Introduction to insect sound production

While many invertebrates produce sounds incidentally, cicadas, crickets, and grasshoppers stand out for their ability to produce loud, purposeful sounds that play vital roles in their communication, reproduction, and survival strategies. The male songs are indeed specific to each species and can even be more reliable indicators for identification than morphological characteristics.

The male’s song is unique to each species and predominantly follows a repetitive and consistent pattern that can be characterized by its tone, volume and rhythm. Graphical representations of these two parameters provide valuable tools for visualizing and analyzing these specific acoustic features, facilitating the comparison of songs across different species or individuals.

PART I. SOUND PRODUCTION IN ORTHOPTERA

Sound production in Orthoptera, known as stridulation, is achieved by the movement of specialized sound-producing structures. Fundamentally, a line (file, series) of teeth (serrations, pegs) located in one body part moves against a rigid structure (vein, ridge) located on another body part. Each tooth impact produces a discrete burst of sound energy (= one pulse). As the entire file of pegs impact against the scraper, many pulses are generated which create a complex of sounds.

Ensifera: elytral-elytral stridulation

The sound production mechanism known as elytral-elytral stridulation is characteristic of crickets and bush crickets (Orthoptera: Ensifera). Here are the key points:

Sound producing structures in Ensifera (Rafael Carbonnel).

1. Involved structures
– Tegmina: The first pair of hardened wings.
– Thickened vein: Located on one of the tegmina (left in Tettigoniidae, right in Gryllidae).
– Posterior margin: Of the opposite tegmina.

2. Mechanism
– The tegmina rub against each other in a scissor-like motion.
– The scraper (thickened vein) rubs against the file (posterior margin) of the other tegmen.
– Typically, the sound is produced during the closing movement of the tegmina.

3. Variations
– The duration of the closing movement can vary significantly among species:
– Uromenus rugosicollis: 0.4 seconds (up to 2 seconds on cold nights)
– Ruspolia nitidula: Up to 100 closures per second

This mechanism allows Ensifera to produce species-specific calls that are essential for intraspecific communication, particularly in mating and territorial contexts. The variation in the speed and pattern of stridulation contributes to the acoustic diversity observed within this group of insects.

Caelifera: leg-to-elytral stridulation

Sound production mechanism in grasshoppers (Orthoptera: Caelifera) involves a completely different system:

Sound producing structures in Caelifera (Rafael Carbonnel).

1. Structures involved
– File (raspa): A row of tiny teeth on the inner surface of the hind femur.
– Scraper: A hardened vein on the tegmina.

2. Mechanism
– Friction between the inner surface of the hind femur and a hardened vein on the tegmina (forewings).
– The hind leg pivots up and down, rubbing the file against the scraper.
– This can occur up to 20 times per second, varying by species:
– Minimum: 1 stroke/second in S. lineatus
– Maximum: 120 syllables/second in S. nigrolineatus

4. Dual mechanism
– Grasshoppers have two hind legs, providing a double stridulation mechanism.
– This allows for subtle variations in signal structure by phasing the movement of the legs.

5. Variations in Oedipodinae
In this subfamily, the arrangement is reversed:
– The tegminal vein has teeth
– The femur has a smooth internal ridge
– This produces a weaker sound.

This femur-tegmina friction mechanism is distinct from the elytral-elytral stridulation used by crickets and katydids, showcasing the diversity of sound production methods within Orthoptera. This mechanism allows grasshoppers to produce a variety of sounds by altering speed of leg movement, pressure applied and coordination between legs.

Other mechanisms in Orthoptera

The Oak bush-cricket (Meconema thalassinum) is unique in its sound production method among European Orthoptera. Instead of using wing-based stridulation, M. thalassinum drums with one of its hind tarsi (feet) on a leaf or branch.

The drumming produces a series of short, percussive sounds. The sound produced (frequency and amplitude) depends on the substrate the insect is drumming on. This results in variable acoustic properties depending on the surface.

