What can roboticist learn from watching spiders run? Well, for starters, robots do not move their legs with joints like humans do. Instead they inflate their joints with haemolymph to straighten them.
Haemolymph is a fluid analogous to blood in many invertebrates.
Like all fluids, haemolymph becomes more viscous as temperature declines and this realization led undergraduate Nick Booster from Pitzer College to wonder whether spiders movements are impacted by temperature change.
The answer to that question could have important implications for the design of robots which use hydraulic fluid to propel their limbs.
Booster approached Anna Ahn and Steve Adolph with an idea for studying the impact of temperature on spider movement and found eager accomplices.
“I’ve always wanted to study spiders because they use hydraulics. This is a fascinating question. We wanted to understand how temperature affects the haemolymph and whether impaired haemolymph movement might influence the spiders’ ability to run,” said Anna Ahn of Harvey Mudd College to Phys.org.
The team chose the Texas brown tarantula (Aphonopelma hentzi), a spider which sees wide temperature swings, for their research. Also known as the Oklahoma Brown tarantula or Missouri tarantula the spiders are about 4 inches wide and weigh 3 ounces. They are considered to be relatively docile and are frequently kept as pets.
The spiders were tested at 15, 24, 31 and 40 degrees celsius (59, 75, 88 and 104 Fahrenheit).
Booster and Frances Su of Harvey Mudd College painted white dots on four locations on the leg of each spider so that the movements could be tracked more easily.
The researchers used a puff of air to get the spiders to move and filmed them on a clear runway. Even with a clear shot however, the spiders movements were not easy to monitor.
“It was difficult when the spiders didn’t run in the right direction,” said Su.
However, after months of analysis, the researchers determined that the spiders movement was strongly influenced by temperature and in interesting ways. The animals moved more quickly at higher temperatures but with less control.
At the lowest temperatures the spiders managed only 20 cm/s, at the highest temperatures they managed more than twice that speed at 53 cm/s. This was matched by their stride count with an average of four strides per second at low temperatures and 10 strides at the warmest temperatures.
However, the faster the spiders went, the less coordinated they came. At the lowest temperatures the animals were able to extend the third and fifth joints of their legs extended almost simultaneously, as the temperatures increased the spiders had a much more difficult time controlling the joints.
The team initially thought that the spiders would be hampered by the increased viscosity of the haemolymph at lower temperatures. They now think that the increased viscosity plays a role in the timing of the spiders movements.
‘Hydraulic extension may allow spiders to save space and mass in their limb, but it may come at the expense of control’, says Ahn.
This helps to explain why the animals have become nocturnal, sacrificing their optimal speed for the increased control of their movements when hunting prey.
