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Page:Popular Science Monthly Volume 72.djvu/189

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185
THE INSTINCT OF FEIGNING DEATH

acting with an intelligent appreciation of his predicament, but it is not to be inferred that he could have reasoned out his course of action did not an innate proclivity in that direction form a part of his instinctive make-up.

The physiological condition in what is called death-feigning is quite different in different forms. In most of the lower animals it is characterized by a tetanic contraction of the muscles. The attitudes assumed by many forms, such as rolling into a ball, keeping the legs and other appendages drawn close to the body, or in some cases holding them straight and rigid, are such as can be maintained only at the cost of considerable muscular effort. If a Ranatra is picked up by one of its slender legs it may be held out horizontally for a considerable time without causing the leg to bend. It is as if a man were seized below the knee and held out straight, face upward, without causing the knee to bend; only the legs of a Ranatra are several times more slender than those of the most attenuated of the human species, and the muscular tension which the insect maintains must therefore be intense.

The death feint of insects and other low forms is not entirely dependent on the brain. It is due rather to a general physiological state of the animal. I have found that the posterior part of the body of a Ranatra can still be induced to feign death, though less perfectly, when entirely removed from the head and prothorax. When it would come out of the feint a few light strokes would cause it to feign again. It has been found that spiders also may still feign after entire destruction of the brain.

The instinct of feigning death is doubtless closely connected with much of what has been called hypnotism in the lower animals. Crayfishes, frogs, lizards, certain snakes and many birds and mammals, may by a very simple process be thrown into an inactive condition from which they are not readily aroused by external stimuli. In ordinary death feigning the animal falls into its immobile state upon slight provocation; a touch, or even a jar is sometimes all that is necessary. In the so-called cases of hypnosis more or less manipulation is necessary. The exciting cause in both cases is generally some form of contact stimulus. In the hypnotism of animals, as Verworn and others have shown, there is diminished reflex irritability, and usually tonic contraction of many at least of the muscles. Similar phenomena are observed in the death feigning of many forms, some of the insects showing a lack of responsiveness that is truly remarkable. In a water-scorpion that is feigning death the legs may be cut off one by one, or the body cut in two without eliciting the least reaction from the unfortunate victim. We can only speculate at present on the condition of the nervous system which makes such a result possible.