683

Original Papers: Braddy, S. J., M. Poschmann and O. E. Tetlie. 2008. Giant claw reveals the largest ever arthropod. Biology Letters 4: 106−109.

Kaiser A., C. J. Klok, J. J. Socha, W-K Lee, M. C. Quinlan and J. F. Harrison. 2007. Increase in tracheal investment with beetle size supports hypothesis of oxygen limitation on insect gigantism. Proceedings of the National Academy of Sciences USA 104: 13198−13203.

British scientists recently came upon something striking: a well-preserved fossil chelicera of a giant eurypterid sea scorpion dating back to the early Devonian period (Figure A). The claw belonged to the largest arthropod ever known on the planet, measuring nearly 2.5 meters in body length (longer than humans are tall). This new discovery was not the only representative of massive arthropods roaming Earth at that time; the Paleozoic fossil record is teeming with massive millipedes, monster cockroaches, and titanic trilobites. Why were these animals so large, and why don’t we see supersized arthropods today?

To find out, we must look back to life in the Paleozoic era, approximately 542–251 million years ago. During this time, seed plants invaded land, land vertebrates became abundant, and winged insects took to the skies for the first time. According to geologic estimates, atmospheric oxygen in the late Paleozoic made up 35 percent of the air, as compared with 21 percent today. Chapter 24 described an experiment examining the relationship between atmospheric oxygen concentration and body size in fruit flies. Experimentally increasing O₂ concentrations resulted in fruit fly lineages that were significantly larger than control lineages, which lends support to the hypothesis that the increase in insect body size may have been due to higher oxygen concentrations. According to the fossil record, as O₂ concentrations declined at the end of the Permian period, so did the body size of arthropods.

Alexander Kaiser and his team wanted to know why oxygen might increase or limit growth. They knew that the larger an animal grows, the greater the distance required for oxygen to be transported to internal tissue. He thought that perhaps after a certain point the network of tracheae that supplies oxygen would start crowding other organs. To address this, the researchers compared mean values of tracheal density of the leg (TrD_leg) and the body (TrD_tot) against body length of living beetles from the literature, as shown in Figure B. They then extrapolated upper limits (dashed horizontal lines) of tracheal density in beetles based on the probable density at which tracheae would start to crowd other tissues.

Questions

Question 1

The graph shows tracheal density of the insects’ legs (red line) and total body (black line) plotted against body length. Researchers wanted to understand the upper limits of tracheal density as a possible limitation for insect size. The tracheal density of the leg is more limiting because it reaches the upper limit threshold at a smaller body size.

Question 2

Insects take in oxygen through pores called spiracles, which open through the exoskeleton to the outside of the body. Oxygen exchange occurs in the tracheal network, an extensive system of tubes that branch into every part of the body. All living cells in the animal are within micrometers of the tracheal network. In contrast, humans and other vertebrates transport oxygen and carbon dioxide in blood and through a network of arteries, veins, and capillaries.

Question 3

Based on the graph, one would predict that the largest living beetle could reach a body length of about 15 cm (with 95% confidence limits of about 12–22 cm). This is the approximate length at which the upper limit of tracheal leg density is reached. The largest living beetle (T. giganteus) is slightly bigger than the regression line would predict, but it is within the confidence limits of the prediction.