When locust become gregarious they are extremely destructive

Locust flight as a model systemCan maintain flight for many hours2 sets of wings:

•Fore wings (second thoracic segment)•Hind wings (third thoracic segment)•Fore/hind wings beat slightly out of phase (7ms)• Beat stroke is complex•Wing beat @ 20 Hz

To fly the locust must solve:•Lift (the ability to maintain a flight force)•Control (the ability to change velocity/direction)•Correction (the ability to respond to external forces)

Comparison of insect wing beat frequencies

Locust (insect) flightBasic behavioral experimental setup (old school)

Locust (insect) flightModern and high tech behavior and neurophysiology

Locust (insect) flightHigh tech behavior and neurophysiology

The wing beat cycle:

•Hindwing precedes the forewing in each wing beat cycle. •Hindwing stroke greater in amplitude•Depressor and elevator muscles initiate each phase of the cycle•Forewing depressor muscle activation lags behind hindwing depressors.

Wing movement and flight forces are complex but rythmicAngle of wing:

•Horizontal on down stroke•Vertical on upstroke•Moves forward on down stroke, back on upstroke•Up/down stroke movements develop vortices•Wing pushes against vortices producing thrust on both strokes

The flight muscles and nervous system involved with flight•Wings are mounted on hinges

•Hinges contain proprioreceptors that indicate wing position/angle•Each wing equipped with several depressor and elevator muscles

•Control up/down strokes and angle of attack•Flight muscle rhythm controlled via TG1-TG3•Brain:

•Initiates flight •Mediates flight rhythm rate and flight surface angle based on sensory feedback

Innervation of the major flight muscles across 3 TGs.

Muscle control: •One to multiple neurons per muscle•Ipsilateral and contralateral innervation•controlled by TG1-3•Motor units are distributed over a broad area

Neural control of flight muscles

•Neural activity from motor neurons are periodic or oscillatory•Forewing lag evident in depressor motor neuron activity•Nevertheless both fore and hindwing motor neurons are coordinated

The concept of the chain reflex•A chain reflex takes sensory input (S) arising from one reflexive behavior (R) and to initiate another behavior:•R1 triggers an S2

•S2 triggers R2 •R2 triggers S1 and so on…

•Assumes sensory input drives patterned responsesThis is not how locust produce wing beat patterns

•Removal of sensory input will reduce wing beat frequency (from 20 to ~10Hz) but does not influence its pattern.

Is it a CPG or something simpler?

Proprioreceptors

“Phase-locked” interneuron motor neuron and muscle activityFigure shows a single muscles activity (M112, a depressor muscle)

•Depressor motor neuron (MN128) positively correlates to M112 contraction•Motor neuron (MN83; elevator neuron) is negatively correlated to M112 contraction•Interneurons (IN301 IN511) are also phasic and correlated to muscle activity

•IN301 and IN511 have different phase relationships to M112 Point: Sensory input mediates wing beat frequency but the generation of the pattern is central.

Simple CPG

•IN301 active during wing elevation•Excites IN501

•IN501 active during wing depression•Inhibits IN301

•Circuit forms the basis of an oscillatory CPG

Locust flight CPG core

Locust flight CPG core with input and output

Locust flight CPG core with input and output and a delay

The CPG circuit (expanded).•Does not show descending input•IN301 indirectly excites IN501 by inhibiting inhibitors of IN501•IN501 directly and indirectly (by inhibiting exciters of IN301) inhibits IN301