The observation of Pliny has passed into current practice. For a long time light traps have been used to capture the destructive Vine Pyralis and it is with traps of this kind or with simple lanterns that collectors of noctuids work during the night in the edges of the woods. However, the night-flying moths are far from being the only insects that fly to light. Very many others have similar habits, as one is easily convinced in the country on beautiful summer nights. Indeed, insects gather in swarms about lighthouses and street lamps, and if one wishes to have some repose in a lighted spot it is necessary to prevent the approach of all tineids, noctuids, June-beetles, scarabs, crane-flies, mosquitos, and other representatives of the entomological class which assemble there. It is at the table in the open air, around the lamp, that travelers in warm countries often collect the most interesting species of insects. All caterpillars-sawfly larvae as well as the larvae of butterflies and moths-are more or less sensitive to light and direct themselves directly toward it. If they are obliquely placed on a plane surface opposite to the source of light they quickly turn their heads, following the axis of the light ray and then the rest of the body moves in this direction.
The same faculty of directive orientation is seen with a great number of animals, even with lower forms which have no eyes. Who does not know the classical experiments with the Euglenes. These flagellate Infusoria progress by turning themselves over and it is by describing a heliooidal trajectory that they direct themselves toward the source of light. Moreover, the directive orientation which we are considering is not at all confined to animals. It is observed in the motile algae of the genus Volvox and we know that with all plants the branches bend and grow toward the light when they are kept in the shade or in the house.
The scientific study of these phenomena was begun with the plants, and the botanists have given the name tropisms to the directive action exerted upon plants by external stimulating agents. To Jacques Loeb 1 must be credited the first proof that the law of tropisms applies to animals as well as plants. In the two kingdoms these phenomena are as varied as the forces that produce them and each one of them has received a special name in relation to the stimulating agent from which it draws its origin. The directive action of light is called phototropism (also heliotropism when it is caused by the sun), and one says that the organism is endowed with positive phototropism when it directs itself toward the source of light as with all the species -cited above.
1 Loeb has synthetized his work in the two volumes: Studies in General Physiology (1905), and The Dynamics of Living Matter (1906).