Friday, 15 August 2014

Constant Cacophany

Ever since the end of May on nights when it is warm and wet, the rainforest pulses with what seems to be a continual din of cricket song.

The songster is a short-winged field cricket, Cephalogryllus species.


This is a pretty large cricket, measuring about 25 mm in length. They seem to be quite common but localised to some extent with small aggregations here and there.

They are subterranean.  Males sing from the burrow entrances after dark on nights when the humidity and temperature is to their liking. It seems that they are silent when the temperature drops to 14C. The short wings serve only to produce the song. They are too short for flight.
There are 14 described species all from the Australian tropics. This one is most likely C. tau Otte and Alexander, but the hind leg is quite different from that described for typical C. tau. So it may be undescrbed.

You can hear a recording of this species at:
https://www.flickr.com/photos/naturenoises/10466004004/in/set-72157636917906914

Although the loud sounds coming from these crickets reminds one of a continuous calling tree cricket (Oecanthinae), the individual calls are just long trills followed by a pause. But they are not co-ordinated so the result to the ear is a continuous trill. They are highly ventriloquial and individuals are very difficult to pinpoint. Once this is successful, it is often a bit embarrassing that the calling male was so close and yet apparently "invisible".

The crickets use their heads to move soil to the surface when they are excavating, hence the origin of the generic name. I have not properly dug the burrows to reveal their structure. This is a tedious task as the burrows meander through a labyrinth of roots and stones. The few that I have dug produce only males. Several had last instar male nymphs.
This is a last instar male. Note the short, undeveloped first pair of wings, the tegmina.

The biology of these neat insects is still poorly known. They re said to drag dead leaves into their burrows. I have never seen an individual abroad at night. Baiting with an oatmeal trail has resulted in only a few cockroaches and beetles but none of these crickets. One would assume that the reason for the males' calling is to attract females. This would seem to necessitate females moving about on the surface of the ground but as noted above, females have not been found. I have checked the oatmeal at several times during the night.
It was noted in Otte and Alexander's treatise on the crickets of Australia that the excavated soil around the entrance to the burrows was peppered with cricket droppings. These can be seen as the black spots above. This may not be happenstance. Could the droppings have some attractive properties that aid in the procurement of females? On the other hand, they may have some repellant function to ward off enemies. Of course, at this point, this is just conjecture and there may be nothing associated with the droppings.

Cephalogryllus crickets seem to be very common and widespread. Similar loud calls are heard each night in the rainforest across the street from the Cairns Botanical Gardens and in the forest near the Gondwanaland section


Literature
Otte, D. Alexander, R. D. 1983. The Australian Crickets (Orthoptera: Gryllidae). Academy of Natural Sciences of Philadelphia. Monograph 22Pp. 1-477.

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