Tarantulas lose coordination and start to act clumsy when exposed to hot conditions, according to a new study, although their speed stays fast and furious.

Some species of these spiders are known for speed but they are not always able to move smoothly, particularly in hot conditions, researchers reveal.

Spiders do not use muscles as their main driving force to move limbs. Instead, a liquid called hemolymph, similar to blood, flows into the legs, causing them to extend. The joint closer to the body usually extends before the other. Then, flexor muscles drive bending at the joints, and push the hemolymph back into the arachnid's body. This liquid can be affected by changes in temperature, potentially altering the spider's ability to walk.

A white dot was painted on each of two joints on the limbs of eight adult Texas brown tarantulas, and researchers monitored the creatures as they ran down a runway. These tests were carried out while the environment was kept at four different temperatures - 59, 75, 88, and 104 degrees Fahrenheit.

Angles between the dots on the spider limbs were recorded as the spiders walked down the incline. Colder temperatures resulted in slower, but more coordinated movements from the spiders, while warmer temperatures produced the opposite effect.

"But at the higher temperatures, and the faster running speed, the two joints were less coupled. The two joints on each leg were a lot less well-controlled at the higher temperatures," Anna Ahn from Harvey Mudd College said. "The spiders increased sprint speed 2.5-fold over the temperature range by changing their stride frequency but not stride length."

Temperature played a significant role in the speed at which the spiders were able to move. At 62 degrees Fahrenheit, the arachnids traveled about 8.5 inches per second, compared with 21 inches per second at 100 degrees F. These equate to an increase from four to 10 times their body length every second. When researchers exposed spiders to temperatures higher or lower than those studied, the arachnids assumed an attack position, standing on their hind legs while lifting their front limbs.

The spider's better stride coordination and stability at lower temperatures may explain why some tarantulas wait until dusk to venture out, Ahn suggests, as their inability to coordinate joint extension at high temperatures may keep them low profile in their burrows when it's hot.

The findings also may be of some help to scientists who study robotic movements, which have a lot in common with spider limb movements as hydraulic fluid moves robot limbs the way spiders inflate their joints with hemolymph fluid. "Hydraulic extension may allow spiders to save space and mass in their limb, but it may come at the expense of control," says Ahn.

The Texas brown tarantula (Aphonopelma hentzi) is common in the far southern regions of the United States, although they have been seen as far north as Colorado. Adults can grow to be more than four inches in length, and weigh over three ounces. While males rarely live more than a year after reaching adulthood, females can live for 30 years or more. The creatures eat crickets and cockroaches, supplemented by a rare treat of a pinky mouse. The animals are sometimes kept as pets by those who appreciate their docile manner. They are harmless to humans, aside from a painful bite if provoked - their venom is mild and weaker than a typical bee's.

Study of the role played by temperature on the movements of A. hentzi was detailed in The Journal of Experimental Biology.

Photo: Michael Wifall | Flickr

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