In that case, which ideas do we want to include?
Expanding radius of damage for multiple critters?
Increased damage over time?
Knocking people prone?
I'm thinking something like this:
Shake the Earth (Ex): A jishin mushi beetle can create a localized earthquake by rhythmically striking the ground with its abdomen as a full-round action. Any creature within 10 feet of a jishin mushi must make a DC X Reflex save each round it uses this ability or fall prone as the ground lurches and buckles beneath it. Jishin mushi are immune to shake the earth generated by themselves or other jishin mushi and never need to save against the effect.
A jishin mushi creates destructive vibrations if it continues to shake the earth. On each round of shaking after the first, all creatures and objects within range (except other jishin mushi) take Y sonic damage, with a DC Z Fortitude save for half damage. Buildings take double damage from shake the earth unless they are specially designed to resist earth tremors.
If multiple jishin mushi use their shake the earth ability in the same round, the radius and intensity of the effect expands as the resonance in the ground combines. For every jishin mushi within 5 feet using shake the earth, the radius of the effect expands by another 5 feet. In addition, for every two additional beetles the damage done by prolonged use of shake the earth increases by Z. Thus, three knell beetles within 5 feet using the ability would generate an effect that affects all creatures within 20 feet of any one of the knell beetles and does 2Z damage.
Creatures with stability or that are considered stable against trip attempts receive a bonus on their Reflex save as if they were being tripped (so a dwarf would receive the +4 bonus on the save to resist being knocked prone). The save DC is Strength-based.
EDIT: Come to think of it, I quite like the original's "fly away to avoid the earthquake schtik". How about this for an alternative:
#1 give the earthquake beetle resistance to sonic damage.
Shake the Earth (Ex) [Take Two]: A jishin mushi beetle can create a localized earthquake by rhythmically striking the ground with its abdomen as a standard action. This process takes up to three rounds. Buildings take double damage from shake the earth unless they are specially designed to resist earth tremors.
On the first round, any creature within 10 feet of the jishin mushi (except for a jishin mushi) must make a DC A Reflex save or fall prone as the ground lurches and buckles beneath it. They must save again on each subsequent round of shake the earth to maintain their footing.
On the second round, the jishin mushi creates destructive vibrations that do X sonic damage to all creatures and objects within range (except other jishin mushi), with a DC B Fortitude save for half damage.
On the third round, the jishin mushi cracks the earth open, doing Y damage to all creatures within range that are standing on the ground (DC C Reflex save for half damage). The jishin mushi is not immune to this damage, so will fly away before the mini-quake strikes. It will only take damage if it unable to flee.
If multiple jishin mushi use their shake the earth ability in the same round, the radius and intensity of the effect expands as the resonance in the ground combines. For every jishin mushi within 5 feet using shake the earth, the radius of the effect expands by another 5 feet. In addition, for every two additional beetles the damage done increases by one dice. Thus, three knell beetles within 5 feet using the ability would generate an effect that affects all creatures within 20 feet of any one of the knell beetles and does 2X sonic damage on the second round and 2Z damage on the third.
Creatures with stability or that are considered stable against trip attempts receive a bonus on their Reflex save as if they were being tripped (so a dwarf would receive the +4 bonus on the save to resist being knocked prone). The save DC is Strength-based.