Heat could help spiders run faster, but also messes up their coordination.

Instead of using muscles, spiders extend their limbs by inflating their joints with haemolymph, The Company of Biologists reported. This movement system is similar to the way hydraulic fluid propels robot limbs, and temperature can have in influence on fluid viscosity.

"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.

The researchers studied Texas brown tarantulas by painting white dots on their leg joints and filming them as they ran across a track. The dots were then used to make digital models that tracked the spiders' movements. After months of analysis, the researchers determined the spiders' hydraulic limbs were influenced by temperature.

At low temperatures of about 60 degrees Fahrenheit, the spiders ran at top speeds of 20 cm s^-1, but at high temperatures of over 100 degrees, they sprinted at high speeds of 53 cm s^-1.  Instead of increasing their stride length, the spiders sped up from four strides s^-1 to 10. The scientists also observed the spiders were less able to coordinate their joints at higher temperatures.

The researchers had initially thought low temperatures would affect the viscosity of the haemolymph, but instead the spiders' coordination was restricted at higher temperatures. This may be because the viscosity does not haemolymph to flow through the leg and back when the spiders run at high speeds in extreme heat.

"Hydraulic extension may allow spiders to save space and mass in their limb, but it may come at the expense of control," Ahn said.

The findings could explain why spiders tend to come out at night when it's cool instead of taking advantage of the heat of the day.

The findings were published in a recent edition of the Journal of Experimental Biology.