Saturday, 1 September 2012

A Cold Night On Mt Baldy

It was a cold late winter's night with the wind blowing, trees creaking and mist blanketing the tree tops. We thought we were in for a poor night.

It seemed that not much was on offer.

A few dung beetles were at the ready:

But they had to be optimistic as we saw only a bandicoot. However, wild pigs had dug up the gutters of the dirt track looking for whatever they could find.

A few frogs were about

I suspect if was slim pickings for the frog population, especially those waiting on leaf surfaces.

Three cockroach species provided the interest for me. All are in the large and varied Australian endemic genus Johnrehnia. [The peculiar generic name is easily explained. It was named in honour of John W. H. Rehn, son of the famous Philadelphia entomologist, James A. G. Rehn. John studied cockroaches, especially their wing venation and how it relates to their classification.]

Two of the three species illustrated below were already known from Mt Baldy. The third was seen for the first time.
 Johnrehnia lakebarrina Roth was described from Lake Barrine, of course. The lake is not far from Mt Baldy. This attractive cockroach lives in leaf litter during the day but emerges after dark to feed on the bits and pieces of particulate matter that accumulates on leaf surfaces.
 Johnrehnia tibrogargana Roth was named for Mt Tibrogargan, one of the peaks in the Glasshouse Mountains of southern Queensland. It was a poor choice of names because even at the time it was described, it ws known from a number of localities along the coast and inland north to Cooktown. J. tibrogargana is common on leaves after dark but on this evening it seemed too cold for it to do much more than wander about in the leaf litter on the ground. On warmer evenings this cockroach is fast-moving and readily flies at the slightest disturbance.
Johnrehnia ruggi Roth was the find of the evening. This species was named in honour of Dr Doug Rugg, former student of Dr H. Rose, University of Sydney. This species appears to be restricted to leaf litter and does not ascend to feed on leaves after dark. That is probably why we had not seen it in the past. All that we saw wereon the ground and several were in cracks and did not appear to be interested moving much farther abroad.

 Johnrehnia species were placed in three species groups by Roth based on the shape of the male's subgenital plate. This is a distinctive structure used in cockroach taxonomy. Both J. tibrogargana and J. ruggi were placed in the Tibrogargana Group.

The subgenital plate consists of all sorts of gadgets having to do with copulation. Below we see the plate itself plus some of the internal structures comprising rods, hooks and the like. One of the neat things about cockroaches is that in many species their genitalia are not bilaterally symmetrical. This means that the left side is different from the right. So if a successful mating is to take place, the bits of the male must fit into specific counterparts in the female. If not, then a successful mating may not be achieved.
 J. tibrogargana, male subgenital plate, dorsal view. (Modified from Roth, 2000)

J. ruggi, male subgenital plate, dorsal view. (Modified from Roth, 2000)

Taxonomists use the shapes of the bits and pieces of the male subgenital plate to help identify species. There are many other features of a cockroach that are distinctive as well. But it is easy to see the differences in the two species above.

Tibrogargana Group species, and there are five of them, have the hind margin of the plate complex with the corners with setal and spine-like structures. In addition, the subgenital plate is not flat but subcylindrical. Here is is illustrated and flattened out.

J. lakebarrina male subgenital plate, dorsal view. (Modified from Roth, 2000)

It is easy to see the differences in the subgenital plate of this species when compared to the two above. It is a member of the Hodgkini Group along with more than twenty other species. This group is distinctive in that its species have the hind margin of the plate simple with the right and left styles (the appendages in the middle) dissimilar, the left one being small and cylindrical. In addition, the plate lies flat and is not subcylindrical.

So there is more to a cockroach than most people realise. They are complex and interesting organisms that have been around long before the dinosaurs. Australia has a remarkably large fauna of these insects. Only a half dozen species cause problems. All of the others (over 500 in Australia!) are at home in their habitats which do not include human dwellings. They are extremely common, especially in forests. They must be very important in the decomposition of leaf litter and the ultimate health of most habitats. Many are very colourful and attractive. You will be seeing more of these in the future!


Roth, LM. 2000. The Australian cockroach genus Johnrehnia Princis (Blattellidae; Blattellinae) Oriental Insects, 34: 83-192.


Camera Trap Codger said...

A bit racy toward the end there, but overall a successful excursion.

Piotr Naskrecki said...

Interesting that three congeneric cockroach species were found in the same habitat. I wonder how they partition their niches. Is there anything known about their food or microhabitat preferences?

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