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Common Mole Cricket and Calling Songs - Gryllotalpa pluvialis

Family GRYLLOTALPIDAE

This page contains information and pictures about Common Mole Crickets that we found in the Brisbane area, Queensland, Australia.
 
Male Mole Cricket - playing its love song, or stridulation, at the entrance of its burrow to call for the female.  

Common Mole Cricket

Common Mole Crickets are everywhere in Brisbane. However, mole crickets cannot be seen easily. We can only hear the males playing songs loudly during a wet summer dusk, with a continuous trilling sound from their burrows. During the 2001 summer season, we heard there were at least three of them hiding in my small backyard (600 m sq.).
 
 
Female and Male, notice that the female has the longer forewings and its folded hind wings extended over the end of its abdomen.  Males have shorter forewings completely covering the even smaller hind wings, its abdomen is relatively longer. Both female and male length 50-55mm.
 
Common Mole crickets are dark brown in colour with a shiny thorax. They have characteristic forelegs. Their forelegs are board and turn outward for digging. When dug up, they do not leap away like other burrow-inhabiting insects but dig their way back underground with powerful strokes of the forelegs. The dirt is simply forced aside.  Their organs to hear, the tympanic cavity, are also located on their forelegs (this information is from the reference book, but I cannot find them in my male specimen, the organ could be in female only for this species). Their hind legs are relatively small comparing with other crickets and grasshopper. Their antennae are shorter than body. And unlike other crickets, females are absent with external ovipositors. 

Mole crickets make their living under the ground. They build deep, permanent burrows and also foraging galleries. It is believed that most of the time they are herbivorous, in rare cases they may be carnivorous on small soil insects. They probably take more than a year to become mature. Some mole cricket species are pests to farmers, and specially to golf lawns. However, I cannot notice their damages in my backyards.

Females have the larger pair of hind wings to fly and look for males. Males are not likely that they can fly. Sometimes they are attracted to lights. Unlike other crickets most have long antennae, their antennae are short, quite a few times I found one of their antennae was broken or missing.

 
The strong shovel-like forelegs for digging burrows. 

In the early winter 2001, one night at about 7:00pm, we found a male mole cricket wandering in my garage. we kept him in a tank with some soil. He died the next day. He could be forced out from his burrow by the female after mating. Usually the female selects a male attracted by his call, then mates with him in his burrow. After mating, she will usually force him out the burrow, and she will stay there and lay eggs. We kept the dead male as specimen for further study and found something important - how the male makes such a loud call. Details please read the follows.

The Loud Calling Songs

Mole crickets make sound by stridulation. Males produce songs and build burrows to amplify their love song to attracted females. Females also stridulate but their calls are lower and most often they do it to protect their burrows. Some reference books suggested that females response to males' love song also by stridulation as well.
 
In mating season, female mole crickets look for males to mate. The males play their love songs at the entry of their burrow. It is believed that females, by judging the sound, can determine the health of the male and the quality of the nest that the male prepared. It seems that the females preferred the wet nest and usually the males will only play their love songs in the wet evening. In a summer evening, if there is rain during the day, we will always hear the loud mole cricket sound everywhere in Brisbane. Sometime We can trigger the mole cricket songs by watering the garden just before evening.

Love songs from male crickets are a series of loud, deep tones repeated regularly about two times per second. But it is not quite regular when start. The mole cricket may have to fine tune his song by trying different positions to get the best sound quality. Press the following buttons to hear the different.  

   Non- regular pattern, recorded at the beginning of the song, 10 sec.
  Click here to hear calling song, 5 sec. 
  Regular pattern song, 10 sec. The mole cricket play it for 30min.
 
Love songs from male crickets are a series of loud, deep-toned chirps, like crick-crick, crick-crick, crick-crick repeated regularly about two times per second. They only play for a short periods of time. They start playing when the sky turns dark and end singing when the sky is dark completely, for about 30 minutes.  They do not play songs if the soil is dry. The shape of their burrow is believed to help amplify their songs. It is difficult to locate them by their songs even if you are within a meter from them. If you come a bit closer, they will stop playing. They may resume the song after a minute or so if no more disturbance. Those songs are so loud that you may think that they are the calls from frogs.
 

