With half as many crickets as roaches, he has 12/2=<<12/2=6>>6 crickets.
With twice as many caterpillars as scorpions, he has 3*2=<<3*2=6>>6 caterpillars.
In total, he has 12+3+6+6=<<12+3+6+6=27>>27 insects in his bug collection.
#### 27