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:
A few frogs were about
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 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.
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.
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.
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