fly which is used similarly to cantharides
蟊 refers to a specific type of fly used similarly to cantharides (blister beetles), likely for medicinal or irritant purposes. Its meaning is narrow and technical, centered on this insect.
蟊 combines 矛 (a spear or halberd) with 䖵 (a doubled insect component). The exact historical development is uncertain, but it likely names a particular insect, possibly one associated with pests or irritants.
A spear (矛) is held over a swarm of insects (䖵), like a fly that irritates like a blister beetle. Imagine a soldier using a spear to fend off a cloud of these medicinal flies.
For ボウ, picture a bow (ボウ) being used to shoot at the irritating fly: bow -> ボウ, and the fly is the target.