Chance plays a predominant role in the two essential activities of insects, the search for nourishment and the sexual approach. It leads the animal to places in which the objective becomes capable of exciting the organs and of producing a directive action.
In the search for nourishment, chance is nearly always lacking with the very numerous larvae which are born at the very spot where they find their food, and therefore, so to speak, quite in the dining-room. With others it is aided by tropisms which very often lead the insect to the feast. The young caterpillars of the gipsy moth on the end of twigs (positive phototropism), and those of the milkweed butterfly on asclepias (negative geotro-pism) are examples. As to adults, it is very often by the play of chance, in fact almost always, that they are led to points from which they can be guided toward food. And mosquitos are directed by odor, bees by the sight of flowers and the perfume of fields covered with flowers, stercopha-gous and sarcophagous insects by the odorous emanations from excrement or from cadavers.
It is also chance which aids sexual approach, by placing the males on the odorous track produced by the emanations of their future mates. With the great peacock moth and with many other nocturnal Lepidoptera these emanations may be perceived at a great distance. In all these cases they exercise an automatic irresistible attraction on the males, and from this fact provoke tropisms rather than true sensations. We are here facing a situation where it is not easy to distinguish between these two kinds of -phenomena. However it may be, both serve as aids to chance in conducting the insect to an unknown end, which is the female.