ON CERTAIN MODIFIED HAIRS PECULIAR TO THE 

 ANTS OF ARID REGIONS. 



WILLIAM MORTON WHEELER. 



While studying the ants of the southwestern States I have been 

 impressed with the series of unusually long hairs, or macrochaetae, 

 which occur on the lower surface of the head and mandibles and 

 on the anterior border of the clypeus in the workers and females 

 of several species peculiar to the arid plains and deserts. Ex- 

 amination of the ants from similar localities in South America and 

 from the still more arid regions of Africa, shows a prevalence of 

 the same structures. There is also observable a tendency for 

 these hairs to reach their greatest development in the species in- 

 habiting the driest deserts. Although these structures have long 

 been known to descriptive myrmecologists, no one, to my know- 

 ledge, has noticed the connection between their presence and a 

 pronounced xerophily, or has endeavored to ascertain their func- 

 tion. These matters are of some interest, because the macrochae- 

 tae occur in several unrelated genera belonging to three of the 

 five subfamilies of the Formicidae, and therefore represent a strik- 

 ing example of convergent development. It is, furthermore, 

 comparatively easy to account for the structures in question, as 

 they are merely elongated and somewhat modified portions of the 

 general hairy investiture of the ant's body. 



Before treating of the function of. these hairs and their occur- 

 rence in the various genera and subgenera, it will be advisable to 

 give a general account of their arrangement. When most com- 

 pletely developed, they may be said to constitute four series, two 

 paired and two unpaired as follows : 



i. Clypeal Macroch<ztce. — Although many ants have a fringe 

 of long hairs or bristles on the anterior border of the clypeus, 

 these are best developed in the desert species. They are curved 

 downwards at their tips, are longest and most projecting on the 

 free median portion of the clypeal border, and gradually grow 

 shorter towards the lateral corners. 



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