CHANGES OF PHOTOSENSITIVITY WITH AGE 597 



SUMMARY 



Larvae of the blowfly, Calliphora erythrocephala Meigen, were 

 tested each day from hatching until pupation to determine what, 

 if any, changes take place in the sign or the degree of their re- 

 action to light. 



The test employed consisted in subjecting a maggot crawling 

 under the influence of a horizontal beam of light to an instanta- 

 neous change of 90° in the direction of the beam. The resulting 

 change in the direction of the animal's locomotion was measured 

 in degrees by means of a protractor. This procedure gave a 

 quantitative index of the reactivity of the larva, for the more 

 sensitive the individual was, the closer did its deflection approach 

 90°. 



Using the same larvae throughout the experiments, one hun- 

 dred trails were run each day. The average deflection of each 

 of these sets of 100 trails was used to locate a point on an 'age- 

 sensitiveness curve.' 



The curve of photosensitivity thus obtained shows that the 

 reactions are negative throughout, with a rapid increase in ampli- 

 tude during the first days of larval life, reaching a maximum on 

 the fourth day with an average deflection of 81°, and then drop- 

 ping steadily till the seventh day, remaining thereafter almost 

 constant until the time of pupation. 



The decrease in sensitivity occurs coincidently with the begin- 

 ning of the migratory period. 



The changes in photosensitivity shown to take place with ad- 

 vancing age indicate the necessity of detennining and recording 

 either the developmental stage or better, the degree of sensitive- 

 ness, of all individuals used for quantitative experiments. 



