Getting to the heart of the arthropod world
A new technique has been developed to measure the heart rate of arthropods – invertebrates with jointed legs, such as grasshoppers, ants, stick insects, moths and spiders – without touching or disturbing them.
The world-first pilot method was designed by University of South Australia PhD candidate Danyi Wang and her supervisor Professor Javaan Chahl.
By analysing video footage of resting animals, the researchers were able to use subtle movements in their body muscles and pumping vessels to accurately measure cardiac activity.
This process allows arthropod heart rates to be measured without capturing the animal or disrupting any natural behaviours.
It could transform how scientists monitor the health and stress levels of arthropods, a group that makes up between 75–85 per cent of known animal species.
Building on previous research
Wang, Chahl and their team have previously pioneered similar remote sensing techniques to conduct health checks on a range of vertebrate species.
High-resolution digital cameras were used to pick up tiny chest movements of giant pandas, Sumatran tigers, koalas, red kangaroos and little blue penguins at the Adelaide Zoo and accurately calculate their heart and breathing rates.
Going one step further, the team then calculated similar health parameters of wild zebras, antelopes and giraffes in Malawi using long-distance drone footage.
This experience gave the team confidence to expand their methods into the invertebrate world.
Arthropod hearts are located on the top side of the abdomen, pumping blood through the entire body cavity in a rhythmic manner.
“Since the heart lies just beneath the cuticle, or outer layer, of the arthropod body, its pumping motion can be seen by the naked eye in some species,” Wang told Australian Geographic.
In recent years, imaging technologies have been developed to observe this cardiac function in arthropods. However, previous observation methods, such as touch sensors or purpose-use microscopes, required some degree of body fixation or physical intervention.
“To the best of our knowledge, this is the first study to extract physiological signals from arthropods using a conventional digital camera,” Wang said.
“Our study shows that subtle body movements can be captured on a standard camera, or even a smartphone, and then analysed to reveal accurate and detailed cardiac activity in a range of arthropod species.”
Giant stick insects were found to have a heart rate of approximately 45 beats per minute and the redback spider about 75 beats per minute. The heart rate of the European honey bee was much faster, at approximately 119 beats per minute. The results aligned closely with reference data from previous studies across a range of species.
The study also detected physiological differences influenced by factors such as wing morphology and temperature.
Exciting real-world applications
“Insects and arthropods provide essential services like pollination and nutrient cycling, and so are vital to our ecosystems. As such, understanding their physiological responses to environmental change is essential,” Wang said.
“Unlike traditional methods that require physical contact or immobilisation, our technique allows study species to remain free.”
The study marks an important step forward in invertebrate research. It’s hoped the method will provide accurate insights into arthropod heart activity by preserving their natural behaviours.
This may be important not just for studying individual insect health but also for understanding environmental stressors, the effects of pesticides, and even colony health and behaviour in social species like ants and bees.
“Non-invasive cardiac monitoring offers enormous potential,” Chahl said.
“There are a lot of possibilities here. Honeybees, for example, are under unprecedented attack from threats like Varroa mite and the overuse of pesticides. Honey bees are not native to Australia, but nonetheless, they’re economically important, particularly to agriculture, and the more accurately we can monitor them the better.
“With insects, sometimes their internal state may not match their external behaviour. Something like a honey bee or an ant is going to behave as it has to behave up until the point where external pressures mean that it can’t anymore. However, we may be able to get ahead of potential physiological conditions by knowing what their inner state is. If heart rates can be used as a proxy for that, five to 10 seconds of video would be enough to tell us lots of things which, at the moment, we absolutely have no knowledge about.
“There is a lot of interesting environmental monitoring that can be done once you can measure these things. There’s no reason why we can’t start remotely monitoring insects and learning all sorts of things that will help us to understand how their cognitive state is connected to their physiological state.”
After all, the more we know, the better we can support these animals that are – quite literally – all around us.
