By Brent Sinclair, University of Western Ontario
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I’m currently on sabbatical in the Department of Zoology, University of Otago in Dunedin New Zealand. This is the department where I did my PhD, so it is an opportunity to come back to familiar territory and re-connect with all sorts of people and places from the past. It’s not a very insect department, but there is a lot of interesting work on ecology, parasites and freshwater biology. A sabbatical is all about recharging scientific and creative batteries, so my main goal here is to write and read and think (and drink coffee and run and hike – but that’s for a different blog), but I felt that I also needed to justify coming all this way by actually gathering some data while I’m here. Respirometry is the perfect answer – once set up, it’s possible to gather data on metabolic rates, breathing patterns and water loss at the expense of only a few minutes at each end of a run, leaving plenty of space for writing and drinking New Zealand’s excellent coffee in between.
What is respirometry?
Respirometry is the science (art?) of measuring the products and substrates of respiration – depending on your strategy, you can measure oxygen consumption and/or carbon dioxide production (to get a handle on metabolic rate) and water loss – among other things. Because I work on generally small insects at generally low temperatures, we mainly measure carbon dioxide production and water loss (the instruments are much more sensitive), and can do some clever calculations to turn this into estimates of metabolic rate.
The equipment itself can look quite intimidating – and certainly like Science – with plenty of tubes and wires (when I calibrate the water channel, there’s even a bubbling flask!), but it’s not that difficult once you figure out what everything is doing, and it looks scary enough that other people generally don’t mess with it. We pass CO2-free, dry air over an insect, and measure the CO2 and water vapour in the excurrent air – all the CO2 and water vapour must have come from the insect, so we can calculate how much it is breathing out. The equipment we use is from a company in Las Vegas called Sable Systems International. Sable Systems’ head honcho, John Lighton, is an insect physiologist who has published in places like Nature and PNAS, which means that when he designs the equipment, he often has insects in mind.
The respirometry system set up in a controlled-temperature room at the University of Otago. CO2-free dry air is supplied by the gas cylinder, and passes through a chamber containing the insect housed in a temperature-controlled chamber (the big grey cooler box), before going on to an infra-red gas analyser (the green box), which uses IR absorbance to measure CO2 and H2O.
What else can we learn from respirometry?
As well as a simple measure of metabolism, it is possible to use respirometry to determine the thermal sensitivity of metabolism (this is important in understanding the effects of climate change), as well as the metabolic costs of various environmental stresses, like freezing or chilling. We can also use respirometry to study how insects breathe (there is much debate surrounding the adaptive significance of the Discontinuous Gas Exchange Cycles observed in some insects), and we can also use respirometry to figure out how much water is being lost across the cuticle of insects – even small ones like individual flies!
What am I …er… respirometing?
After 65 million years of evolution without mammals, New Zealand has an amazing array of endemism and some pretty weird insects. My favourites are the alpine insects, which include impressive radiations of cockroaches, stick insects and weta – large Orthoptera related to the Jerusalem crickets of North America. The mountains are fairly young (<3 million years old), so it’s possible to do all sorts of work comparing alpine species with their lowland relatives .
A group of alpine weta, Hemideina maori found under a stone at 1400 m a.s.l. on the Rock and Pillar Range, Central Otago, New Zealand. The males defend harems of 2-7 females. Female weta can weigh over 5 g, and males over 7 g, making them the heaviest insect known to survive internal ice formation. Photo by B. Sinclair.
Of course, it is the most fun to work on the big, weird insects. So far I’ve been putting alpine weta (Hemideina maori, Orthoptera: Anostostomatidae) and New Zealand’s longest insect, the gloriously-named phasmid Argosarchus horridus through their paces. Male alpine weta can weigh up to 7 g, and are the largest insect species known to withstand internal ice formation. The stick insects can easily reach 4 g, and posed some unique challenges in respirometry – with a body form so long and stick-like, it makes perfect sense to use a converted spaghetti-storage container!
A large female Argosarchus horridus (this one weighs a shade over 3 g) ready to go in her respirometry chamber. Photo by B. Sinclair.
The main questions I will be addressing will be about the evolution of thermal sensitivity and water loss in alpine insects, but the great thing about respirometry is that I never know what I’ll find along the way!
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Brent Sinclair is an Associate Professor at the University of Western Ontario. He is the 2012 recipient of the Entomological Society of Canada’s C. Gordon Hewitt Award.
