We recently interviewed Meredith Johnson (graduate student), Jordan Glass (graduate student), and Dr. Jon Harrison from the School of Life Sciences at Arizona State University about the research they presented at the 2021 annual Experimental Biology conference.
Q: You mentioned in one of your presentations that insects have an ‘unusual’ respiratory system. Can you explain how it differs from mammals?
Dr. Harrison: Insects exchange gases through blind-ended air-filled tubes called tracheae. There are small holes in the body wall called spiracles, and the air-filled tracheae branch throughout the insect’s tissues, delivering oxygen to the cells. While there are oxygen-binding pigments in the blood of some insects, especially the more primitive species, these are generally not quantitatively important in most insects. In contrast to older views, air is commonly flowing through the large tracheae, often unidirectionally, with diffusion occurring in the final steps in gas exchange. So, quite different from mammalian gas exchange, though with some similarities to birds.
Q: How does this difference limit insect size and prevent bugs from taking over the world?
Dr. Harrison: What we found is that larger species need to invest more in the tracheal system within the legs. The legs combine high oxygen need with long distance from the spiracles, which is why we likely see this response in the legs but not more generally in the body. It appears that if an insect approached the size of a normal mammal, it would have to completely fill their legs with tracheae in order to adequate oxygen transport, leaving no room for muscles, nerves, etc. In other words, just not a workable solution. Of course, it is plausible that larger insects could have a ventilatory system that might overcome any such limitations. But, based on the patterns we have documented showing that the legs become increasingly filled with tracheae as insects enlarge, it appears that this could be a real limitation. Cleveland is safe!
Q: Your lab studies a variety of insects. Which do you find to be the most interesting from a physiological perspective?
Dr. Harrison: Tough question. I am currently fascinated by the solitary bees that Meredith has been studying. These bees spend an entire year a few inches beneath the soil surface in the Sonoran Desert, emerging synchronously for annual mating leks.
Meredith: I can add to that. I have recently begun studying bees in the Centris genera. They range from the US Southwest to the tip of South America. I focus on the Sonoran Desert species. As you may know, summertime is incredibly hot here, with ground temperatures reaching near 60°C at times. Despite the heat, the female bees dig anywhere from 2-8 inches below the desert soil and deposit an egg, which develops into a prepupae (basically a pre-adult bee). The prepupae wait underground for a staggering 10-11 months. When the adult bees emerge at the end of this waiting period, they face high summer temperatures and desiccation. Males congregate together to intensely compete for a female to mate with.
Q: A couple of your studies looked at how bees regulate body temperature. How do you measure thermoregulation in a bee?
Meredith: Using rapid-response thermal probes and the “grab and stab method”, I found that males have high flight muscle temperatures in the cool mornings; up to 18.5 °C above air temperatures, enabled by their flight metabolic rates that are 6.4 times higher, per gram, than the highest rates measured in humans. Remarkably, they maintain a relatively constant flight muscle temperature as the day warms up. When the air is cool, the blood warmed by the flight muscle stays near the flight muscle in the thorax, but as the air warms, they increase shunting of hot hemolymph (blood) from the thorax to the large, relatively uninsulated abdomen, from which heat is lost by radiation and convection. This mechanism is quite analogous to vasoconstriction and vasodilation in mammals.
Jordan: Flying endothermic insects have evolved several behavioral and physiological mechanisms to regulate their body temperatures within relatively narrow ranges. Honey bees can also thermoregulate during flight, but they cannot vary their heat loss from thorax to the abdomen. Honey bees vary their flight metabolic rate to thermoregulate, and also rely heavily on evaporative cooling (from fluid regurgitation) to dissipate excess heat when flying at high air temperatures. The mechanisms enabling them to alter flight metabolic rate when air temperature varies remain unclear.
Thermal performance curves can also help us understand why endotherms like mammals and bees thermoregulate. I found that maximal flight metabolic rate occurs when honey bees are flying at a flight muscle temperature of 39°C, approximately the body temperature of birds. Combining thermal performance data with heat budget models and body temperature measurements from animals in the field can enable us to predict how climatic warming may affect pollinators.
Q: Which do you find to be the most important from an agricultural perspective?
Dr. Harrison: We have been studying locusts for many years to try to understand the links between ecology, nutrition, and outbreaks because locusts are likely the most important insects from an agricultural perspective, worldwide. The ongoing locust outbreak in Africa has not received much coverage in the US, but early assessments from the FAO suggest that in Ethiopia alone, locusts have destroyed 800 square miles of cropland, resulting in more than 1 million people requiring food aid.