101 SCIENCE
CNN Indonesia
Friday, 18 Aug 2023 08:16 WIB
Illustration. Experts reveal why spiders have eight legs. (Photo: Web screenshot cisr.ucr.edu)
Jakarta, CNNIndonesia —
Living beings have variations in the number of legs as a result of evolution. Humans have two legs, dogs have four, insects have six, and millipedes can have more than 1,000 legs. Why two by two have eight legs?
Fredonia Thomas Hegna, assistant professor of invertebrate paleontology at the State University of New York, said the reason spiders have eight legs is due to genetic ancestry.
“I think the best and simplest answer is that spiders have eight legs because their parents have eight,” Hegna told LiveScience.
“But then it went into decline, and somewhere it all had to start.”
If we follow the parental journey of the eight-legged spider some 500 million years ago, during the middle Cambrian Period, we will arrive at the roots of the lineage chelicerata, the group of arthropods that contains the spiders.
Even further back, 541 million years ago, we will find lobopods living in the oceans, the ancestors of all arthropods. The name “lobopods” does not refer to a single species, but to a wide variety of species with rather simple bodies.
Basically, they are worm-like creatures with segmented bodies. Each segment bears a pair of more or less identical short, stubby legs, and this pattern continues throughout their body.
As lobopods evolved, they began to specialize in their legs and attach their body segments together. Then the chelicerates appear to have fused their small body segments into two large body segments: the head and the abdomen.
Scientists aren’t sure what the reason is. But by the time spiders appeared 315 million years ago, they had inherited a body plan that was perhaps 150 million years old.
It is not clear which environmental pressures cause the chelicerates to settle on the eight-foot array. However, we know a lot about where their feet came from and it’s just weird.
“The legs are actually part of their mouth,” says Nipam Patel, a developmental biologist and director of the Marine Biology Laboratory, affiliated with the University of Chicago.
Since spiders, insects, crustaceans, and millipedes all evolved from a probable ancestor that had segmented bodies with a set of appendages in each segment, these species are simply highly modified riffs of that basic plan.
According to Patel, all arthropod appendages including legs, antennae, and even mandibles (jaws) can be traced back to the stout lobopod appendages.
For example, praying mantis shrimp. This shrimp swims with a group of small legs on a segmented abdomen.
The cephalothorax (head and thorax combined) have legs for walking, and near the mouth are small appendages that not only form the jaws, but also sweep food into the mouth to help it eat.
Contrast that with insects, whose stomach has no appendages. However, it has six legs on its chest, while its head and mouth are basically shaped like a praying mantis shrimp.
“If you look at a spider embryo, it looks exactly like an insect embryo,” Patel said. “Except he only grows legs on his head.
Instead of using it as a mouth, the spider uses it to walk.
The reason spiders walk with appendages on their faces goes back to lobopods and the original chelicerate body plan.
Meanwhile, modern arthropods are spoiled with special appendages. Lobopods are worm-like creatures with many more or less similar sets of appendages.
“At first, all feet were the same. But then the first appendages became different as they became sensory appendages, such as for tasting and taking in food,” said Heather Bruce, a research associate in the Marine Biology Laboratory.
From that time on, the ancestors of chelicerate spiders began to differ from other groups. In the ancestors of insects and crustaceans, the dual-function front appendages of lobopods lost their capturing and feeding abilities and became specialized sensory structures called antennae.
However, in chelicerates, those same appendages lose their sensory abilities and become fangs.
Meanwhile, the chelicerates’ second pair of legs evolved into a set of gripping appendages called pedipalps; the next four pairs of legs remain in their role as walking legs, and all appendages after that are lost.
(can/dmi)
2023-08-18 01:16:00
#Spiders #Legs
