Flood Bugs on the Move

On two occasions in western Queensland, I've come across vast numbers of what we in Australia call 'slaters' moving along roadsides in the cracking black-soil country; both occasions followed rains. (I'd not expect to see this very often - in that country you don't venture off the bitumen after a few mm of rain, and there's not much bitumen about.)

Swarming Flood Bugs Australiodillo bifrons, Diamantina River west of Winton, Queensland.

These are the so-called Flood Bugs, which live in moist soil crevices in flood plains of inland eastern Australia. Their remarkable mass movements are well-documented, but as far as I can determine their purpose is not understood; it is certainly most unusual in their group.

Slaters (apparently a Scottish term originally) are known elsewhere as pill-bugs or woodlice (among many more localised terms). They are not insects, but crustacea; like insects they belong to the phylum Arthropoda, but within a different sub-phylum that comprises mostly marine animals, including crabs and prawns. The slaters form a wholly terrestrial sub-order of some 5000 species in the Order Isopoda; there is also a similar number of aquatic Isopod species, mostly marine.

They are a very ancient group, going back at least 300 million years, when they appeared in a similar form in the fossil record. They tend to be flattened dorso-ventrally (ie top to bottom) and have seven pairs of legs, each on a body segment, which number immediately seems counter-intuitive to our prejudiced ideas of how things ought to be. What seems even odder is that immatures begin life with six segments and leg pairs, and add another one later as they moult towards adulthood. Another unique aspect of slater moulting is that they do it in two stages; while virtually all other arthropods moult all their exoskeleton at once, the slater does the back half first, and the front segments a couple of days later; the new shell is paler and pinker than the old one.

The big hind legs (or pleopods) contain gill-like breathing organs; this is one reason why they are restricted to moist areas. Up to 100 young are carried in the mother's pouch (known as a marsupium, just as a kangaroo's is).

Some species are very familiar in gardens - indeed several common Australian garden species are introduced, including the familiar Porcellio scaber from Europe. On the other hand many species live far from gardens in the dry inland (albeit in sheltered situations).

Porcellio scaber, Canberra. The big pleopods are clearly visible; other slater characteristics include
the obvious antennae (plus a very inconspicuous pair) and the eyes, which are never stalked.
The first body segment is fused to the head.

As any inspection of a compost heap will confirm, slaters play an important role in recycling dead vegetable material. A curious aspect however has emerged in western New South Wales and Victoria from as recently as 2006, where there are reports, for the first time, of slaters - including it seems our friends the Flood Bugs - eating crop seedlings, including wheat and canola. Explanations for this behavioural change tentatively include changed farming practices and climate change, but we certainly don't know for sure. We can only hope that indiscriminate poisoning doesn't follow before we properly understand what's going on.

Meantime, just a couple more images of the magnificent march of the Flood Bugs.

Above, the massed animals crossing a concrete bridge, spilling from the gutter
into the roadway. References speak of more than 100,00 individuals moving, but
I think we saw many more than that, over hundreds of metres.
Below, swarming up a road reflector sign.

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