on the vision of Arthropods. 405 



species observed he was able to touch, except two species 

 of Ichneumonidce, of the genus Pmipla. 



Twenty-eight species of Diptera were observed ; the 

 distance at which they perceived movement was found 

 to be rather greater than in the case of Hymenoptera, 

 being on the average about 26 in. ; seven or eight species 

 he was not able to touch. 



Diurnal Lepidoptera were found to be still more 

 acute ; the average distance at which they perceived 

 a movement of the body of the observer being four or 

 five feet : of the fourteen species tabulated he was able 

 to touch twelve. 



Odonata, or dragon-flies, are endowed with magnificent 

 optical organs, and are most active creatures ; hence 

 entomologists assign to them acute powers of sight. 

 Plateau discusses this reputation, and decides that it is 

 exaggerated ; the larger species perceive movement at a 

 distance of three to six feet, rarely did they seem to 

 detect it at a distance of nine or ten feet. 



Amongst Coleoptera, Cicindela campestris and hyhrida 

 were found to be as well endowed as Diurnal Lepi- 

 doptera, while species of Carabus were very stupid, and 

 Dytisciis dimidiatus did not apparently perceive move- 

 ment at all. 



We have next a series of interesting observations 

 made in a room lighted by two windows at one end : 

 these windows were closed by shutters, in one of which 

 there was, for the admission of light, a square orifice of 

 about four inches in each direction ; while in the other 

 shutter there was a similar orifice so arranged that it 

 moved round in a circle by means of a revolving 

 apparatus. The size of the orifices was capable of 

 variation, either separately or together, and by these 

 means the Professor ascertained some interesting facts. 

 Insects, when set at liberty in the room, flew towards 

 these lights ; they perceived that the one of them moved, 

 but they did not object to this, and directed their flight 

 towards it as much as towards the stationary orifice ; so 

 perfect, indeed, was the perception of movement, that 

 the motion of the insect in flight was controlled by the 

 movement of the orifice to such an extent that the 

 direction of the flight became spiral. The results of 

 this series of observations may be thus arranged : — 



