CHAPTER XXIII. 



THE COMPOUND NESTS. 



" Le but unique de tous les actes comme de toutes les representations des 

 insectes sociaux, c'est 1' elevage des jeunes; mais si le but est unique, les moyens 

 sont nombreux." Espinas, " Des Societes Animales," 1877. 



In several of the foregoing chapters I have discussed the ethological 

 relations of ants to a variety of other organisms flowering plants, 

 fungi, Homoptera, caterpillars and a host of other arthropods belong- 

 ing to the most diverse natural orders. These chapters, however, did 

 not include an account of some of the most interesting symbiotic rela- 

 tions, namely, those of the ants to other species of their own taxonomic 

 group and to termites. This living together of colonies of different 

 species may be properly designated as social symbiosis to distinguish 

 it from the simple symbiosis that obtains between individual organisms 

 of different species and the intermediate form of symbiosis exhibited 

 by individual organisms, like the myrmecophiles and termitophiles that 

 live in ant or termite colonies. 



The researches of the past forty years have brought to light a 

 remarkable array of instances of social symbiosis, varying so much in 

 intimacy and complexity that it is possible to construct a series ranging 

 from mere simultaneous occupancy of a very narrow ethological sta- 

 tion, or mere contiguity of domicile, to an actual fusion, involving the 

 vital dependence or parasitism of a colony of one species on that of 

 another. Such a series is, of course, purely conceptual and does not 

 represent the actual course of development in nature, where, as in the 

 animal and vegetable kingdoms in general, development has not fol- 

 lowed a simple linear course, but has branched out repeatedly and 

 terminated in the varied types existing at the present time. 



It is convenient to follow the European writers, von Hagens, Forel, 

 Wasmann and others, in grouping all the cases of social symbiosis under 

 two heads, the compound nests and the mixed colonies. Different spe- 

 cies of ants or of ants and termites are said to form compound nests 

 when their galleries are merely contiguous or actually interpenetrate 

 and open into one another, although the colonies which inhabit them 

 bring up their respective offspring in different apartments. In mixed 

 colonies, on the other hand, which, in a state of nature, can be formed 

 only by species of ants of close taxonomic affinities, the insects live 



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