THE SOCIAL INSECTS 



is adjusted to the width of the gap at different points along 

 the leaf. When the edges of the leaf are drawn together, 

 another ant comes along bringing one of the larger grubs in 

 its mouth. With the silk which is emitted from the lower lip 

 of the grub, the leaves are sewn together by a regular criss- 

 cross of threads. Eventually enough silk is produced to close 

 the gap with a solid sheet. 



This silk is the material which in Ponerine ants serves to 

 form the cocoon when the grubs change into the pupa. In 

 many of the higher ants no cocoon is formed, the pupa 

 lying naked in the nest. In forms like Oecophylla, the pupa 

 is also naked, but the ability to produce silk has been retained 

 and diverted to another use. The method of construction can 

 be seen if a hole is made in the nest and if the ants are 

 observed repairing it. This seems to provide one of the 

 best examples of the co-operation of individuals to perform 

 a complicated operation. 



One further type of nest is interesting as an example of 

 the adaptability of ants. Many species build small nests in 

 natural cavities in plants, such as hollow stems or empty oak- 

 apple galls. A number of plants have natural preformed 

 cavities which are regularly used by ants. Such are many 

 species of Acacia, in which there are hollow stems or large 

 hollow thorns ; other plants have swollen, hollow bases to 

 their leaves. Certain kinds of ants are regularly and exclu- 

 sively associated with such plants, nesting in no other situa- 

 tion. It has been suggested that the plants have evolved in 

 this way in order to attract the ants ; the ants in turn are 

 supposed to protect them from the other insects which would 

 feed on their leaves. There has however been a good deal 

 of evidence that the leaves of such plants are no less eaten 

 than those of plants not inhabited by ants, and the question 

 needs much more study. 



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