Charles H. Spurgeon on Psalms 105:31

Flies. This term serves to denote a kind of insect that alights on the skin or leaves of plants, by its bite inflicting pain in t}fe one case, and causing destruction in the other. The swarms of flies in Egypt are usually numerous, and excessively annoying. They alight on the moist part of the eyelids and nostrils, and inflict wounds that produce great pain, swelling and inflammation. They are also ruinous to the plants in which they lay their eggs. Philo (vit. Mos. 2 pg 110) describes the dog fly or gad fly as a grievous pest of Egypt. Gnats and mosquitoes are also abundant and virulent. A plague of such creatures would cause immense suffering and desolation. James G. Murphy.