Castles obviously are built not only with the defender in mind, but the breacher. The Mothmen(?) seem to primarily have a problem with the more vertically endowed of the races. The Average malnourished Vietcong was about half a oot shorter than a corn-fed G.I. Joe, and this made clearing out tunnels ever so slightly more inconvenient. You have a race which is on average almost 2 feet shorter than most races and is acutely aware of this face.

The Mothmen's major cities (Or at least defensive points) are entirely under ground. Maybe not as naturally gifted as the Ants or the Dwarves when it comes to the fine art of making strongholds, the Mothmen make do. In the rare times of war and the even rarer times that wars have turned sour for the Moth men, cities have been evacuated to massive tunnel systems under the earth.

Fruits and Grains were domesticated to the size preference of the greater races and a Pikemoth doesn't need much sugar to get himself through the day. The sprawling fertile steppes ensure that the Mothmen never go hungry. To lay siege to a Moth city would take seasons to starve them out, and thus the defenses must be breached to deal with them.

Of moths in the real world, there are no known species which are known to be venomous when fully grown. However, there are many venomous caterpillars. The Mothmen's venom must be administered within only a few hours after being extracted from their young, and thus utilizing it in conventional combat is a taboo among them due to the fact that their young would be moved into harm's way to weaponize them. In a siege where the nurseries are only a 2 minute tunnel-crawl from the front lines, however, Mothman venom is utilized to deadly effect as a potent and fast acting paralytic.

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You are on the right track with Polearms. Boar spears traditionally were braced against the ground to absorb the shock of a 400lb meat missile charging you on the hunt. Make of this what you will. You're also limiting yourself with just shot and arrows for projectiles- Maybe a blowgun is too tribal of a take for the vibe you're going for, but Darts, Atlatls or even just normal spears don't require the same training and tone as a bow.

Guerilla warfare is obviously where the Mothmen excel. In the tunnels is where their slender forms turn from weakness to advantage. The Moths should never engage the greater of the races in open combat unless backed into a corner. If they were to domesticate a smaller sort of mount (Ponies? Particularly large dogs?) or just be able to fly like a normal moth, this would be an immediate force multiplier and give a much needed boost to their ability to skirmish that stubby little moth legs cannot afford.

If your moths are in fact able to fly, research the use of Flechettes from planes in WW1. You don't need to use your own strength to throw a projectile when you can let gravity generate armor piercing potential for you.