Heat is the result of molecular vibrations which increase in number in proportion as the temperature rises. We know that it is one of the principal agents of all chemical phenomena. In different ways it plays a part of the first importance in the modifications of living matter. Most insects perish when submitted to temperatures of 38° to 43° (Centigrade) and if they resist very low temperatures (Pictet states that the eggs of the silkworm moth stand-40°), is it at the expense of their vitality, which is lessened to the most extreme limit. It is, then, not astonishing that organisms respond with a directive orientation to stimulations by heat, and this is due to thermotropism as it is called.

Like all tropic manifestations, this reaction is particularly apparent with the lower organisms, like the myxomycete fungi among the plants, and the Infusoria among the animals. But it is found equally in the articulates.

Loeb put the caterpillar of the Brown-tail Moth in a dark box one end of which was near a warm stove, and after some instants found them all together in the warm end of the box. Here the reaction is positive, although much more poorly marked than are the reactions caused by luminous stimuli, for the caterpillars do not form a straight line beside the source. As we shall see, very few insects escape this reaction to heat. Every one has noticed that most ants go into their nests when cold weather comes, and leave them when it he-comes warmer. This varies somewhat with the species. At Royan I have seen the harvest ants retire to their nests during the hot hours of the day, just as in the cold spells, and then they return to work, even at night, in moderate temperatures. In this case the thermotropism is positive or negative according to the intensity of the heat rays.