle every 24 hours.  Mutations in the genes that encode these four clock proteins can change the duration of the cycle or completely disable the clock mechanism.

Periodic behavior is certainly not unique to insects — it occurs, to some extent, in nearly every living organism on earth.  To a biologist, this fact suggests that there are strong selective pressures acting to preserve the cells’ genetic mechanisms for periodicity.  For insects, daily periods of inactivity serve as an opportunity to conserve energy and resources.  Many mosquito species, for instance, seek a humid, shady habitat during the day to avoid dehydration.  Likewise, honeybees (Apis mellifera) return to the hive at dusk and pass the night in a quiescent state that has been described as “sleep”.  For other species (e.g. cockroaches and stoneflies) circadian rhythms in emergence and/or molting may reduce the threat of predation.  And finally, synchronized periods of activity within a species can expedite the timing of courtship behavior and reduce the metabolic costs of reproduction.  It’s all a matter of timing!