164 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



conditions under which the tests were carried out were not always 

 above criticism. The reader who is interested in this aspect of 

 the subject is advised to consult the papers by Hoskins and 

 Craig and by Marshall. 



Reactions to Light. In many countries practical entomologists 

 have made use of light traps as a means for the quantitative 

 attraction and destruction of noxious species of Lepidoptera, and 

 in some cases of Coleoptera also. These traps are usually so 

 constructed that insects flying to the light fall into a surrounding 

 receptacle containing either oil, or water covered by a film of oil. 

 On the whole, the results derived from the application of this 

 method cannot be regarded as commensurate with the trouble 

 and expense involved. In temperate countries the success of such 

 a method, as with chemotropic traps, is considerably influenced 

 by meteorological conditions. Temperature, for example, is 

 significant in that when it falls below certain limits, varying 

 according to the time of the year and the species concerned, 

 phototropic responses almost cease. Such factors as precipitation, 

 general humidity, wind, etc., appear to exercise a variable effect, 

 but little has been determined with regard to their respective 

 influences. In the tropics, on the other hand, where much more 

 uniform conditions prevail, such variable factors are far less in 

 evidence. It is frequently claimed that the preponderance of 

 male insects that are attracted discounts the value of light traps. 

 A further disadvantage that has been emphasised is that the 

 female insects have, for the most part, already deposited a quota 

 of their eggs before being trapped. In some cases these drawbacks 

 are obviously well founded, but in others premature conclusions 

 have been draw n on account of insufficiency of experimental data. 

 Due allowance needs to be made for the fact that the males of a 

 number of species tend to appear earlier in the season than the 

 females. Also, cases are known where the males are on the wing 

 during one part of the night, and the females during another. 

 There seems, also, little doubt that more can be done towards 

 improving the efficiency of the light source itself. More attention 

 needs to be given to light rich in violet and ultra-violet rays (vide 

 p. 121). In this connection it may be added that very large 



