As we have just seen, it is by the responses of the organism to the variations of luminous intensity that work concerning differential sensitiveness began, and the responses to these variations have been the subject of very numerous and very profound studies. It is through these that we approach the present study in the group which we are considering, in order to examine afterward the responses to the variation of other stimulating forces.
The responses of the positively phototropic articulates to variations of light have been the subject of interesting observations in the work of Parker on the mourning-cloak butterfly ( Vanessa antiopa). When resting on a surface in the bright sunlight, this diurnal butterfly turns its back to the light. In flight, or when walking, under the same conditions as to light, it shows, on the contrary, a positive phototropism, and directs itself with its face to the rays. If, then, a shadow is thrown upon the insect when it is walking, it stops, says Parker, momentarily closes its wings, and rapidly takes flight. Our thistle butterfly (Vanessa car dui) undoubtly reacts in the same way, for it is, according to Bohn, positively phototropic; but experiments do not give the same result with Vanessa io or with Satyrus jcrnira, for the same author has observed that the photot-ropism of these two diurnal butterflies is negative.
One can get similar responses, or even more marked ones, with all insects whose positive pho-totropism is strong. The currant-worm (Nematus ribesi) is one of these. Captured in the garden, where it causes serious damage, there were many of the worms which, placed on a sheet of paper, directed themselves in a straight line toward the window of my room. I cast on them the shadow of a book held in the hand, and they immediately stopped, raised themselves up, and moved the anterior part of their body about, then made a half-turn, or less, and directed themselves to the side or in a reverse direction. Like Vanessa antiopa, these positively phototropic insects react by a half-turn to a quick diminution of the light.
Insects having negative phototropism show an identical reaction, but only when they are subjected to a rapid increase of light. The bedbug is, as we know, energetically photophobic; it hides in the darkest cracks and leaves only during the night. In the daytime, says Bohn, if an accident has brought it from the shade into the light, it immediately executes a rotation of 180 degrees, which brings it into the shadow. On a sheet of paper it walks away from the luminous rays and turns about immédiately when one holds a candle toward it. "It is very difficult," adds Bohn, "to make a bedbug which is on black paper pass over upon a piece of white paper.19
All the insects which are negatively phototropic lend themselves to similar experiments, among others, the meal-worms, or larvae of Tenebrio moli-tor, and the maggots of different flies. Placed upon a sheet of white paper which receives the sun's rays from a window, these larvae orient themselves in the direction of the rays and march the other way ; they unhesitatingly enter a shadow thrown upon the sheet and they follow it until they reach its limit, when they walk in the shadow and make a half-turn if one throws a luminous ray before them. A negative variation of intensity has no effect upon them, while a positive variation makes them turn or reverse.
Thus, positively phototropic animals respond to a diminution of light,-that is to say, to a negative variation of luminous intensity,-and those whose phototropism is negative respond to a variation of positive intensity. Experiment shows also that animals having no phototropism (at least apparently none) respond, nevertheless, to variations of intensity, but some to positive variations and others to negative ones.
In all cases the response is independent of the direction of the rays. It reacts especially to sudden variations and manifests itself by rotation of 180 degrees or a digression in this direction, and often, as with many swimming organisms, by one or several rotations of the individual upon itself.
Established for light, the preceding rule really applies to all external stimuli. Animals of negative geotropism climb vertical or inclined surfaces. When they are at rest, or marching, it suffices to diminish quickly the incline to make them stop, to make -a turn, and almost always to go in the opposite direction. Bohn tried this experiment with the mollusks of the genus Littorina, with starfish, with the larvae of the sand-dune butterfly (Tyria jacob) ; I have tried it with the larvae of Nematus ribesi, and always with the same result. Upon a glass plate Jennings (1887) exposed a drop of water charged with the common infusorian (Paramecium aurelia) and put in the center of the drop a droplet acidulated with carbonic acid. The infusorian, endowed with positive chemotropism by the influence of this acid, penetrated into the droplet, recoiled and tried to leave, but finished by staying imprisoned.
In fact, whatever the stimulations of external forces, the laws of differential sensitiveness remain the same, and the principle may be formulated by saying, with Bohn: "When the tropism is positive, the animal reacts only to a negative variation of the excitant; when the tropism is negative the reverse is true." One would say that the animal abhors variation. It results from this law that sensitive animals accumulate in the places favorable to the exercise of their tropism, but they depart from the opposite kind of places. Parker observes that he has never seen Vanessa antiopa fly in shady spots when the sun was brilliant, and every one has noticed that little flies, at the hour when they dance in swarms in the sun, avoid the shady places in the glades. When maggots or meal-worms flee from the light under a leaf traversed by a large dark spot, they penetrate into the spot and do not leave it, »and are finally all gathered there together. When one puts them into an open box, one of the walls of which is in the light, they all cluster against the opposite side. One can, as Bohn justly says, compare these favorable areas to traps. The shadow is a trap for maggots and meal-worms, and the light for Vanessa antiopa; and in* the experiment of Jennings, cited above, the acidulated droplet plays the role of a chemical trap. The other rules of differential sensibility. We can go further into the study of the rules of differential sensibility.