Rubbing Forewings and Touching Ceiling

Most field crickets and katydids play their love songs by rubbing their left forewing with their right forewing. Common Mole Cricket play their love songs in a sightly different way. The songs from field crickets and katydids usually consist of a carrier frequency and then modulated with pulses. This is generated by rubbing the forewings, each pulse correspond to a stroke of rubbing forewings. The songs from Mole Cricket are characteristic by the addition series of chirps, the further modulation on the pulses and carrier frequency. The chirps are generated by the forewings touching the burrow ceiling. Details of carrier frequency, pulses and chirps are explained in the following section.

With the Mole Cricket's forewings under the microscope, we can see the file vein with stridulatory teeth on left forewing and the peg vein on the right forewing. The file vein is on the wing bottom while the peg is on wing top, and they are facing each other. By a stroke of rubbing both forewings, this generate one pulse of modulated carrier frequency. 

   
 
First picture shows the bottom view of the left forewing, where the file vein with stridulatory teeth locate. The second picture shows the top view the right forewing and the location of the peg.
 
 
 
The Mole Cricket playing the love song at the entrance of its burrow. When a stroke of opening  and closing of its forewings, this generate one pulse of carrier frequency. The frequency depend on the number of teeth on the teeth vein and the speed of moving the forewings.
 
 
 
Besides the pulses, the songs from Mole Cricket are characteristic by the addition series of chirps, the further modulation on the pulses and carrier frequency. The chirps are generated by the forewings touching the burrow ceiling. The male mole cricket plays the love song standing at the entrance of his burrow and facing inwards. By rubbing two forewings together, the mole cricket generates the pulses. By touching the forewings to the ceiling, the mole cricket generates the chirps.
 
  
 
The first picture shows the left forewing of a male Mole Cricket. Notice the area which in contact with the ceiling, it had been worn out a bit,  showing that part was rubbed with the ceiling. The second picture shows when the Mole Cricket rubbing its forewings, with the wings in contact with the burrow ceiling, This enhance the vibration transmitted to the hard shell, which join between the forewings and the abdomen. 

By the inspection of the male Mole Cricket specimen, we found that 2/3 of it abdomen under the wing's base is hollow. There is a hard shell that connects between the wing's base to the hollow cavity. If the Mole Cricket rubs its forewings WITHOUT touching the ceiling, the sound generate is comparatively small. If the Mole Cricket rubs its forewings AND touching the ceiling, which enforce the transmission of vibrations to the abdomen. The empty space inside the abdomen acts as the resonance chamber for the 1st amplification of the sound. By touching the forewings to the burrow ceiling in a constant interval, this generate the chirps. 

Three Stages of Amplification

The Mole Cricket Songs are so loud because there are three stages of amplification. Field crickets and katydids rub their wings and amplify the sound by the wings surface. The loudness is beyond compare with Mole Crickets' songs.

As mentioned above, by transmitting the vibrations to  the empty space of the abdomen ,  the hollow abdomen is used as the resonance chamber for the 1st amplification of the sound.

Mole Crickets build their nest chambers with volume, i.e., the resonance frequency close to the pitch of the love song they play. The thorax of the  mole cricket is just like a plug at the entrance of its burrow. By standing inside or outside a bit, the mole cricket can fine tune the volume of the chamber, i.e. the resonance frequency. Then the sound is further in resonance with their burrow chamber. This is the 2nd amplification stage of the love song. 

Mole crickets build the entrance of their burrows in horn shaped. This acts as the third stages of the amplification. The horn also direct the sound to improve the efficiency.

 
 
The mole cricket playing their love songs by its forewings rubbing together and  touching the ceiling of the burrow. The second picture shows the three stages of amplification -  the hollow abdomen, the resonance chamber and the horn shape entrance.
 
