The Bionomics of South African Insects. 457 



are therofoie of especial value. The weights of the 

 eighteen bred specimens are given on p. 451, and it will 

 be seen that the diiforence between the phases is very 

 marked, althougli not nearly e([ual to that between tlic 

 two forms of artaxv(. 



There is no escape from the conclusion that the larvto of 

 the dry phase of these species must be much larger than 

 those of the wet, and must eat a great deal more food. 

 This inevitable conclusion suggests that in experimenting 

 on this most interesting of all known examples of seasonal 

 change, it will be well to keep an open mind on all con- 

 ceivable stimuli : on the abundance and character of the 

 food-plant as well as the inorganic conditions of humidity 

 and temperature, the latter of which has been proved by 

 Dorfmeister, Weismann, Merrifield and Standfuss to be 

 an effective stimulus in the case of certain Pala^arctic 

 seasonally dimorphic species. It is possible that the 

 parched state of the food-plant towards the end of the dry 

 season may be the stimulus which determines development 

 in the direction of the smaller summer phase. The different 

 sizes and weights render it nearly certain, as I have argued 

 above, that the phase is predetermined in the larval stage. 

 Now the larval stage of the first dry-season brood is passed 

 in the wet season, and that of the first wet-season brood 

 probably in the dry. We must look to some condition 

 affecting one or both of these larval stages, or the eggs 

 from which they arose, as the stimulus which sets in 

 motion the organic processes resulting in a change of 

 phase. Some colour of support is lent to the suggestion 

 that the condition of tlie food-plant may afford the neces- 

 sary stimulus by the fact that the wet phase of P. cirtaxia 

 is unknown in certain forest regions, where it is probable 

 that the food is not subject to the same alternation of 

 condition as in more exposed stations. But forests would 

 also act as moderating influences for extreme differences 

 in temperature and humidity, and thus tend to prevent 

 these from acting as stimuli for the species in question ; 

 for we know that some stimuli must be effective in pro- 

 ducing such seasonal changes as occur in other forest 

 species of Precis (see p. 423). Finally, c[uantity as con- 

 trasted with condition of food would be well worth trying. 

 The unusually low weight of the imagines bred from the 

 egg (Expts. 11, 12, and 13 on p. 451) was a probable 

 result of difiiculty in obtaining a constant supply of fresh 



