The means by which insects resist chemicals probably vary and as yet are not thoroughly understood. Some of the insects that defy chemical control are thought to be aided by a structural advantage, but there seems to be little actual proof of this. That immunity exists in some strains is clear, however, from observations like those of Dr. Briejèr, who reports watching flies at the Pest Control Institute at Springforbi, Denmark, “disporting themselves in DDT as much at home as primitive sorcerers cavorting over red-hot coals.”
