24 ENTOMOLOGY. 



obvious movements of bodies. On spiders, which do not 

 possess compound eyes, Plateau has experimented, and he 

 quite fully proves that in general they are near-sighted, 

 only perceiving at a distance the displacement of large 

 bodies; while the hunting spiders (Attidae and Lycosidae) 

 are probably the only kinds which see the movements of 

 small bodies, and that only at a distance of between 2 and 

 20 centimeters (.80 to 8 inches); while the distance at which 

 their prey is seen well enough to be captured is from 1 to 

 2 centimeters (. 40 to . 80 inch) ; and he adds, even at this 

 distance spiders cannot see distinctly, because they often 

 miss grasping their booty. 



It is so with all larvae, grubs, and caterpillars, as well as 

 such perfect insects as do not have compound eyes; they 

 can only tell daylight from darkness, and indistinctly see 

 moving objects near at hand. 



Even insects with compound eyes have a less perfect 

 vision than formerly supposed. Both Exner* and Plateau 

 have discarded the mosaic theory of vision, and the latter 

 claims that insects, such as flies and bees, see the outline of 

 objects indistinctly, and only when in motion. Plateau ex- 

 perimented in the following way: In a darkened room, with 

 two differently shaped but nearly equal light-openings, one 

 square and open, the other subdivided into a number of 

 small holes, and therefore of more difficult egress, he ob- 

 served the choices of opening made by insects flying from 

 the other end of the room. Careful practical provisions 

 were made to eliminate error; the light-intensity of the two 

 openings was as far as possible equalized or else noted, and 

 no trees or other external objects were in view. The room 

 was not darkened beyond the limit at which ordinary type 

 ceases to be readable, otherwise the insects refused to fly (it 

 is well known that during the passage of a thick cloud in- 

 sects usually cease to fly). These observations were made on 



* Exner finds that the focus of a corneal lens in the compound eye 

 of Hydrophilus is about 3 mm. away, and some distance behind the 

 eye. (Miall and Denny's " The Cockroach," p. 105, note.) 



