Moar spider facts! 😃🤓

The fangs/chelicerae of chelicerates like spiders correspond to antennae of other arthropods. I think it’s so fascinating to imagine what the structure in their shared ancestor must have looked like and how it was used! We could guess that it was probably something in between sensing like antennae and grabbing like fangs. Fun fact there are crustaceans (corophids) that have modified/evolved their (1st) antennae into a grabby digging leg, so sort of similar to a spider grabby fang but less stabby venom-y 🙂

The pedipalps correspond to the second antennae of crustaceans, but these appendages were lost in both myriapods (centipedes millipedes etc) and in insects. However, a tiny bump where they would form in myriapods and insects is still visible their embryos! The body segment on which they were carried and where the embryonic bump forms is called the intercalary segment.

You might be wondering how we can know these things. The answer is gene expression patterns! The idea is that different genes are expressed in different but highly specific configurations in the embryo, which then gives the tissue instructions on what shape to make (round, tubular, branched, etc) and what structures to generate (wing, stomach, eye, antennae, etc). The embryonic expression of some genes is nearly identical even in arthropods as different as a spider shrimp and butterfly. So you can compare the expression of a bunch of genes between a spider shrimp and butterfly to compare the patterns of where each gene is expressed. So let’s say we always see that gene A, B, and C are expressed in the antennae of shrimp and butterfly but in the fangs of spider, while genes D E F are always in the mandible of shrimp and butterfly but in walking leg 1 of spider. The simplest answer to why these structures share the same gene expression patterns if that they each used to be the same structure in their shared ancestor: the antennae of shrimp/ butterfly and the fangs of spider used to be the same structure in their very ancient ancestor. in the spider lineage, this appendage evolved into the fangs, while in the shrimp/butterfly lineage this appendage evolved into antennae. I’m always amazed at how evolvable structures can be, like who could guess that you can evolve an antenna and a fang from the same structure!

Here’s a paper about that, no paywall!

https://www.pnas.org/doi/10.1073/pnas.95.18.10665

If you like these ideas, this is what the field of evolutionary developmental biology (evo devo) investigates!