144 THE FACTORS [Part I 



At the phylogenetic commencement of symbiosis, the ants bored an 

 entrance through the groove, evidently because the wall was somewhat 

 thinner there, and in particular, in accordance with a custom that is 

 almost always followed and is connected with the domestic arrangements, 

 they bored as much as possible in the upper part of their dwelling. All 

 features that facilitate boring through this place must have been retained 

 in the struggle for existence, and been further added to through selection 1 . 

 They finally led to the differentiation of the thin weak diaphragm that 

 has been described. 



r 1 



Fig. 79. Cecropia adenopns. Part of a transverse section of the wall of an internode, showing 

 the diaphragm. Slightly enlarged. 



The ants of the Cecropia devote themselves in their dwellings to the 

 tending of Aphidae ; they would seldom leave this work, and would rarely or 

 never visit the foliage, were it not that the latter merited continued attention. 

 The base of the petiole is covered on its dorsal surface with a brown 

 velvety coating upon which, in uninhabited trees, ovoid whitish bodies, 

 about 2 mm. long, lie quite loose (Fig. 80). The presence of such bodies, 

 which are named Midler's corpuscles after Fritz Muller their discoverer, 

 is a sure sign that the tree is uninhabited, and they are always visible 

 in our conservatories. They are absolutely wanting on the surface of 

 the pulvinus of inhabited trees, because they are continually carried away 

 and eaten by the ants that are always looking for them, just as in Acacia 

 cornigera. Muller's corpuscles, like those of Belt, consist of delicate 

 parenchyma rich in proteids and oil. 



1 I have observed on the mountains between the provinces Rio and Minas a species of 

 Cecropia which appears to have attained a lower stage of adaptation, since, among other 

 points, the entrance is absent in younger plants. The observations, however, were but 

 fragmentary. 