 
We have studied a mole cricket calling site and opened the chamber. The left picture shows the horn and the entrance. The right picture shows the internal of the chamber. 

Love songs from male crickets are a series of loud, deep tones repeated regularly. The sound consists of carrier frequency, pluses and chirps. It is worth to go back and listen again. 

Analyse the Sound Waveforms

The Mole Cricket Song repeated regularly in a pattern. To know exactly how Mole Cricket play the song we need an expensive high speed camera, and a bit of luck. The Mole Cricket may not be willing to play under the heavy equipment set up. However, by carefully analyse the sound waveforms patterns and what we have observed above,  we can reasonably describe how the Mole Cricket play the song.

We recorded the sound by a hand-held tape recorder 200mm - 300mm above the Mole Cricket. Then we play back to the PC sound card via a condenser  microphone and recorded by the 'Sound Recorder' program within Windows98. We analysed the sound waveforms using a free software 'Cool Edit' downloaded from http://www.syntrillium.com.

The sound is a series of chirps, with about 5 chirps per second. Within each chirp, there is seven pulses. Within each pulse, there is the 2.1KHz carrier frequency.

 
Waveform 1. In the waveform picture, 'A' is one chirp. The picture shows two second, i.e., ten chirps of the sound. The second picture shows the frequency spectrum of the sound, indicates the base frequency is 2.1KHz. 
 
Waveform 2. The chirp 'A', with seven pulse 'B'. The pulse rate is about 90 pulses per second.
 
 
Waveform 3. One of the seven pulse 'B', and the 2.1KHz base frequency. 
 
 
Waveform 4. The picture shows the waveform at the end of a chirp. Notice that the last pulse with the amplitude about 1/4 of the normal pulses. This is sometime happen at the last pulse of the chirp. I think this is due to the Mole Cricket low down its wings too early, i.e., rubbing its wings without touching the ceiling. This made the amplitude significantly lower.
 
Now we can summarize what we found and describe how the Mole Cricket generate its love song. The Mole Cricket rubs its forewings in a constant speed. Each stroke generates a pulse of carrier frequency 2.1KHz. The Mole Cricket rubs its wings at the rate of 90 cycles per second. This generates the pulses within the chirp. The Mole Cricket move its whole body slightly up and down five times a second, so it forewings in contact with the ceiling five times a second and generate the chirps pattern. The whole process make the  crick-crick, crick-crick sound.
 

The Tunnel System

We have studied a mole cricket calling horn and the connected tunnel system. Around the calling site, we found at least three horizontal tunnels which link to three vertical tunnels. The horizontal tunnels are built on the top soil which is loose and easy to dig with. The horizontal tunnels are filled with loose soil and they look no different to the other top soil. we have to find these tunnels by following the entrance and feel by our fingers. Some parts of the horizontal tunnels are just 'U' shaped tracks filled with loose soil. Those horizontal tunnels link to the vertical tunnels and they sink to the ground vertically. We cannot follow the vertical tunnels for they are deep and in the hard part of the ground.  

The mole cricket tunnel system around the calling site. 

By looking at the tunnels map, it shows that the mole cricket is well planed for escape. In case being found by the predator during playing the love song, the mole cricket can run away from front or from the back. The map also shows that this is just a site for calling, and not its nest. It is reasonable to guess that the mole cricket comes to the site only in calling time. When we studied the site, we dug around the area and found no sign of the any mole cricket. 


Questions for Discussion

1. Why the horizontal tunnels fill with loose soil?

2. Why does the mole cricket call for only 30 minutes and not longer time?


Reference:
1. Grasshopper Country - the Abundant Orthopteroid Insects of Australia, D Rentz, UNSW Press, 1996, p150.
2. Wildlife of Greater Brisbane - Queensland Museum 1995, p77. 
3. Mole cricket Gryllotalpa pluvialis - Queensland Environmental Protection Agency 2007.

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Last updated: April 23, 2007.