56 Transactions. 



With the few exceptions shortly to be specified, it will be seen that a 

 striking agreement exists between the New Zealand and the British species 

 in the following respects : — 



(1.) General Distribution of the Food Plant of the Larva in the Region 

 ivhere the Insect is found. — There is no exception to this rule, which holds 

 good in every case where the female is semi-apterous and the food plant 

 is known. It is, in fact, obvious that the semi-apterous state would be 

 absolutely fatal to an insect feeding on a scarce or local plant, as the females 

 would require to travel over extensive areas in order to deposit their eggs. 

 Any advantage which a species might obtain by possessing a semi-apterous 

 female would, therefore, be wholly neutralized unless its food plant were 

 very common and generally distributed. 



(2.) Appearance of the Imago in Winter, or in Very Early Spring. — It is 

 in this circumstance, which is common to all the species having semi- 

 apterous females, with the exception of the three species of Metacrias in 

 New Zealand and the two species of Orgyia in the British Isles, that, in 

 my opinion, an explanation of the semi-apterous condition of the female is 

 to be sought. As a matter of fact, M. erichrysa and M. huttonii can hardly 

 be regarded as exceptions, seeing that they are both strictly alpine insects, 

 and therefore practically exist under winter conditions. One of the species 

 of Orgyia also occasionally appears in winter, and may therefore have 

 recently changed its habits. The only important exceptions are, there- 

 fore, M. strategica (New Zealand) and 0. antiqua (Britain), and of the latter 

 insect there appear to be a succession of broods throughout the entire 

 summer. As corroborative evidence from insects belonging to an order 

 other than the Lepidoptera, two of our common crane-flies {Tipula 

 obscuripennis and T. heterogama) occur to me. They have semi-apterous 

 females, and appear often in considerable numbers late in April and during 

 May, a period which must, of course, be regarded as winter in New Zealand. 

 I think, 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 cause 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 pro- 

 duces 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 autumn or in the winter are frequently so overpowered by the 

 cold as to render them incapable of flight, and it is equally 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- 

 toi-pid 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 per- 



