THE TOBACCO BEETLE. 25 



PHOTOTROPISM. 



Adults of the tobacco beetle are accustomed to darkness or semi- 

 darkness. Up to a certain degree of intensity they respond positively 

 toward light, but they are negatively phototropic if the light is too 

 intense. Observations made in tobacco warehouses and on beetles in 

 specially constructed cages at the laboratory showed that they avoid 

 intense sunlight, but toward sunset, or when the light intensity is 

 lowered, they move toward the source of light. 



REACTION TOWARD COLORED LIGHT. 



Laboratory experiments made with apparatus which transmitted 

 light through color screens or ray filters which made it practically 

 monochromatic showed conclusively that the tobacco beetle, in com- 

 mon with other insects, reacts most strongly to colors of shortest 

 wave length. The movement toward blue or blue-violet is most pro- 

 nounced, and the movement toward red least of all. When a series 

 of traps was operated with the light transmitted through color screens 

 placed in regular order from red to violet the number of beetles at- 

 tracted, in the majority of instances, increased in fairly regular order 

 from red to violet. Experiments made with electric lights showed 

 that the beetles were attracted toward a bulb of clear glass transmit- 

 ting light rich in rays of short wave length, and scarcely at all toward 

 a bulb of red glass, which transmitted rays of long wave length giving 

 light at the lower or red end of the spectrum. 



The adults, in common with other insects reacting negatively 

 toward intense sunlight, are only slightly sensitive to light at the 

 lower end of the spectrum, and rays of longer wave length, limited 

 to red and orange, seem to act on them in much the same manner as 

 darkness. Adults exposed to bright sunlight under color screens of 

 red and blue were observed to collect under the red screen almost as 

 readily as they did when an opaque screen was used in place of the 

 red, although the apparent intensity of light under the two screens 

 was the same. This shows a reaction directly opposite to that ob- 

 served when the beetles, in darkness, are exposed to lights of low 

 intensity. 



EFFECT OF TEMPERATURE ON ACTIVITY OF ADULTS. 



A series of laboratory experiments was conducted to ascertain the 

 effect of ascending and descending temperatures on the activity of 

 adults. Beetles were confined in the lower part of a long glass tube 

 20 millimeters in diameter. A thermometer was passed through an 

 opening in the cork and the tube lowered into an inverted bell jar filled 

 with water. A support was arranged in such a manner that the bell 

 jar could be lowered into melting ice, or into a basin of water which 



