Signal generation - acoustic Flashcards
Why do insects generate acoustic signals?
Useful in dense vegetation or at night
Calling songs (uni-directional in crickets as females are mute, bidirectional in bush crickets, grasshoppers and cicadas where female response is often v simple)
Mating songs (in grasshoppers, the elaborate leg movements are often accompanied by mouth and antennae movements to generate a courtship ritual)
Rivalry songs
All of these may generate phonotaxis
What kinds of sounds do insects make?
High frequency (because the sound producer is a frequency transducer - slow frequency muscle movements transduced into high frequency sounds), can be high amplitude if there’s a resonant chamber (e.g. abdomen in cicadas –>120dB, wing harp in crickets –> 100dB) or sounds are concentrated into a narrow frequency band
Often a complex temporal pattern e.g. from elaborate leg movements in grasshoppers
Species specific - some grasshopper species are morphologically identical, and can only be distinguished by sound
Selection on acoustic signals
Acoustic communication developed separately in various insect groups (convergent evolution)
The signal is produced by a ‘fixed action pattern’, a motor program. This cannot be learnt as in many cases the parent is dead before the offspring starts generating signals.
The sound it produces is a key stimulus that identifies conspecifics, and activates an ‘innate releasing mechanism’ in the receiver.
[key stimulus is a well conserved caricature of the species, e.g. 1:3 head:rump ratio of blackbird]
IRM functions as a filter, to decide whether the stimulus should indeed elicit the particular FAP. IRMs can be altered by experience (Gestalt perception = identifying something by its integrated configuration rather than key stimulus). Cf Aquired release mechanisms.
The FAPs of males are under selection by the IRMs of females
Tymbal system - cicadas
A tymbal is a cuticular spring, in cicadas on the side of the abdomen. When the timbal muscle contracts, the timbal bends inwards, then moves back outwards under spring forces, generating a sound pulse.
Cicadas use this inward and outward motion to generate pulses that eventually merge, resulting in complex patterns
Their air-filled abdomen acts as a resonator, allowing sounds up to 120dB
Stridulation - most grasshoppers
File (with cuticular pegs) on legs is scraped against raised cuticular vein on wing
Each peg makes a sound pulse, pulses merge to allow a broad frequency range, from 2-40kHz, but only about 65dB amplitude as there’s no resonator (except in bladder grasshoppers, have an air filled abdomen and position themselves high up to take advantage of temperature gradients, can signal over 1 mile)
Most can only travel over a couple of metres
Stridulation - crickets
File on one wing, scraper on the other
Every time they close their wings, there’s a sound pulse or ‘syllable’
3-5 pulses are grouped into chirps
Pattern of syllables and chirps is species-specific
Temporal pattern is crucial, and controlled by different motor patterns (just like in grasshoppers)
Wing harp acts as an oscillator to produce constant frequency around 4.8kHz. This narrow frequency range allows sound to 100dB.
Measuring basis of sound generation
Can measure motor patterns using electromyogram with v fine wires
Can use glass electrodes to get intracellular recordings
The simplest machinery is in cicadas, because they only need to control the timbal muscle (no antagonistic muscle required, because of the spring forces), and do it using only one motoneuron
The most complex is in grasshoppers, because of their elaborate leg movements
In crickets there are open (M99) and close (M90) muscles for the wings, so there’s speculation that the CPG for singing evolved from the CPG for flying, though no interneurons connecting the two have been found. CPG is located in the abdominal ganglia, determined by systematic lesioning of abdominal nerve cord.
In grasshoppers the CPG is two hemi-ganglionic oscillators. If you separate them, each leg still generates sound but they are not coupled.
Descending control of signal generation
Brain ultimately controls it all - stimulation or injection of neuroactive substances can release the behaviour.
In crickets, a neuropil in the anterior proto-cerebrum, near the alpha lobe controls the behaviour
In grasshoppers, a region posterior to the central body complex is involved
These areas hold the dendrites of command neurons, which are sufficient and necessary for the behaviour, driving CPGs by tonic activity.
Stimulating them will cause singing, blocking them will prevent singing.
They each control different parts of the motor pattern, apparently in labelled lines.
We don’t know how they’re activated.
CPG for stridulation - a closer look, how was it found?
Crickets are best studied… in grasshoppers the CPG is in the metathoracic ganglion, in crickets the abdominal ganglia
Initially we assumed thoracic ganglia, because that’s where the motoneurons are.
But males didn’t sing when connectives behind the thoracic ganglia were cut, and selective warming experiments in male field crickets showed that syllable period is influenced but not totally determined by thoracic input. [Female song preference was shown to be plurisegmentally regulated] All this implicated the abdominal ganglia.
Cutting between A5-A6 has no effect
Cutting between A4-A5 (nearer head) causes chirps of different duration, spacing irregular
Cutting between A3-A4, chirp structure breaks down - sounds produced, but no chirp pattern
SO, A4-6 = chirp timer
A3-4 = pulse timer
T3 (metathoracic ganglion) = feedback loop
Maybe it’s there because ventilatory pattern contributed to evolution?
Abdominal Opener interneuron
No evidence of direct synaptic coupling between opener and closer motorneurons in cricket. So how do they coordinate?
In the field cricket:
IN fires just before MN, v tightly coupled
Stimulating the interneuron shifted the timing of the singing so it was out of phase
