1981] 
Ward — Rhytidoponera impressa. II 
119 
Table 5. Mean colony size (number of workers) for nests with alates and for those 
without alates at the time of year (February- September) when winged reproductives 
are normally present. 
Mean no. workers (± S.D.) 
Species Alates present Alates absent 
confusa 203. 1 + 179.9 (n= 132) 83.9+ 65.7 (n=41) 
chalybaea 270.7+206.2 (n=68) 146.7+122.4 (n=41) 
mental (ergonomic) factors influence the production of alates. 
The available information on impressa and purpurea indicates a 
seasonal brood cycle similar to that of confusa and chalybaea. Nests 
of the two former species collected in the winter in Queensland 
generally had small larvae (sometimes eggs), alates, and few or no 
cocoons (sample of 8 impressa colonies, 16 purpurea colonies). A 
lowland population of purpurea from near Cape Tribulation, north 
Queensland, was exceptional in overwintering with mature larvae, 
as well as worker cocoons and adult alates. Nothing is known of the 
brood cycle in New Guinea populations of purpurea which inhabit 
much less seasonal environments. 
Thus, in Australia at least, four species in the impressa group 
produce one brood of sexuals a year, most or all of which are 
overwintered in the nest and released in the spring. This occurs 
despite contrasting climatic regimes at the north-south extremes of 
range (summer rainy season in the north, and winter rains in the 
south) (cf. Brown, 1954). 
Collections of enigmatica suggest a similar brood cycle (i.e. small, 
overwintering larvae; cocoons present only in summer), with one 
important distinction: alates are usually absent from nests in the 
winter. Of 13 nests collected in the early winter (April 30- July 1) 
only one contained alates (all males); on the other hand, four out of 
five nests collected in the summer (January 12- March 7) contained 
alate pupae (also all males). The differences are significant (p < .02, 
two-tailed Fisher’s exact test), and suggest that alates fly principally 
in the fall. If this is so, there would appear to be considerable 
temporal isolation between enigmatica and its two sympatric 
congeners (< chalybaea and confusa). 
Mating Flights 
Two pieces of indirect evidence suggest that reproductives of 
confusa and chalybaea normally mate in the spring: 
