I like the idea of them having differences, but perhaps not by weight.
Giving desert myrmex armor decreased saturation drain [if this is possible], light fire resistance, and slightly increased movement speed would be more suitable to fit the theme of the open desert incentivizing certain evolutionary traits.
Jungle Myrmex offering faster regen but more saturation consumption, small haste and step up buffs, would fit more in line with the bugs in a moist environment that's dense and thick.
Desert myrmex weapons offering increased reach and inflicting slowness, and reduced attack speed would probably fit
Jungle myrmex with slightly buffed attack speed and a brief but rapid poison [wither? Due to witherbone handle?] Effect might be better.
Jungle myrmex presents benefits for rapid movement through difficult terrain and heavily prioritizes rapid devastating ambushes.
Desert myrmex would prioritize steadfast and steady weathering of an enemy.
But this is just a game theory and also just random ideas given for your inspiration and consideration, citizen.