A Dung Beetle that Rarely Eats Dung

While searching for weeds in the regenerating vegetation in Ku-ring-gai Flying-fox Reserve at Gordon, I noticed some little black beetles about 12 – 15 mm long. What were they? What did they do? Did they have a name that we could add to the fauna list for the Reserve. Liz Cameron, in Search and Discover at the Australian Museum, not only had it identified as Cephalodesmius armiger but sent a fascinating paper from the Memoirs of the Queensland Museum, describing its life cycle.

This native (endemic) dung beetle occurs in rainforests and adjacent sclerophyll forests throughout its range from the Bunya Mountains (Qld) to Wollongong. You may have wondered about the 10 -12mm diameter holes in the ground which are surrounded by a circular ‘push up’ of soil. Animal dung is only a minor component of this beetle’s diet. Instead it collects soft plant material, drags it into its burrow, and turns it into food by a process of mastication and decomposition with the aid of fungus.

Annual cycle
In late summer the new generation of adults emerges from the pupal balls in the previous season’s nest burrow. Initially each beetle forms a small burrow but then gradually they pair up. By the start of winter virtually the whole population consists of pairs of beetles in large burrows which they share for the rest of their life.

In early spring the beetles enlarge a chamber at the end of the burrow. Into this they drag leaves and other vegetable detritus. The male soon takes over most of the foraging while the female manufactures a ‘brood mass’ from the plant material.

In late September the female starts to construct small ‘egg balls’ from the brood mass and lays an egg in each. Larvae hatch inside these tiny balls within a few days. Intensive foraging and brood mass manufacture continues. The female plasters brood mass material on the outside of the enlarged brood balls, as the larvae feeds and grows inside.

By mid summer the larvae are full grown and begin to pupate within the large balls which the female continues to tend. The male by now has ceased to forage and is excluded from the brood chamber of the burrow. Soon after, the new generation emerges by which time the original male and female are usually dead.

In late summer there is some overlap of two generations when newly hatched young beetles may be foraging with surviving adults of the previous generation. The latter show excessive wear on their clypeal prongs (spines on front of the head) and tibial teeth (spines on legs). Sometimes copulation between individuals of the old and new generation takes place during above ground foraging.

Foraging
Foraging for food and brood mass material is mainly carried out by the male. It is done during periods of high humidity or direct moisture in the litter layer, usually from dawn to midday. The beetles forage up to 1 metre from the burrow. The beetle carries objects with its forelegs and walks backwards in a direct line to its burrow. The great proportion of items taken into burrows are small leaves or leaf pieces, though any soft organic matter will do, including small flowers and fruits. Where food material is fragmented for transport to the burrow the beetles use a combination of slashing movements of their saw-toothed fore tibiae and thrusting with their clypeal prongs.

The study on the Tooloom Plateau in northern NSW found recognisable dung pellets in 2.3% of the 426 burrows excavated. The researchers noted that at night the beetles sat at the top of their burrows with antennae extended. This nocturnal behaviour may enable the beetles to immediately detect dung pellets dropped by possums or wallabies.

Brood mass
The materials dragged into the burrow by the male form the basic ingredients for the female to manufacture a processed, dung-like substance which is used both to make brood balls and as a food supply for adults. The female compresses the plant material with her forelegs, constantly shaping, squeezing and mashing it. New material and the faeces of both male and female are added. The brood mass grows slowly until it reaches three to four times the size of the female. By this stage fungal activity has commenced and larvae of the phoretic sphaerocerid fly, Leptocera myrmecophila tunnel through the mass.

Eggs and egg balls
Over a period of three hours the female forms an egg ball from the brood mass using all her legs. Once the male has copulated with her she lays one egg into the hollowed out ball. Then it is smoothed to a sphere. Having commenced egg laying she produces one egg ball per day until between 4 and 7 eggs balls, 5mm in diameter, are formed each containing an oval egg 2 – 3mm long. Eggs hatch after a week and the larva feeds on the inner surface of the egg ball. The female adds additional brood material to the outside of each ball as the larva eats and grows inside. The body of the larva is strongly arched so that it is able to progressively rotate within the brood ball. It also enables it to ‘scratch its chin with its tail’ to make sounds (stridulate). The researchers think that the larvae communicates with the mother beetle from within the ball. They believe the message may be just the age-old cry from progeny to parent – “Bring more food!”. No stridulation was observed in adults of these beetles.

Pupation
When the larval brood balls reach a maximum of 16-18 mm diameter, the females cease provisioning and the larva inside pupates. At this time the male stops foraging and becomes sealed off from the nest chamber. The female remains in the nest chamber with the pupal balls. Pupal life is of the order of 2 – 3 weeks. By the time the young adult beetles emerge from their pupal ball most parents have died and the young leave the nest chamber to dig an individual burrow.

Unusual among dung beetles
Cephalodesmius is unique among the dung beetles for processing plant material to feed its young, for permanent pair bonding and for progressive provisioning of the larval balls. The manipulative skills of the female are also unusual among dung beetles. Although this genus is believed to be “primitive” in its subfamily there is no doubt that its immediate ancestors were dung feeders and that its ability to compost vegetable matter is a secondary adaptation.

Acknowledgement
This foray into the world of beetles has been a delightful interlude for a bush regenerator with bats on her mind. It is thanks to Liz Cameron at the Australian Museum and Tom Weir in the Division of Entomology at CSIRO Canberra that we now know about these little animals in Ku-ring-gai Flying-fox Reserve. Thank you Geoff Monteith (Queensland Museum) and Ross Storey (Department of Primary Industries, Mareeba) for doing the original research and publishing it with excellent photographs and drawings.

Note from Ross Storey
Although Cephalodesmius burrows can be very numerous where they are active, not all “push-up burrows” seen will be made be this genus. Other beetle groups, earthworms and scorpions make similar homes.