# Biology question: Crabs and Scorpions **URL:** \ **Category:** Factual Questions\ **Created:** [June 21, 2022, 1:12pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551 "2022-06-21T13:12:02Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![MrAtoz](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MrAtoz](https://boards.straightdope.com/u/MrAtoz)\ **Post date:** [June 21, 2022, 1:12pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/1 "2022-06-21T13:12:02Z")
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. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [June 21, 2022, 1:14pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/2 "2022-06-21T13:14:03Z")
Are you a quadruped? Stranger ---
**Author:** ![What\_Exit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/what_exit/32/10652_2.png) [@What\_Exit](https://boards.straightdope.com/u/What_Exit)\ **Post date:** [June 21, 2022, 1:33pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/3 "2022-06-21T13:33:51Z")
**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. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [June 21, 2022, 1:43pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/4 "2022-06-21T13:43:39Z")
> [@MrAtoz](#): > > So my question is, what’s the difference? Both animals clearly have 5 pairs of 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. [![Imgur](https://live.staticflickr.com/1735/42663301292_ea1db28212_b.jpg "Scorpion Chelicerae") ](https://www.flickr.com/photos/mygale/42663301292) ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [June 21, 2022, 1:44pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/5 "2022-06-21T13:44:17Z")
This falls in line with the dumb/insightful riddle “how many legs does a dog have, if you call its tail a leg?” “Four, because a tail isn’t a leg even if you call it that.” So, arachnid pincers or palps aren’t legs even if you would be inclined to count them as such. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 21, 2022, 1:49pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/6 "2022-06-21T13:49:21Z")
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. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [June 21, 2022, 1:56pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/7 "2022-06-21T13:56:53Z")
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. ---
**Author:** ![MrAtoz](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MrAtoz](https://boards.straightdope.com/u/MrAtoz)\ **Post date:** [June 21, 2022, 2:00pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/8 "2022-06-21T14:00:12Z")
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. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [June 21, 2022, 2:14pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/9 "2022-06-21T14:14:02Z")
> [@MrAtoz](#): > > scorpion pincers are more accurately described as specialized…something else. Pedipalps. > **[Pedipalp](https://en.m.wikipedia.org/wiki/Pedipalp)** > > Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs. > Pedipalps are composed of six segments or articles. From the proximal end (where they are attached to the spider) to the distal, they are: the coxa, the trochanter, the femur, the short patella, the... Apparently, specialized antennae. > [@](#): > > more recent studies (e.g. using Hox genes) suggest they are probably homologous with the crustacean second antennae. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [June 21, 2022, 2:21pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/10 "2022-06-21T14:21:29Z")
> [@MrAtoz](#): > > 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. Nope, they are all specialized legs. (Or all appendages derive from the same base appendage.) > **[Arthropod leg | Evolution and homology of arthropod legs](https://en.wikipedia.org/wiki/Arthropod_leg#Evolution_and_homology_of_arthropod_legs)** > > The embryonic body segments (somites) of different arthropods taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages. The homologies between these have been discovered by comparing genes in evolutionary developmental biology. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [June 21, 2022, 3:10pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/11 "2022-06-21T15:10:26Z")
> [@Darren\_Garrison](#): > > Nope, they are all specialized legs.(Or all appendages derive from the same base appendage.) 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. ---
**Author:** ![Richardpurvey](https://avatars.discourse-cdn.com/v4/letter/r/3ec8ea/32.png) [@Richardpurvey](https://boards.straightdope.com/u/Richardpurvey)\ **Post date:** [August 14, 2024, 2:54pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/12 "2024-08-14T14:54:37Z")
You are, indeed, correct, Chronos, and thank you! They ARE really from a mutual ancestor. See > **[Why do spiders have 8 legs?](https://www.livescience.com/animals/spiders/why-do-spiders-have-8-legs)** > > Spiders' ancestors evolved to use their appendages in very weird ways. “all arthropod appendages — including legs, antennae and even mandibles (the jaws) — can be traced back to a stubby lobopod appendage” ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [August 14, 2024, 3:14pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/13 "2024-08-14T15:14:22Z")
> [@gnoitall](#): > > Pedipalps. 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? ---
**Author:** ![mozchron](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mozchron](https://boards.straightdope.com/u/mozchron)\ **Post date:** [August 14, 2024, 3:17pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/14 "2024-08-14T15:17:22Z")
The “claws” on a scorpion are modified mouthparts (chelicerae), not legs. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [August 14, 2024, 3:40pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/15 "2024-08-14T15:40:13Z")
The mouthparts on scorpions are modified legs. > **[Arthropod mouthparts](https://en.wikipedia.org/wiki/Arthropod_mouthparts)** > > The mouthparts of arthropods have evolved into a number of forms, each adapted to a different style or mode of feeding. Most mouthparts represent modified, paired appendages, which in ancestral forms would have appeared more like legs than mouthparts. In general, arthropods have mouthparts for cutting, chewing, piercing, sucking, shredding, siphoning, and filtering. This article outlines the basic elements of four arthropod groups: insects, myriapods, crustaceans and chelicerates. Insects are... ---
**Author:** ![mozchron](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mozchron](https://boards.straightdope.com/u/mozchron)\ **Post date:** [August 14, 2024, 3:42pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/16 "2024-08-14T15:42:41Z")
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. I’m an entomologist I do this for a living 🙂 ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [August 14, 2024, 3:58pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/17 "2024-08-14T15:58:09Z")
> [@mozchron](#): > > 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. A leg by any other name… ---
**Author:** ![Q.Q.Switcheroo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/q.q.switcheroo/32/5713_2.png) [@Q.Q.Switcheroo](https://boards.straightdope.com/u/Q.Q.Switcheroo)\ **Post date:** [August 14, 2024, 3:59pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/18 "2024-08-14T15:59:52Z")
> [@Babale](#): > > A leg by any other name… I should have been a pair of ragged claws… ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [August 14, 2024, 4:18pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/19 "2024-08-14T16:18:53Z")
> [@mozchron](#): > > I’m an entomologist I do this for a living Does this kind of imprecision bug you? ---
**Author:** ![mozchron](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mozchron](https://boards.straightdope.com/u/mozchron)\ **Post date:** [August 14, 2024, 4:19pm UTC](https://boards.straightdope.com/t/biology-question-crabs-and-scorpions/966551/20 "2024-08-14T16:19:48Z")
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