Reflex immobility and the central nervous system.

Abstract

A thoroughgoing theory of animal conduct must be prepared to deal successfully not merely with the orienting movements of animals, as Loeb's tropism conception and the work related thereto seems capable of doing; it must also be in a position to treat the problem of quiescence at one moment, movement at another, and in general the whole matter of “physiological states” in which the central nervous system functions as adjustor organ. The inducible tonic immobility markedly developed among arthropods, often termed “feigning of death” and commonly referred to as an “instinct,” provides opportunity for study of just this kind of central nervous function.

The length of an act of immobility initiated by appropriate stimulation of an insect or crustacean is in reality determined by the duration of a sustained condition in the central nervous system. It is easily shown that in a variety of Hexapods and Isopods with which these experiments have been concerned, the animal gives other well defined reflexes without emergence from the specific immobile state. With Ranatra in particular several striking proofs are obtained of the reciprocal innervation of the appendages and of the orientations of their central connections.

Nevertheless, as Raboud has insisted, in each organism there are certain peripheral loci stimulation of which at once arouses an immobile specimen at the will of the experimenter.

The duration of successive acts of immobility is rhythmic. In Isopods at least, the rhythm does not exist as a general metabolic rhythm on which the observations are, so to speak, superimposed. It is initiated by the first stimulations of a series. Once initiated, however, a cycle rises to a maximum of possible duration, then falls to practically zero duration, independently of further excitations.