38 

 PSYCHOLOGY. 



The function of The larva of Chironoiniis pusio has already been shown to possess 



a pair of bifurcating fringed hairs on segments 5 to 9. The problem 

 now to be considered is what function do they normally perform in 

 those larvae which possess them ; and are they functionally active in 

 this species, or only persistent, though useless, organs like the vibrissae 

 on the striated flaps of larva No. 7. 



I think I shall be able to show that in this larva, and also in the 

 stalk-cased larva No. i, they have a very important part to play in the 

 economy of the insect. 



From the position of the bifurcators it is obvious that they must 

 be continually coming in contact with the sides of the tube as the 

 larva moves about, but if the larva begins to come out of its case 

 some of the bifurcators will no longer be touched, and by the time 

 the larva has extended the first nine segments no hairs will be left 

 within the tube at all. Supposing the stimulation of these hairs gave 

 rise to a painful sensation, we should expect the larva to remain 

 extended as far as possible from its case so that nothing might be in 

 contact with the hairs ; but if on the other hand the feeling-tone of the 

 hairs was one of pleasure, then we should expect the larva to retain as 

 many pairs of bifurcators as possible in contact with the tube so as to 

 enjoy to the full the agreeable sensation set up by the stimulation of 

 these hairs. The facts which I have seem to argue in favour of the latter 

 supposition. When at rest all the segments bearing bifurcators are kept 

 within the tube and the head alone appears outside, with perhaps one 

 or two of the anterior segments bearing no bifurcators. When actively 

 employed it comes far out of its tube, but only for a moment at a time, 

 and even then never exposes all the segments bearing these sensory 

 organs, the ninth segment at least remaining within, so that it may 

 enjoy an occasional stimulus from at least the last pair of bifurcators. 

 If it occasionally oversteps the mark the absence of a pleasant stimulus 

 will immediately cause it to retreat until a pleasurable feeling-tone is 

 once more started. In this way the bifurcators will act as a continual 

 check upon the over-activity of the larva, and prevent it from advancing 



