1916] The Sleep of Insects 273 



speculatively— these apparently most complicated manifesta- 

 tions further. One thing I should like to call attention to 

 particularly, however, and that is the significance of the organs 

 of sight. Sight seems to be the sense which is most intensely 

 affected by sleep, judging from the susceptibility of a sleeping 

 organism to light stimuH, particularly the dependence of sleep 

 on the amount of light. We are forced to the conclusion that 

 it is the optic nerves which usher in sleep, which perceive and 

 transmit the first intimation of sleepiness, which give the body 

 the signal that it is time to retire. (Just think of the drooping 

 eyelids of tired persons.) The same is true, as we have seen, of 

 the sleep of insects. On comparing the organs which bring 

 this about — the eyes — which transmit the sleep producing 

 sensations, we find a very material difference. On the one side 

 we have the one lens eye (closed by means of a lid) of the verte- 

 brates, and on the other the facet eye of the insects. Are these 

 differently constructed organs of sight in any way connected 

 with the variations in sleep manifested by these two widely 

 separated groups? Picture the effect of Hght, positive or 

 negative, on insects, and its effect on the nervous system of the 

 articulates with relatively few ganglia: many eyes and few 

 nerve cells opposed to one pair of eyes and a single nerve center 

 with a highly diversified nervous system among mammals and 

 birds. Would it not be conceivable that, since sleep manifes- 

 tations are so closely connected with the eyes (the optic nerves) , 

 they would assume certain definite forms according to the kind 

 and construction of the organs of sight? According to my 

 judgment all the great variations in the organization* of the 

 widely differing groups of animals which are expressed in various 

 forms of sleep, also cause the various uses of the Hmbs, especially 

 of the muscles ; we have only to call to mind the great difference 

 in the sleep of warm and cold blooded vertebrates, which, 

 undoubtedly, is caused primarily by a difference in respiration, 

 i. e., conditions of blood circulation. The manifestations, 

 which. among cold blooded vertebrates have a certain resem- 

 blance to sleep, are so different from those conditions among 

 mammals and birds, not even a sensitiveness to light seeming 

 to be present, that it is doubtful whether among cold blooded 

 animals conditions occur which can be designated as "sleep" 

 at all. 



*It would perhaps be simplest to ascribe the specific manifestations of sleep 

 among insects to the little developed circulatory system. 



