I was reading this National Geographic article, 50 New Spiders Discovered In Australia when I became confused by the two trapdoor spider species photographed. I looked up trapdoor spiders and looking at pictures, they all seem to me to have five pairs of legs. I know spiders have eight legs; can someone tell me what am I missing here?


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Cork lid:

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(OK, so I found out that the things next to the mandibles are pedipalps. How are they different from legs on these beasts?)

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    $\begingroup$ Antennae​, mandibles, maxillae, chelicerae, pedipalpi and legs are all appendages of arthropod metamers. However, they have different functions and embryological/evolutionary origins.Regarding your specific question, palpi don't have the metatarsus, i.e., they have one segment less than the legs. Besides that, they are functionally used to manipulate objects (you can clearly see that in the big palpi scorpions have), mating (sperm transfer) etc, but not walking. $\endgroup$
    – user24284
    Apr 29 '17 at 12:10
  • $\begingroup$ Oh, by the way, spiders don't have mandibles, as you said in your last paragraph. $\endgroup$
    – user24284
    Apr 30 '17 at 7:47
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    $\begingroup$ All spiders have eight legs. The two on front are called Palps $\endgroup$
    – waltjh
    Jul 9 '17 at 17:52

The definition of a leg from Merriam-Webster is

"One of the rather generalized segmental appendages of an arthropod used in walking and crawling"

As Gerardo Furtado has already stated, the front pair of appendages are called palpi and are not used for walking, but for mating and manipulating objects. By this definition, palpi are not legs and therefore trap door spiders have 8 legs, not 10. More information on the rather large pali found on trap door spiders can be found in P.D. Hillyard's The Private Life of Spiders.

enter image description here Image from Biodiversityexplorer.org.


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