COMPOUND EYES 109 



It will be obvious that in conducting experiments relative to 

 insect vision it is important to take into account the type of com- 

 pound eye present in the subjects utihsed. So far no comparative 

 studies have been made with respect to the photic reactions in the 

 two types mentioned, and a wide field for enquiry remains for 

 future investigators. Apparently, most observations have been 

 made with insects whose eyes give apposition images. Such types 

 are essentially adapted for diurnal vision only. Their focal range 

 as regards form-perception is very limited, and, in the case of 

 Vmiessa urticce, Eltringham gives reasons for concluding that it 

 does not exceed 3 feet. Perception of movement, it would appear, 

 may be effective over a greater distance, and, in the case of dragon- 

 flies, Tillyard has observed that the species Petalvra gigantea can 

 be frightened away b}' waving a net at a distance of 20 yards. 

 In this connection the observations of Vigier with regard to the 

 existence of a capacity for focal accommodation may be significant. 

 While appreciation of form is probably of great importance in the 

 pairing of sjiecies, there is a considerable evidence that compound 

 eyes are endowed more especially for the perception of movement, 

 and that insects rely upon this capacity to escape from their 

 enemies. With eyes adapted for daylight, the movement of even 

 small objects in the field of vision would be registered as a fleeting 

 impression on the retinal cells, and it becomes transmitted to the 

 brain of the insect, which responds accordingly. It is often possible 

 to approach an insect without occasioning its alarm. If the 

 approach be sufficiently slov^ and gradual the change of position 

 passes apparently unnoticed. The explanation appears to be that 

 an object as large as a human being affects all the ommatidia more 

 or less equally and simultaneously. Since the movement occurs 

 very slowly, the change of position entails only very slight and 

 gradual changes in the character of the retinal image as a whole. 

 A sudden movement of a part of the body of the person, however, 

 is sufficient to bring about immediate departure of the insect, for 

 the reason that it has caused an abrupt change in the retinal 

 image by affecting a number of ommatidia in rapid succession. 



Ocelli. Ocelli present so many different types that they are 

 not definable upon any common structural basis. In some few 



