374 ants' nests. 



among the ants in the first half of the summer, when the nests 

 have to be enlarged for the brood. It then ceases, and truces 

 follow ; in the autumn there is abundant space for all, and peace 

 prevails. It is not without reason that the females and males of 

 Solenopsis fugax do not swarm until September, when the s warm- 

 ing-time of their host ant (July and August) has long been past. 

 They can then, in spite of their size, go to the upper surface of 

 the nest and swarm undisturbed, as I have seen myself, whereas 

 they could not have done so earlier without great danger. 



A peculiar variety of the compound nest is formed by the 

 dwelling of the guest ant, Formicoxenus nitidulus, Nyl., with For- 

 mica rufa and Formica pratetzsis, which I first discovered in a 

 fragmentary condition, and which Adlerz subsequently found and 

 described more fully. Formicoxenus hunts the large Formica^ and 

 even follows it up closely throughout its changes of abode, as 

 Wasmann first noticed, and as I have verified. By Formica^ on 

 the other hand, it is merely tolerated and superciliously ignored. 

 The peaceable guest constructs in the walls of the nest of its large 

 host ant little chambers and passages, which are, however, only 

 imperfectly closed, and open freely into the chambers of the 

 Formica. In these little chambers lie the brood of the Formi- 

 coxenus. The Formicoxenus' s mode of subsistence is still unknown. 



8. — Nests of Mixed Colonies. 



The mixed colonies of the slaveholding ants and parasite ants 

 {Polyergus rufescens^ Latr., Stro?igylognathus testaceus, Schenk, and 

 ^S". huberi, Forel, Anergates airatulus, Schenk, Xenomyrmex stollii^ 

 Forel) have nests which always display the architecture of the 

 working ant (slave or host), and have no further interest for us 

 here. When Polyergus rufescens seizes Formica rufibarbis and 

 keeps it as its slave, its nest resembles a larger nest of that species; 

 if, on the other hand, it enslaves Formica fusca^ its nest looks like 

 the nest of Formica fusca, because the so-called slave or auxiliary 

 ants are the only builders. 



The case appears to be somewhat different in the rare, natural, 

 fortuitous mixed colonies {Formica pratensis or truncicola or 

 exsecta, with Formica fusca ; Tapinoma erraticum with Boihrio- 

 myrtncx meridionalis) discovered by me (Fourniis de la Suisse), as 



