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tropical America from Brazil to Southern Texas, ncarh' always 

 nests in hollow twigs and shows merely local preferences for cer- 

 tain kinds of trees and bushes ; but in Nicaragua, where Belt 

 made his observations, and in one locality in Quirigua, Guatemala, 

 as previously stated, this ant has taken to nesting in the acacia 

 thorns. It hollows these out in precisely the same manner as 

 do the regular Psendomyrmce, making the aperture at the 

 same point near the tip of one thorn of each pair. The aperture, 

 however, is larger because it has to admit larger ants, and for the 

 same reason it takes very few gracilis workers, laivie, and pupœ, 

 to fill the cavity of a pair of thorns. Such rapid and perfect 

 adaptation on the part of Ps. gracilis indicates that no special 

 hereditary instinct modification may have been required to in- 

 duce the same adaptation in Ps. belli, spinicola, and nigrocincta, 

 for these three species, like gracilis and many other Pseudo- 

 myrmas, very probably once nested in all kinds of trees. 



Belt's observations, however, suffer from an erroneous sup- 

 position and an important lacuna. After correctly describing 

 the way in which the ants hollow out the thorns, he says (1874, 

 p. 221) : " Strange to say, this treatment seems to favour the 

 development of the thorn ; whilst in my plants that were not 

 touched by the ants, the thorns turned 3'ellow and dried up into 

 dead but persistent prickles. I am not sure, however, that this 

 may not have been due to the habitat of the plant not suiting it." 

 This cautious statement in regard to the enlargement of the 

 thorns becomes a very positive one in Beccari's account of the 

 ant acacias (1884-86), where he says : "Mi sembra indubitato 

 che tale maggiore rigonfiamento debba attribuirse alia irritazione 

 prodotta dalle formiche." He reached this conclusion, which 

 has also been repeated by more recent writers, from finding that 

 the thorns of A. cornigera grown in Itah', and therefore free 

 from ants, were less curved and dilated at the base than the 

 thorns of some herbarium specimens that had been inhabited 

 by ants. It never occurs to him that the thorns ma}- be highly 

 variable, even on the same tree, as indeed the}- are ; just as it 

 seems never to have been observed by Belt that a certain number 

 of thorns often remain small and turn yellow even on large, 

 healthy trees. This, together with the fact recorded above, that 

 the thorns are entered b\- the ants onh' after they have attained 

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