THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 169 



surfaces of these buds, gives the animal the sense of direction, and 

 that it is possible in consequence that the vision of the perfect 

 insect may be reduced to this simple faculty. 



Loeb ('90) in his studies on heliotropism worked upon the larva 

 of Musca vomitoria because of its negative reaction to light and 

 its eyeless condition. He found that the extreme anterior por- 

 tion of the larva is highly sensitive to light. 



In a recent paper by Holmes ('05a) the phototactic movements 

 of individual fly-larvae (blow-fly), together with those of other 

 species receive attention. In the words of the author (p. 98) : 

 ''No one has attempted to work out in detail the exact mode of 

 response in any of these forms, although the fact of their orienta- 

 tion to the direction of the rays of light has been described by 

 several different observers." The conclusions which Holmes 

 (p. 105) reaches in reference to the flj^-larvae are as follows: 

 '' In the animals here described there is, so far as I can discover, no 

 forced orientation brought about by the unequal stimulation of 

 the two sides of the body, but an orientation is produced indirectly 

 by following up those chance movements which bring respite from 

 the stimulus. I do not deny that there may be an orientating 

 tendency of the usual kind, but if there is, it plays only a sub- 

 ordinate role in directing the movements of the animal." 



The writer (Herms, '07) in a preliminary study of the tropisms 

 of fly-larvae as a factor in food habits and migration (pp. 77-82) 

 pointed out a positive reaction of the larvae of Lucilia caesar. This 

 reaction was further discussed in a paper read at the Seventh 

 International Zoological Congress (Herms '11). 



2. Anatomical 



It is a generally known fact that fly-larvae have no obvious, 

 external visual organs, and this fact alone has made interesting 

 the observations relative to their locomotion and accommodation 

 to external stimuh. Weismann ('64) and Lowne ('90-92, '93-95) 

 have treated quite exhaustively the anatomy of the 'blow-fly' 

 through all stages of its development, and it is largely on the work 

 of these two authors that the following statements are based. 



