COLE. — IMAGE-FOKMING POWERS OF VARIOUS TVPES OF EYES. 371 



large light are 77 records ; on that towards the small light 80, giving 

 -an excess of only 3 records in the direction of the small light, which is 

 only 1.0 per cent of the total number of trials. 



Polygons ^1 and Ji are plotted from the data of the experiments 

 with one-sided illumination, when the large (^1) and small (//) lights 

 were used separately. 



The conclusions to be drawn from the reactions of the larvae of 

 Tenebrio moUtor are, it would seem, as follows. These animals are 

 almost uniformly negative to unilateral illumination in light of mod- 

 erate intensity. The ability of the eyes to form distinctive images of 

 objects differing considerably in size is wholly lacking, or at least prac- 

 tically so, as is to be inferred from the reactions of the animals when 

 exposed to the simultaneous influence of the two light areas used in 

 these experiments. It is perhaps worth noting that the excess of 1.5 

 per cent is away from the large light, and that in no case did the larvae 

 turn towards the large light when that light was used alone, whereas 

 a few did turn towards the small light when that was the only one 

 used. Still, it would not be very surprising if in a repetition of this 

 series of experiments the balance should lie on the opposite side of the 

 zero class. The magnitude of this ditference is probably too small to 

 be of significance. 



The above results are what would be expected in view of the rudi- 

 mentary condition of the eyes in the mealworm. The eyes apparently 

 consist of only two or three ocelli on each side of the head, arranged 

 in a vertical row immediately behind the base of the antennae. Exami- 

 nation of the cbitin immediately overlying them, even after it had 

 been boiled in caustic potash, has shown no thickenings or differen- 

 tiations that might serve as lenses. The eyes are so small that they 

 can scarcely be seen without the aid of a lens. 



Loeb ('90) has remarked on the relation existing between the reac- 

 tions of these animals to light and moisture and their natural habits. 



4. Soiv Bug (Oniscus asellus Linn.). 



This widely distributed, active little isopod may be found in abun- 

 dance beneath stones, bark, pieces of wood, etc., in the woods in the 

 vicinity of Cambridge. Its retiring habits, indicating an avoidance 

 of light, and the ease with which it can be kept and handled in the 

 laboratory, suggested that it might be a suitable form for use in these 

 experiments. Handling often has at first an inhibitory effect as re- 

 gards locomotion, but this is usually overcome in a short time, and the 

 animals then normally start off as soon as released. The eyes are 



