This question is prompted by a clue on last night’s episode of Jeopardy!, but I’m putting it here because it’s really a factual scientific question.
The category was Zodiac signs, and the clue was something like which zodiac sign had the greatest number of legs. The correct response was Cancer, the crab. I immediately thought “What about Scorpio?” Don’t scorpions have the same number of legs as crabs do? So off to wikipedia I went.
From what I can find (and this seems to be verified by other online sources as well), crabs are called decapods. They are considered to have 10 legs, the front pair of legs being their pincers. Scorpions, by contrast, are said to have 8 legs, plus 2 pincers. Their pincers apparently are not regarded as legs.
So my question is, what’s the difference? Both animals clearly have 5 pairs of appendages. Why are crabs’ pincers considered to be legs, but scorpions’ aren’t? Any biologists in the house able to weigh in on this? Thanks in advance.
Crabs are part of the Decapoda Order with Hermit Crabs, Lobsters, Shrimps & Prawns. Decapoda means 10 footed. So those front claws rarely used for walking are considered legs as they are specialized legs.
Scorpions are part of the Arachnida Class along with Spiders and Ticks. The pinchers are not specialize legs but weird head appendages.
If you start including mouthparts and antennae, they may have more than that.
The difference between legs and pedipalps (the pincers) in scorpions includes number of segments. And if you actually look at a scorpion’s mouthparts, you’ll see its chelicerae are like the pincers, but definitely not legs.
Eh, all of an arthropods’s protrusions (legs, claws, antennae, mouthparts, wings, etc.) are all homologous. The ancestral arthropod was something like a centipede, with one pair of appendages per segment, every segment. In a centipede, all of the appendages are very similar, but from there, they specialized, and some of the segments grouped together into larger body parts like “head” and “abdomen” (that therefore had multiple pairs of appendages each). But with some very minor gene tweaks, you can, for instance, cause a fruit fly to grow extra legs out of its head where you’d expect its antennae to be.
Sure, but once specialization of segments started happening, the pedipalps appear to be homologous to sensory appendages in other related groups, and not legs. So they were not walking legs that developed claws, they were antennae that did.
Thanks, all. So if I’m following correctly (no guarantee of that!), there is some definite biological difference between the pincers of a crab and the pincers of a scorpion, such that crab pincers can be described as specialized legs, whereas scorpion pincers are more accurately described as specialized…something else.
The second one. The ancestral original here is most likely a biramous leg/paddle-gill-chewing appendage.
The comparison to centipedes originally made is off-point, I’d say, since even your cite shows the scorpion pedipalps are derived from segments that are in front of appendages that even in centipedes and millipedes are mouthparts, not legs.
Wasn’t Sarah Jane “Pedipalp” Hamilton the aunt of Melanie Wilkes in Gone with the Wind, who lived in Atlanta and feared the coming of the dreaded Yankees?
I mean, if we’re being technical all appendages on an arthropod are modified legs if you go back far enough…
The point here is that scorpion claws are “legs” that were modified into mouthparts (long before scorpions were scorpions), which were then modified into (in the case of scorpions) claws. Different derivation to the claws on a crab even though they look similar.
