Phototropism in Insects—an Indictment of the Light-trap Method

Extract

The attraction of insects to light is proverbial, and allusions to it occur throughout the literature, showing that it has always excited the interest and curiosity of mankind. Yet it has not received the scientific treatment it deserves. Entomologists have, it is true, investigated it in recent years, but with no great success. The reason for their failure is not far to seek. The method adopted in the beginning was, unfortunately, that of the light-trap, which led immediately to a utilitarian and unscientific approach. The literature that ensued so far from elucidating, merely obscured, the true nature of the phenomenon.

This is clearly shown by the fact that in nearly every investigation the method is to expose one or more traps of various design and complexity in fields at night, the catch being counted and classified in the morning. This is quite a harmless hobby and may even be of some use in indicating whether one trap is better than another and, so long as it is used for this purpose, there is no fault to be found with this method. But this is not the case. The prevailing tendency has been to proceed from observations of the catch not merely to conclusions regarding the relative efficacy of light-traps, which would be legitimate, but to deductions concerning the laws and nature of phototropism which is fallacious. Thus, to take a common instance, a species A is caught in considerable numbers while another species B, though known to be present, is not trapped in any quantity, and the conclusion frequently drawn is that A is more strongly photo-positive than B. The objection is that the investigator knows nothing about the density and distribution of the species concerned in the vicinity of the traps ; he is, therefore, unable to go beyond the observed fact that his trap took more of A than of B, and is not entitled to draw conclusions regarding the relative photo-positiveness of the two species since one may have been more favourably situated or far more numerous than the other in the field.