Zebstrika: Evolves at 27, stats: 75/100/63/80/63/113 ground weakness
Galvantula: Evolves at 36, stats: 70/77/60/97/60/108, rock/fire weakness
You'd be carrying Joltik around for an extra 9 levels, and even then Galvantula only outdoes Zebstrika in Special Attack, though Zebstrika's physical does more damage. Zebstrika can also switch in on a known Electric-type move to boost damage output or speed through its ability. Galvantula can't learn fire-type moves, which was a way my Zebstrika saved me on numerous occasions. Galvantula's bug typing is redundant in this generation as so many good pokemon get U-Turn and X-Scissor. Get Excadrill and keep Scolipede until you get something better. It'll become useless too.