Cicadas: a different insect order = a different sound mechanism

Cicadas (Hemiptera: Cicadidae and Tibicinidae) are close relatives to bugs and aphids. They produce sound using two membranes called tymbals located on their abdomen. These membranes are stretched by small muscles that cause them to vibrate rapidly, while the abdominal cavity acts as a resonance chamber to amplify the sound. When the muscles contract and relax quickly, the tymbals buckle and unbuckle, producing a series of clicks or pops that create the characteristic cicada song. The abdominal cavity enhances this sound, and some species also possess additional air sacs for further amplification. Cicada songs can reach extremely loud volumes, sometimes up to 120 decibels, and the frequency and pattern of the clicks are specific to each species. Typically, males are the primary sound producers, using these calls for mate attraction. They can modulate their songs by controlling muscle contractions, allowing variations in volume, frequency, and pattern. This tymbal mechanism is unique to cicadas among Hemiptera and differs fundamentally from the stridulation used by many other insects. Additionally, it is highly energy-efficient, enabling cicadas to produce loud sounds for extended periods, showcasing a remarkable adaptation for acoustic communication.

PART II. PARAMETERS TO DESCRIBE SOUNDS IN ORTHOPTERA

The main physical parameters used to analyze and distinguish the songs of different species are:

Tone of an insect song

The frequency of the song refers to the tone or pitch of the sound and is measured in Hertz (Hz). While low pitched sounds produced by Orthopterafall within the hearing of humans, high pitch sounds are inaudible and must be recorded with specific microphones.

Some Orthoptera (crickets and grasshoppers) produce low-pitched sounds that are audible to humans and others (bush-crickets) emit high-pitched sounds that require specialized microphones to detect. Note that insects can produce sounds of different frequencies at the same time that are variable to some extent during the song.

The frequency of sound over time is visually displayed in graphs called spectrograms.

Spectrogram of an Orthoptera sound. X = time. Y = Frequency (kHz). In red, higher amplitudes.

Volume of the sound

Sound amplitude, commonly known as loudness, is expressed in decibels (dB) and represents the intensity or strength of an acoustic signal. Decibels use a logarithmic scale to represent the wide range of sound intensities that the human ear can perceive. The human ear perceives an increase of 10 dB as a doubling of volume.

Of course, the volume of a song is not constant and oscillogram represents the amplitude of the sound as a function of time.

Oscillogram an Orthoptera sound. X = time. Y = Amplitude (dB).

Rhythm of the song

The rhythm is visible on the oscillogram, especially examining records at very slow motion. Rhythm or repetition rate refers to the temporal pattern of the sounds, generally measured in pulses per second or per minute.

> Echeme is the visible unit of the song pattern

An echeme is a discrete unit of sound production in insect calls. It’s often described as a “chirp” or a “trill” in common language. Echemes are separated by periods of silence and form the basic building blocks of many Orthopteran acoustic signals. Therefore, it can be considered as the basic unit of the song pattern although it is a complex of individual sounds.

Oscilogram showing a succession of echemes. In red: one echeme.

> Syllable is the sounds produced by one complete movement of the stridulatory system

A syllable is the unit of sound production in insect calls corresponding to one complete cycle of wing movement in crickets or one leg stroke in grasshoppers. Syllables are the components that make up echemes.

The following drawing is even slower down and also shows in the top part both legs movement of a grasshopper along a time line, resulting in sound production (displayed here as an oscilogram). A syllable (S) is the unit which represent an entire movement of wings/legs.

Slow-down oscilogram, showing a series of syllables (S).

> Pulse is one single tooth impact

A pulse is a fundamental unit of sound production, a brief and discrete burst of sound energy produced by a single movement of the finer sound-producing structure (= one tooth stroke).

We can distinguish the duration (D) of the pulses, time that each individual sound lasts within the song, and the intervals (I) between pulses, the time of silence between each sound. Period (P) is D+I.

Slow-down oscilogram, showing the duration (D), interval (I) and period (P) of a pulse.

Revisited spectrogram

Another way to represent a stridulation is to not consider the time as the X axis, but instead analyze the characteristics of the sound during an entire lapse of time. In this case, the frequency is represented against the amplitude, showing quickly which is the dominant pitch during the song.

Another spectrogram representation. X = Frequency (kHz) instead of Y axis as in the usual graph. Y = Amplitude.

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