Sunday, July 26, 2009

Akimidis caterpillars

People often ask me what I do with caterpillars and poke their head toward my various apparatus. Well, this one is obvious! "Why are you dipping caterpillars into water?"

Together with a hard working undergraduate research student, we perform an simple experiment what Akimidis would have appreciated very much. By dipping the caterpillar progressively, we could map the volume contribution from head to tail by quantifying the buoyancy. This simple method easily beats the fancy 3D laser scanning, MRI imaging, and X-ray data our predecessors attempted before us in this lab. The only special skill is to tam the caterpillars to stay still during the measurment. Well, this is what caterpillars do in nature -- "I'm not here!!" Nevertheless, cold water can be irritating, thus controling the temperature is critical. The total emersion directly gives the overal body density of any given caterpillar.

Another collateral experiment was something Sir Issac Newton would have been quite interested. We balance a lightweight beam with the caterpillar on it by providing a pivot support below the beam. By measuring the moment force at one end, and shift the pivot across the animal, the mass distribution can be easiliy calculated. The first immediate result is the center of mass for the animal. Combining this data with the above volume map, we can reconstruct a density map across the animal body length.

Density map and mass distribution are two crucial parameters for any biomehcnaical model of a soft body. In addition, the density change across different animal sizes indirectly reflect the trachael volume and respiratory capacity, since most other tissues in the body are similar to water density. Open gas cavities also affect the use of hydrostatic skeleton for a soft-bodied animal. If some caterpillars don't use their hydrostatic skeleton for locomotion, maybe their bodies are too leaky for economic pressurization.

PS: The caterpillar in the lower photo was groomming after a water bath!!

1 comment:

  1. Di;

    I do my best to translate it. Whatever, somethings wrong, Pls revise it.



    我與一個勤奮工作的大學生,作了一系列連阿基米德都極為讚賞的簡單實驗:藉由浸在水裡行進中的毛蟲,通過從頭到尾量化的浮力可以映像出體積分佈,如此簡單方法所量測的數據,輕易地打敗花俏三維激光掃描、核磁共振成像、X光所蒐集的數據,這些數據是前輩們以前在實驗室努力的;這樣的設計唯一的特殊技巧是,在測量期間內,馴服毛蟲始終進行於量具上;同時,這又恰恰是毛蟲的天性 ─ 「呵!我不在這裡!」;不過,冷水會刺激蟲體,從而控制水溫是至關重要的。完全泡浸法,對於任何毛蟲,都能夠直接量出整體〈體密度〉。