Judging by the number and variety of `giant insect' horror movies, there is something primal about the human fear of bugs, especially really big ones. Imagine having to dodge the attack of an ancient dragonfly with a wingspan of nearly a meter! Giant insects were common during the late Paleozoic era. Luckily, there appears to be a size limit on modern insects that keeps them from becoming the stuff of science fiction, but the nature of this limitation is as of yet unknown. One major difference between the late Paleozoic and modern times is the amount of oxygen in the Earth's atmosphere. Back in the time of giant insects, oxygen comprised about 30% of the Earth's atmosphere,compared with about 20% today. Some believe that the higher levels of atmospheric oxygen in the Paleozoic period allowed insects and other creatures to grow to sizes much larger than currently possible by allowing higher rates of oxygen delivery to the tissues. In their recent PNAS paper, Alexander Kaiser and coworkers from Midwestern University and Arizona State University set out to test the validity of this `oxygen limitation hypothesis' by measuring the relationship between insect body size and the tracheal system to see if the oxygen delivery system could limit insect size.

The team used synchrotron x-ray phase contrast imaging to non-invasively measure the volume of the tracheal system in live, but immobilized beetles. They investigated four species of darkling beetles that differed in their body mass by three orders of magnitude.

The team found that in the largest beetles about 4.8% of their total body volume is composed of the tracheal system, while in the smallest beetles the tracheal system accounts for only about 0.5%. They also noted that different body compartments appeared to be under different constraints. Tracheal volume increases in a roughly equal relationship to body mass in the head region, and they concluded from this that oxygen delivery to the head is not likely to limit body size. However, oxygen delivery to the legs is a different story. In smaller beetles the tracheal tube occupies only 2% of the leg orifice, while in larger beetles about 18% of the opening is occupied by the tracheal tube,leaving precious little space for all the other parts to operate.

These results suggest that larger beetles must devote a greater proportion of their body volume than small beetles to the tracheal system for gas exchange purposes. This relationship appears to be especially important in the supply of oxygen to distant and isolated parts of the body such as the legs. The team suggests that the increased proportion of space occupied by the tracheal system in large beetles will likely impose tradeoffs in other physiological systems, and may eventually lead to a constraint on maximal body size. Based on the data presented for oxygen supply to the legs, the authors predict a maximal beetle body length of around 16 cm, which closely matches the 17 cm size of the largest living beetle Titanus giganteus. An increase in atmospheric oxygen concentration such as that experienced in the late Paleozoic era would certainly help to alleviate constraints on body size imposed by the tracheal system in modern insects, by simply delivering more oxygen to the tissues per unit of air exchanged.

Maximal body size in insects may very well be limited by oxygen concentrations in the atmosphere. So for now we can all breathe easy knowing that giant insects are a thing of the past. However, don't be surprised if the opening scene of the next big horror movie spotlights an ordinary cockroach scurrying into an oxygen bar!

Kaiser, A., Klok, C. J., Socha, J. J., Lee, W.-K., Quinlan, M. C. and Harrison, J. F. (
2007
). Increase in tracheal investment with beetle size supports hypothesis of oxygen limitation on insect gigantism.
Proc. Natl. Acad. Sci. USA
104
,
13198
-13203.