|MadSci Network: Zoology|
Exoskeletons imply internal connections for muscles, as well as a rigid body that has to be molted periodically. The former may cause some problems for the organism as it gets larger (molting requires a period when the organism is soft and vulnerable and the muscles will not work well with a soft exoskeleton), but the mere weight that a heavy exoskeleton adds as size increase makes it impossible for land-dwelling arthropods to get much larger than 250-300 mm. In order to be as strong as the exoskeleton for smaller arthropods a very large arthropod would have to have a much thicker exoskeleton. The associated weight would make movement difficult and above a certain size, virtually impossible. Another constraint to large size is the inefficient oxygen transport method through tracheae. Arthropods do not need to invest much energy in oxygen transport simply because of their size constraints associated with an exoskeleton. Thus every thing is connected to everything else. Water is more supportive than air (it is heaver and non-compressible) and thus arthropods can push the size limitations further. Even the largest living animal with an internal skeleton (the blue whale) and the largest without a skeleton (e.g. the lionís mane jellyfish) are aquatic, so the support provided by water helps ALL creatures reach larger sizes, not just those with exoskeletons. The larger dinosaurs may have been partially aquatic, but even if not they were able to support their huge masses with huge internal skeletons, especially noticeable in their massive leg bones. The arthropod leg size varies, but even heavier legs with an exoskeleton and internally attached muscles would not help much because of the problems associated with thickness of the cuticle, oxygen transport and muscle function that occur with larger size. All of these systems work well at small sizes and arthropods are quite successful as evidenced by the well over one million species. They just cannot get huge. References: Price, P. W. 1997. Insect Ecology 3rd ed. John Wiley & Sons, New York. Arthropods: http://www.backyardnature.net/arthropd.htm The Skeleton: http://www.backyardnature.net/arthropd.htm
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