As Alice discovered during her trials in Wonderland, being small isn't easy. No one can see you, you're easily squashed, and even the tiniest drop of water poses a threat. But maybe it's not all bad if you're a newly hatched American locust. Kendra Greenlee and Jon Harrison wondered whether their minute stature gave the young insects the respiratory edge. The team suspected that the insects' scaled down respiratory systems would deliver oxygen efficiently to tissues by diffusion, making the youngsters less vulnerable to hypoxia than larger older grasshoppers. But would this turn out to be the case? The team put several insect generations through their respiratory paces, testing how well the insects coped with hypoxia during each instar period. But Greenlee and Harrison were in for a surprise. While the adults were very tolerant, the youngsters were extremely susceptible, which was completely unexpected (p. 497).
First, Greenlee needed a supply of freshly hatched grasshoppers to track their respiratory development through to adulthood. Fortunately, the adult grasshoppers were extremely cooperative, laying their eggs in cups of sand where the youngsters could hatch safely. `When they climb out they're clearly grasshoppers' says Greenlee `but about the size of an ant'.
Working with the older insects was relatively straightforward, but the tiny insects were particularly tricky to handle. Once safely secured inside their respiratory chambers, Greenlee began monitoring each insect's breathing rate and respiratory volume, as she gently dropped the oxygen level from 21% to 0%. Although Greenlee couldn't detect the minute levels of oxygen that the tiny insects consumed, she had more success measuring the carbon dioxide they produced as she tracked their metabolic rate. Not surprisingly, the larger insect's metabolic rates were much higher then the youngster's, but when she adjusted their metabolic rates for their sizes, the tiny insects were using far more energy than their elders. So how did their respiration compare?
As Greenlee dropped the oxygen levels, she saw that the adult insects increased their abdominal pumping rates to breath faster, and as she monitored their abdominal movements, Greenlee realised that the insects had also increased their tidal volumes. They were definitely breathing harder. But when she monitored the tiny first instar insects, their breathing didn't alter at all no matter how much oxygen was available. They always breathed at the same rate. The first instar insects didn't seem able to respond to the hypoxic conditions.
More surprisingly, the larger insects with their extensive tracheolar systems successfully maintained their metabolic rates even when oxygen levels plummeted to 2%, while the first instar youngsters couldn't sustain their metabolic rates, even when the oxygen levels were still relatively high at 14%! So instead of being better prepared, the tiny youngsters with their smaller diffusion distances were finding it much more difficult to breath. Greenlee and Harrison suspect that these immature locusts can't sense the falling oxygen levels and so fail to react, where as the elderly insects can, and do.
Next, Greenlee wondered how the growing insects cope when they're on the verge of bursting out of their shells (p. 509). Realising that the insect's tracheolar systems become more compressed as the insects expand inside their exoskeletons, Greenlee monitored their respiration to see how it changed towards the end of an instar. Sure enough, the insects' breathing frequencies increased as conditions became more cramped and they couldn't carry as much air in their compressed tracheolar systems. They also became more sensitive to falling oxygen levels, needing more oxygen to maintain their metabolic rates as they became more restricted in their undersized exoskeletons. In fact, Greenlee suspects that getting breathless could be the trigger that drives the insect's upgrade to a looser skin.
- © The Company of Biologists Limited 2004