516 CAROLINE BURLING THOMPSON 



INTRODUCTION 



This subject was suggested to me by Prof. W. M. Wheeler of 

 Harvard University as one that needed investigation. After my 

 work was begun, Pietschker ('11) pubUshed his paper ''Das Gehirn 

 der Ameise." Although I can corroborate much of Pietschker 's 

 work, I differ from him in several important pomts. Professor 

 Wheeler has told me that Pietschker's Camponotus ligniperdis is, 

 in regard to internal structure, one and the same as the Cam- 

 ponotus pennsylvanicus of this paper, the two forms differing 

 only in very slight external characters. I am greatly indebted 

 to Professor Wheeler for his interest in my work, for the iden- 

 tification of specimens, and for material from his collections which 

 will be used in a second paper on the brains of other genera of 

 ants. 



MATERIAL AND METHODS 



The genera and species described in this paper are: Cam- 

 ponotus herculeanus L., subsp. pennsylvanicus, Formica pal- 

 lidefulva, Latreille, subsp. Schaufussi, Lasius niger L., var. amer- 

 icana. 



The material was collected in 1911 and 1912 during June and 

 July, at which time the developing stages are very abundant. 

 Camponotus colonies were found by chopping open partly de- 

 cayed logs, Formica and Lasius were found in the ground under 

 stones or logs. Following the example of Wheeler ('10) and 

 Pietschker ('11) the pupa was the only stage used in this work, 

 for at this period the skin is so soft and free from chitin that the 

 entire head and body can be sectioned. The brain of the oldest 

 pupae differs very little from that of the adult, and the difficulty 

 of dissecting out this delicate organ is avoided. Both very 

 young and nearly mature pupae were used, the approximate age 

 being determined by the pigmentation of the eyes. The young- 

 est pupae have colorless eyes, which later become pinkish, red- 

 dish brown, and finally black. The pupae are enveloped in 

 tough brown cocoons which were removed with fine needles 

 under a dissecting lens. The heads were then detached from the 

 bodies and dropped into the fixative. The size of the cocoons 



