This parallel need not be pushed too far, because the mechanism and the physical nature of simulation present special characters.
Let us note first the muscular tetanus which gives to articulates in this condition the appearance of a dead body. "If a Eanatra," says Holmes, "is seized by one of its slender legs, one can hold it a long time horizontally without the legs bending in the least. This is as if a man were seized below the knee and held straight up without the knee bending, except that the legs of the Eanatra are incomparably more delicate than the leg of the thinnest man, and the muscular tension which holds the insect out must in consequence be intense." Eabaud has established the fact that this simulating rigidity is the result of a direct contact which differs from the ordinary physiological contraction in that it is produced instantly and not by steps, and also because the immobilized muscle, instead of being more excitable, is always infinitely less so.
Speaking in general of this tetanus, the simulating creatures are distinguished by a characteristic insensibility and by the very considerable lessening or entire disappearance of the reflexes; one may try to excite them in many ways, mutilate them, cut oft" a leg, without the least reaction except sometimes a slight palpitation of the feet. The opinion of biologists is unanimous on this point, as on the preceding.
But these two phenomena, muscular tetanus and lessening of reflexes, are equally characteristic of catalepsy. "We may, then, regard simulation as a cataleptic state. This was the opinion of Fabre, who has been followed by Holmes, by Bohn, and very recently by Rabaud.
May one say as much of the sudden inertia which is shown, before any movement, in the normal phenomena of differential sensitiveness? It is possible, but hardly probable. For want of experiments, we are ignorant if that state of inertia is produced by a muscular tetanus, but it is not impossible to show that it is accompanied by a considerable lessening of the excitability of the subject. From this point of view, the observations of Rabaud (1916) should be cited, for they will allow us, perhaps to reach the solution of the problem. This biologist distinguishes with our articulates two kinds of inertia, of which the praying Mantis and Dixippus morosus offer perfect examples. These two Orthoptera may be thrown into a condition of simulation and then, immobilized, acquire the characteristic insensibility; but the second during the day and the first when it lies in the foliage, seem equally inert and are simply motionless, since they respond then to the least excitation. But Rabaud has shown that our freshwater sand-flea (Gammarus fluviatilis) takes on this last state by differential sensitiveness when it finds itself shaded from light rays, so that the inertia produced by this sensibility is of quite another nature than simulating rigidity. After observations upon the seashore beach-flea (Talor-chestia), Holmes arrives at quite another conclusion, and thinks that the ordinary immobility may lead to immobilization. The problem is not yet solved, but the studies of Rabaud seem indeed to lead to its solution.
Other studies by the same author establish the fact that the simulating reflex with insects is provoked "by non-sensorial excitations directly upon certain points on the surface of the body" and that the antagonistic reflex, that which returns its mobility to the animal, has also for its seat a localized spot. Most often the excitable zone of the immobilizing reflex is found in a vein of the wing (among butterflies, dragon-flies, etc.), on the legs, on the sternal plate ; and that of the mobilizing reflex at the end of the abdomen, at times also (in Lepidoptera) at the extremity of the antennae.
Rabaud has not always fixed the excitable points with sufficient precision, and we may believe that all are not narrowly localized. In any case, it does not seem exact to say that simulation is independent of sensorial organs, at least with the articulates. The action of the light rays and of ether shows that this is not always so, and the pressure exercised by Rabaud upon the excitable areas must certainly have some influence upon the ends of the tactile nerves. From this point of view, simulation does not seem to differ much from differential sensitiveness.
"We can explain in the same way the simulation which is independently produced by the cervical ganglia. Holmes has stated that Ranatras, deprived of the head and prothorax, become rigid when they are excited by slight shocks, and he adds that he obtained the normal simulation with spiders whose brain had been destroyed. Evidently the eyes play no role in either case, but the provoking shock must certainly touch the terminal tactile nerves.