In comparison with the more heavily constructed wasp, with its four wings, the house-fly, with its two wings, is the more alert and active flier. The wasp is more robust than the fly and will be active in weather too inclement for the latter; however, some of the frail and slender gnats will brave cold temperatures impossible for the wasp. CHAPTER VII DISTRIBUTION AND CONCENTRATION OF FLIES It might be supposed that a strongly developed house haunting proclivity would not be consistent with a disposition to roam far afield from the locality of birth. Many clever experiments have been made with marked flies released and recaptured within measured distances and times. After an immensity of pains taken, very little profitable knowledge has been arrived at thereby. Little of what we really want to know is indicated by such a fact as that, out of hundreds or of thousands of marked flies released, one per cent was recaptured at spots as remote as a mile within two or three days, or by such a fact as that a large percentage should be observed to remain within a more limited home circuit. The variable factors of temperature, wind, sunshine and rain inevitably tend to discredit the reliability of the observed results following any such experiments. Close observations of the habits of the house-fly reveal the very appurtenant fact that the movements of newly-hatched flies, for their first six or seven days' active life, differ from those of a more mature age, when the breeding instinct has grown strong. The latter are disposed to locate themselves for the rest of their lives in and about one attractive spot, and they are indisposed to fly high above ground or to travel far, unless it be with the object of leaving an unsatisfying locality and discovering a better place. However, the younger flies seem to feel no such restrictive influence, for, as soon as they have become fit and the weather suits, they show an inclination to fly high and thus may travel to very remote places. It is just the same with peacock, red admiral, and tortoise-shell butterflies, which I have often reared and released for adding to the interest and beauty of a flower garden. In sunny weather many or most will soon wander never to return; those which have remained a few days continue residence close round about, especially if nettles, the food plant, grow in the neighbourhood. It would be of great interest if we could discover how far a plague of flies arising from unsanitory surroundings in one locality is liable to spread to the injury of other localities.