INFLUENCK OF CLI^IATE ON WnfAL DISEASE. 499 



an attractive or repulsive influence on the i)arricular form of 

 life studied, two forms of heliotropism. a positive and a negative 

 form respectively, have come to be recognised. A very interesting 

 discussion of the purposefulness of heliotropism may be found 

 by those interested in the subject, in Jacques Loeb's essays, col- 

 lected under the title of " A Mechanistic Conception of Life," 

 and many references to its possession by different living forms 

 are there met with. So far as is known, however, it has not been 

 before noted in regard to the larv?e of H. contortns. 



X'eglia found that in the case of these larvae the phenomenon 

 of negative heliotropism was exhibited and first noted 

 this when watching the development of the larvse in 

 cultures kept in glass jars. In keeping them under such 

 conditions and exposing them to lights of different 

 intensities, he has noted that whilst in the ])resence of 

 dift'use light or darkness they tend to crawl up the sides of the 

 vessel in which they are kept (temperature and moisture condi- 

 tions of course being suitable), and to heights increasing in a 

 certain proportion as the light supply is lessened, they, on the 

 other hand, are caused to quickly move down the sides of the con- 

 taining vessel, into the layer of faeces at the bottom, when this 

 glass vessel is exposed to a bright light such as sunlight. He has 

 further observed that alternate upward and downward move- 

 ments of the larvae can be induced by alternating exposure to 

 subdued light or darkness, and bright light respectively but after 

 repeated excursions of the larvae induced in this manner, they 

 appear to become exhausted, for some time at any rate, and 

 they retire into the layer of faeces, or earth containing faeces, 

 placed at the bottom of the jar, and i)lacing the vessels in dark- 

 ness no longer stimulates them to moA^e up the walls as before. 

 The duration of this phase or its ultimate termination have not 

 yet been fully determined. 



The actual destructive eff'ect of sunlight on the larvae is also 

 included in Veglia's observations, and he attaches considerable 

 importance to it. Detailed reference, however, to all of these 

 points will appear later in his publication on the subject. 



This effect of light on the migrations of the larvae will thus 

 be seen to be of extreme interest, but the part which it plays m 

 the biology of the v/orm under natural conditions, and how far it 

 acts as a factor in regulating, along with temperature and moist- 

 ure, the migrations of the larvae up the grass blades during the 

 night or in cloudy weather, and how far the harmful efi"ect of 

 direct sunlight on the larvse may come to be exerted under natural 

 conditions, are all matters requiring further investigation. 



We may leave this part of the subject here, however, and go 

 back to our former remarks on the effect of heat and moisture 

 on the rate of development of the worm. 



Recalling these facts and bearing them in mind, it is hardly 

 necessary to emphasise their importance any further, and the 

 importance of the part played by wet or damp pastures and wet 

 weather is made evident. The facts, however, which have been 



