Common Mole Cricket
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.
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.
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.
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.
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.
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.
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.
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.
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?
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