To get an idea of why scientists would want to study daddy longlegs, try playing a game of “One of these things is not like the others” the next time you see one.
“If you watch a daddy longlegs move, it will effectively walk on just three pairs of its legs,” said Guilherme Gainett, a graduate student at the University of Wisconsin–Madison. The remaining pair of legs, he adds, wave around in the air, probing the arachnid’s surroundings.
And when they find something interesting, they can grab it.
“One of the unique things about daddy longlegs is the ability to coil the tips of their legs, achieved by the subdivision into dozens of small articulating pieces called tarsomeres,” Vanessa L. González, a computational genomics scientist at the Smithsonian Museum of Natural History, told NPR.
You can think of tarsomeres as similar to the segment between the knuckles on your fingers, González said.
A single leg can have dozens of tarsomeres, making it incredibly versatile. To discover how the arachnid developed this ability, González, Gainett and their fellow researchers sequenced the daddy longlegs’ genome — and eventually, they succeeded in creating a daddy shortlegs, by manipulating its genes.
It’s another step in understanding arachnids’ genome and how their body plans evolved, González said, including their unique grasping legs. She and Gainett are co-lead authors of the study, which was recently published in the journal Proceedings of the Royal Society B.
An evolution based on recycling genes
The researchers found that the daddy longlegs, also known as harvestmen, evolved their special legs not through a genetic secret ingredient but by reusing parts of the same genetic recipe found in other arthropods, such as genes that affect tarsomeres — subsegments of the leg.
“Other arthropods have tarsomeres,” said researcher Prashant Sharma, a professor at University of Wisconsin–Madison, “but only harvestmen use them in such a broad range of behaviors — sensing, climbing, fighting, courtship.”
The scientists reached their findings by interfering with, or “knocking down” the arachnid’s Hox genes. They’re a type of homeobox genes — which determine animals’ body plans. To picture how that works, González suggests thinking of a dimmer switch on a light. Rather than trying to turn a gene off entirely, the researchers wanted to turn it down to a very low setting.
When the researchers turned down the expression of two specific genes, the daddy longlegs developed much shorter legs than usual. Instead, the legs became pedipalps — appendages that are used in feeding.
In the future, similar studies could look at the genes responsible for other interesting features, such as spiders’ fangs, González said.
Tarsomeres help daddy longlegs catch on
“Tarsomeres are small joints on the distal part (tips) of the legs, that allow the daddy longlegs to curl and coil their legs around objects, like twigs or even the legs of other daddy longlegs,” González said.
“If you had more segments on your fingers you would be able to curl your finger much tighter,” she added. “Then if your finger had both lots more segments and was also much longer, you could hold tight to pretty much anything. These tarsomeres help the legs achieve the capability of prehensility, like that of a curling marsupial tail.”
The legs also have another special characteristic, a survival tactic that scientists refer to as autotomy. The arachnid’s legs can “detach themselves from the body, a trick used to distract predators while the daddy longlegs runs away with the remaining legs,” González said.
What you should do if you meet a daddy longlegs
People in the U.S. are likely very familiar with daddy longlegs. When asked what they should they do when they encounter one, González says they’re harmless.
“Phalangium opilio is one of the most widespread harvestman species in the world and can be easily seen in cities. One of the biggest misconceptions is that they are venomous, but are too ‘weak’ to inflict a bite,” she said, adding that the arachnids neither have have venom nor do they bite.
“If you were to encounter one in the forest or your home, look at them in awe, and let them go on their merry way,” she said. “If you feel you must remove it from inside your home, pick it up gently and put it outside.”