Chelicerates have an interesting amount of provings. Many groups such as scorpions and spiders have an abundance of provings. Others have none. Eurypterids have no provings, and the Xiphosura have only one species proven. As such any provings of the Eurypterids or Xiphosura would be well received. 
The arachnids have some very well proven groups, and some very poorly proven groups. The Acari, Scorpions and Spiders are all well proven, but the Amblypygi, tailless whip scorpions, Palpigradi (microwhip scorpions, Psuedoscorpions, Ricinulei, hooded tickspiders, Schizomida, split whip scorpions, Solifugae, sun spiders, Thelyphonida (vinegaroons) are all unproven, lacking even any potentised remedies. The Opiliones have one remedy, and no provings, and thus could easily be explored.
Among the potential provings of Eurypterids would be Jaekelopterus, the largest known arthropods at roughly 8 feet long. There are currently 4 living species of Xiphosura, one of which has been proven. Any of the three remaining species, or any of the fossil species would be excellent additions.
The Spiders are one of the most well proven Groups in Homeopathy, but some major gaps in the spider Materia Medica exist. The Mesothelae are completely unrepresented, and would be well received. The Mygalomorphae are reasonably well known, as are the Araneomorphae, but the Araneomorphae lack any remedies made from either the Hypochilidae and Austrochiloidea both have no remedies proven or made whatsoever.
One purely incidental interesting spider species (the only herbivorous spider) is Bagheera kiplingi.
Image Credit: https://en.wikipedia.org/wiki/File:DirkvdM_Nephila_clavipes.jpg
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