The Diving Beetle

Female H. shuckardii
Female granulose form of Hyderodes shuckardii from the South East of South Australia.

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.

Male H. shuckardii
Male H. shuckardii from the South East of South Australia.
blank-australia-maps-thread-map-what-im-doin-inside-outline Swamp Wallaby
South East of South Australia

The Tiger Beetle

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The Tiger Beetle Pseudotetracha whelani on Lake Gairdner, South Australia

Driving along a remote outback road with dust bellowing up into the clear night sky, the rocky hills are lit with the faint blue light of the moon which slowly wanders across the sky. We arrive at our destination and turn the head lights off to be confronted by a wide glistening expanse before us, a surface encrusted with salt crystals stretching almost to the horizon.

This is Lake Gairdner, the third largest salt lake in Australia, which is accessible through Mt Ive Station in the Gawler Ranges in South Australia. Salt lakes are one of the most bizarre and beautiful landscapes I have ever visited. There is such an expanse of flatness which you rarely encounter in day to day life, and the brightness of the sunlight reflecting off the surface during the day can be astounding. They seem like a weird moonscape, apart from I imagine the moon is not as flat. At night, with a clear sky, they are breath-takingly beautiful. The moonlight glistens off the salt crystals and the stars above don’t feel so far away.

While wandering around the edge of Lake Gairdner admiring the wide expanse of sky studded with the diamond like stars, something speedily scuttled past my foot. My torch zips about across the salt, a flash of iridescent green, then gone into the darkness. My light finally locks onto one of these creatures to reveal a large brilliant green shining beetle with long yellow legs and mandibles longer than its head.

This voracious looking creature is a tiger beetle of the genus Pseudotetracha, which is found mainly on salt lakes, and unsurprisingly is a nocturnal predator. These beetles dig deep burrows where they spend their scorching outback days, and then come out at night to hunt. When I say speedily scuttled past my foot I was not joking, Pseudotetracha whelani, the species we encountered, has a body length of around 17.5 mm and run at a maximum of 0.72 m per second (2.6 km/h). That’s 41 body lengths per second. This is like me running at about 265 km/h! Another Tiger Beetle Cicindela hudsoni (approximately 21 mm long), is thought to be the fastest running insect in the world and can run at an astonishing max speed of 2.49 m per second (9 km/h) and 120 body length per second. C. eburneola although having a lower max velocity (1.86 m/s) can move at 171 body lengths per second. These are some amazing stats.

The group of Tiger Beetles within the genus Pseudotetracha appears to have evolved into several different species associated with Australia becoming more arid between 10 – 5 million years ago. As Australia dried up, the ancestral beetle populations became isolated from each other at different salt lake drainages. They begun to change over time eventually becoming different species. The salt lakes act like islands surrounded by water where the area in-between the habitable island or lake is mostly impassable and the beetles from one lake cannot come into contact with beetles from another and breed. Despite being able to run fast, many of these Tiger Beetles are poor fliers or are unable to fly, like P. whelani the species I saw, contributing to their isolation.

Salt lakes are stunning places to visit, and Australia has a good number of them. Being out on one at night you may encounter some of these speedy beautiful Tiger beetles going about their nightly hunting. However, these extremely interesting insects are not just found on salt lakes but in all sorts of habitats all over the world. So keep an eye out for these voracious and fascinating beetles.

I would also like to thank Dr Peter Hudson from the South Australian Museum for verifying the ID of this Tiger Beetle given the location and photograph. Very much appreciated.

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Lake Gairdner, South Australia