I am going to start this post a little differently today because there is a little more back story to this encounter. Part of my PhD research involves investigating the relationships between size, metabolic rate and dive characteristics in Diving Beetles, the family Dytiscidae. Diving beetles, of which there are more than 4,000 described species, are found in most freshwater habitats and are often the top predators in fishless environments. Both the larvae, sometimes called water tigers, and the adults beetles have veracious appetites with some of the larger species being know to attack tadpoles and small fish.
To investigate the relationships between size, metabolism and dive characteristics I needed to collect a number of diving beetle species across a wide range of sizes, so I travelled to three areas in South Australia. Initially I had to travel away from Adelaide to the South East of South Australia and into the Flinders Ranges to collect the large species, then small beetles could be collected in the Adelaide Hills. This post describes collecting in the South East.
We arrived at the edge of a road to be greeted by a swamp partly surrounded by teatree and reeds. Near the swamp large cryptically coloured grasshoppers leapt from the ground flashing their bright yellow wings, which were hidden while sitting on their blade of grass munching away. There was a small opening on one side of the swamp where we could get to the water’s edge. It was a beautiful sunny day, and a dragonfly was sitting on a stem of the incredibly diverse aquatic vegetation just after it had emerged from its nymphal stage, still soft and not ready to fly.
Despite the beauty we had to get to work and placed a few bottle traps into the water with a little bit of fish as bait. Within minutes large dark figures begun to zip around near the traps in the tea coloured water, some almost as big as your thumb. Moments later diving beetles were appearing in the traps and were voraciously consuming the bits of fish.
We pulled up the traps to find two species of diving beetle, Onychohydrus scutellaris, a whopper of a beetles at almost 3 cm long and either in iridescent emerald green or sometimes brown, and Hyderodes shuckardii which was about 2 cm long and much darker in colour. These two species represent two of the three largest diving beetles in South Australia.
It was not until you looked at H. shuckardii up close that you could see their more subtle beauty. The surfaces of the wing covers (elytra) on the males were smooth and dark green in colour and their legs a dark amber. The females on the other hand were not smooth, instead their wing covers and pronotum (bit between the head and wing covers) were rough showing mottled green and amber. These females are the “granulose form” of females. The other type of female looks like the males with smooth wing covers and pronotum.
This granulose condition in the females is thought to give the females more control over which male to mate with. The males have pads on their forelegs with heaps of tiny suckers that they use to grab onto and hold onto the females. The roughness of the granulose females’ surfaces reduce the number of suckers that can attach and makes it easier for her to get away from the male if she wants to. The ratio of the smooth and granulose females in an environment may relate to the mating intensity, and all of the females we caught were the granulose form suggesting mating intensity was high in that particular swamp.
As with other insects, diving beetles have an air filled respiratory system, like ours but instead of lungs they have a network of branching tubes running from holes on the outside of their bodies (spiracles) to the cells where the oxygen is needed and carbon dioxide is produced. Diving beetles overcome this problem of having a gas filled respiratory system in water by holding a bubble of air collected from the water’s surface underneath their wing covers which supplies oxygen for them while they dive. This is where my research comes in. I’m trying to find out how much air different sized beetles can hold, how quickly they consume oxygen from the bubble and how this effects the time the beetles can stay underwater.
Diving beetles are very widespread and can often be found in freshwater habitats all over Australia and the world in various sizes from a few millimetres to several centimetres. They are fascinating creatures for more reasons than I have included here, so keep an eye out for them, either while dabbing around your local pond with a net or you may find them flying around lights at night.