BRAINS OF THREE GENERA OF ANTS 541 



mushroom bodies. The fibers of fl unite the inner and outer 

 lobes. H, gl., and gr. are efferent fiber tracts occupying the core 

 of the stalk, and originating respectively from the cells of Groups 

 I and II. 



More tracts are found in the brain of the queen than in the 

 worker or male. This is not due to size, since the worker mush- 

 room bodies are larger than those of the queen in Camponotus 

 and Formica, and about equal in Lasius; it therefore seems to 

 indicate that the queen possesses the most complex and highest 

 type of brain. The tracts that are characteristic of the queen 

 are tracts c, ex, and d; c, however, being present in one male, 

 Camponotus. R is found only in the queens of Formica and 

 Lasius. E is absent from the queen brains of Formica and 

 Lasius, but is present in the Camponotus queen. It is interest- 

 ing to note from table 2 and from the figures that the brain 

 of the queen of Lasius niger, which has already been spoken of 

 as possessing a more generalized type of structure than Campo- 

 notus or Formica, has a few more tracts than either. But the 

 same table and figures make it evident that the decreased 

 size of the mushroom bodies of any caste is due to a decrease in 

 quantity, common to all cells and fibers, rather than to the dis- 

 appearance of particular cell groups or fiber tracts. 



Camponotus queen: figure 32. The study of the mushroom 

 bodies by means of serial sections cut in the frontal plane shows the 

 following fiber tracts named from the anterior surface backward. 

 Tract 6 lies anterior to the main stalk of the mushroom body, 

 connecting the outer and inner parts of the protocerebral lobes, 

 and on the dorsal surface is connected with other fibers of the mush- 

 room body, although these fibers are not shown in the figure. 

 This tract, which is three sections thick in its central part, is a 

 broad very prominent bundle of fibers, curving from the outer 

 part of the protocerebral lobe up as far as the base of the cellular 

 envelope of the mushroom body, then down again into the inner 

 part of the protocerebral tissue on one side of and dorsal to the 

 central body. In the queen of Camponotus there is nothing to 

 indicate the direction of the fibers, but in the workers of Campo- 

 notus and Formica (figs. 33 and 36), the origin of h can be traced 



