Gnat, mosquito
Archimandrite Nikifor’s Biblical Encyclopedia
(Matt 23:24) - a small but troublesome insect of the same general class as the mosquito. It is especially abundant in damp places and seldom appears by day except in dense forests. The irritation of its bite comes from a caustic fluid introduced into the skin. Scripture names it only in the Lord's figurative rebuke to the Pharisees: 'Ye blind guides, which strain out a gnat, and swallow a camel.' The meaning is that they attended scrupulously to minor demands of the Law while disregarding its weightiest requirements.