4y8 INFLUKNCE OF CLniATE oX ANIMAL DISEASE. 



The shortest time observed by Veglia as occurring between 

 the eg",s^ and the final infective stage larvse was three days, and 

 this was noted at a temperature of from 22° C. to 35° C. The 

 effect of decreasing temperature in retarding the rate of develop- 

 ment is again seen in the fact that the same developmental period 

 as that just referred to occupied six days at 15° to 18° C. ; 

 eight days at 15° C. ; and 12 days at 15° to 13° C. In making 

 field experiments, he noted in one of these, made in March, 1915, 

 that the final stage infective larvae were to be met with after four 

 days, the maximum temperature recorded during this time being 

 26° C. whilst the minimum was 15° C. ; in another field experi- 

 ment, where these limits were 26° C. and 12° C. respectively, it 

 was only on the seventh day that the larvae were found crawling 

 up the grass blades ; whilst in a third and similar experiment, but 

 with temperature limits of 26° C. and 8° C. only 10 per cent, of 

 the larvae were found on the tenth day to have reached the final 

 and infective stage. 



In regard to the eft"ects of drying and freezing, he has noted 

 that eggs resisted freezing up to 36 hours, and some for even 

 48 hours, and that, kept at 4° C, they died in from 40 to 50 

 davs. llie immature larvtC in some cases ap])ear to resist freezing 

 for 24 hours, but at tem])erature descending from 10° C. in- 

 creasing numbers of them die when exi:)6sed for a similar length 

 of time. He also noted that this larval stage in one ex])eriment 

 was not able. Vx-h.en present in faeces, to resist drying for mf)re 

 than ten days. 



The resistance of the infective larval stage is evidence by 

 the fact that after an exposure to a tem])erature of 0° C. from 

 the 30th July, 1914. to the 21st December, 1914. 30 per cent of 

 this stage larvae were still found to survive. In regard to the 

 effect of drying on this stage. Veglia remarks that when spread 

 out in thin layers on either glass plates or dry grass blades, and 

 not allowed to collect in the form of clusters (in which formation 

 original moisture is retained longer than when the individual 

 larvfe are well separated), they are invariably found dead in 

 four dav's time. In faeces, ho^\•ever, kept in the field, and thus 

 deriving a certain amount of moisture from the soil, he has 

 found 50 per cent, of this stage larvae surviving from the 2nd 

 April. 1915, to the 22nd Jtme, 191 5, but in another similar experi- 

 ment, however, he found the same stage larvae placed in a similar 

 position on the 8th September. 19] 4. to have died out in June. 



1915- 



Another observation to which, reference is made here be- 

 cause of its general biological interest and importance is the 

 extremely interesting observation in regard to the eff'ect of light 

 on this larval stage. 



The writer may point out that the effect of light in deter- 

 mining movements and activity of certain animals is an effect 

 which has been before recognised, and especially in regard to 

 larval forms. This subject is one which has been studied under 

 the name of heliotropism. and according to whether light exerts 



