As Bohn has shown, the preceding phenomena seem tributary to differential sensitiveness. At first they manifest themselves identically under the influence of variations of a stimulant, whatever this stimulant may be. Fabre used shocks, pressures, but he reached the same result by submitting his insects for some seconds to vapors of ether. Holmes (1903, 1908) has stated that the water-bugs of the genus Ranatra do not react in the water, which is their normal habitat, but that they fall into rigidity when they are exposed to the air. Currant-worms are very sensitive to touch, and I have frequently seen them fall on the ground when I have thrown insecticidal liquids on them. This rigidity may be obtained by a sudden exposure to light or to shade, by sonorous vibrations, by an electric shock, even a light one. This may, and does, vary with one or the other species. In his interesting studies of these phenomena Rabaud has principally used pressure.
Like the responses of differential sensitiveness, the phenomena of simulation appear in the most widely different organisms. Holmes states that he does not seem to be able to reproduce them with the lower invertebrates "such as Protozoa, c-lenterates, mollusks, and worms, although certain of these animals show Reactions prophetic of this instinct." This observation is corroborated by Bohn, who notes phenomena which may be precursors of simulations with certain tube-inhabiting worms, with the amoebas, and a contraction into a spherical mass with the rhizopods of the genus Pelomyxa. It is among the articulates that these phenomena are most often observed. The myriapods of the Julus group ordinarily roll up, and those of the Glomeris group into a perfect sphere. Among the Crustacea this rolling into a ball is noticed with certain isopods and amphipods above 'all with our species of Arrnadillidium, which then resemble Glomeris, the Talitres or sand-fleas curl themselves into an arc and bring their appendages to the sides of the body; it is noticeable that many spiders react to stimulating variations, and hold themselves motionless and press their legs against the wall of the céphalothorax-a phenomenon observed especially with the species which spin orb webs. Ra-baud observes that most insects, if not all, are capable of taking on rigidity when a certain region of the body, variable according to species, is excited. But with many, this region is closely localized, difficult to reach, while with others, it is easily accessible to all stimuli, so that the animal readily takes on the characteristic rigidity. These latter insects, in which simulation of death is easily produced, belong to all orders, but they seem to be rather rare among the Lepidoptera, the Diptera, and the Hymenoptera ; otherwise they do not seem to be related by any zoological affinity. Let us add, finally, that simulation is possible with certain vertebrates of all classes, as one notes even with the fox, and that similar phenomena are observed in sensitive plants, such as the Drosera, which responds by certain attitudes to various stimuli.
To end this parallel between simulation and differential sensitiveness, we must add that the agents capable of modifying the second (luminous rays, heat) act also upon the first, as the above-mentioned experiments of Fabre show. The same experiments have shown us, also, that simulation may be provoked several times in succession with one individual, but that the subject ends by becoming temporarily insensible to the stimuli. It shows then a kind of fatigue which is noticed also with animals successively submitted at close intervals to the productive variations of differential sensibility.