on the vision of Arthropods, 397 



In observing caterpillars the methods used were those 

 I have already mentioned in the case of the Myriopoda, 

 in addition to another that I have not yet alluded to, 

 viz., placing a caterpillar on an apparatus somewhat 

 like a parrot-perch, a transverse rod supported by a very 

 slender uj^right stem, such as a stick held in the hori- 

 zontal position by a needle. When placed on such an 

 apparatus the caterpillar seeks to escape, and when it 

 arrives at one end of the rod extends itself in various 

 directions to find another object on to which it can pass. 

 When the caterpillar was in this position Prof. Plateau 

 placed objects near to it with a view to ascertaining if 

 they were perceived by the eyes of the insect, and if so, 

 at what distance. He found that the hairs on the cater- 

 pillars were very important aids to them in detecting the 

 proximity of objects, so that it was necessary to pay 

 attention to their existence and see that they were not 

 touched ; he found the distance at which the existence 

 of an object was perceived by the caterpillar to vary 

 from something less than a \ in. to fin. ; no caterpillar 

 experimented with perceived a rod \ in. in diameter at 

 a distance of 8 centimetres {1^ in.). Under similar 

 circumstances a Julus distinguished nothing, and Prof. 

 Plateau considers therefore that we are justified in 

 attributing to caterpillars formation of an image on the 

 retina, and as a consequence a true vision. 



[This, however, is, I think, going too far. Professor 

 Plateau's experiments have shown that a stick is per- 

 ceived by a caterpillar at a short distance, and render it 

 highly probable that it was perceived by the aid of the 

 ocelli ; but in what way the ocelli operated there is no 

 evidence at all, and that the perception was by means 

 of an image on a retina is rendered improbable by the 

 extremely short distance at which alone the existence of 

 an object was detected. — D. S.] 



Professor Plateau gives a summary of the results of 

 his observations on caterpillars, which I translate as 

 follows : — 



1. Either the structure of the eyes in caterpillars is 

 still imperfectly known, or the deductions made from it 

 are partly erroneous, for the eyes have a more important 

 function than that of simply distinguishing between 

 light and darkness. Caterpillars see, but see badly. 



2e 2 



