274 [December, 



therefore, there can be no doubt that the semi-apterous condition of the 

 female is in some way connected with the appearance of the species 

 during cold periods. 



The Effects of Cold on Insects. 

 In considering the cau.se of semi-apterous females amongst moths 

 appearing in winter the effect of cold on insects generally must be 

 briefly considered. A slight lowering of the temperature below the 

 normal produces torpidity, which is first manifested by inability to fly. 

 Further cold results in inability to walk, then in suspended animation, 

 and if prolonged, in death. It is a matter of common observation that 

 species appearing late in the autunui, or in the winter, are frequently 

 so overpowered by the cold as to render them incapable of flight, and 

 it is eqiially obvious that a female so incapacitated when away from 

 the food-plant of the larva would fail to leave offspring owing to her 

 eggs being deposited in a position where the young larvae would be 

 unable to obtain food. In this way the loss of the power of flight 

 would be a distinct advantage to the female as she would be prevented 

 from straying from the food-plant, and although semi-torpid through 

 cold, would in most cases have sufficient vitality to deposit her eggs in 

 a fitting spot before death. On the other hand the power of flight 

 would still be necessary for the male in order to enable him to seek out 

 the female and to prevent the evil effects of prolonged interbreeding. 

 The fact that a certain number of males would no doubt perish from 

 the cold without pairing would be of little importance so far as the 

 perpetuation of the species is concerned, especially if the males outnum- 

 bered the females, which is known to be the case in many insects. It 

 may, of course, be urged that a far simpler explanation of the semi- 

 apterous condition is to be found in a lowered vitality, induced by cold, 

 preventing the full development of the organs of flight, but the winged 

 condition of the male in my opinion negatives this explanation, and the 

 existence of many winter insects with fully developed wings in both 

 sexes is also against it. Briefly stated then, I believe that the semi- 

 apterous females have been evolved in many insects appearing in the 

 winter or very early spring because such a condition prevents them from 

 leaving the food-plant, and being afterwards unable to return to it to 

 deposit their eggs when overcome by the effects of cold. Such insects 

 would naturally feed on widely distributed plants, as otherwise 

 the R. nii-apterous condition of the female would be a fatal disadvantage, 

 and this fact is in complete agreement with actual observation. In 

 view of the restricted data on wliich this theory is based, it is put for- 



