550 CAROLINE BURLING THOMPSON 



SUMMARY 

 I. STRUCTURE 



1. The mushroom bodies 



a. Cells. Four kinds of cells, the cells of Groups I, II, III, IV, 

 are present in the mushroom bodies arranged in four zones, but 

 seen in section in seven groups. These cells differ in size and ar- 

 rangement. Group I, in the center of the cup, consists of the 

 largest cells; Group II, forming the apex of each lobe, consists of 

 smaller cells in radial rows; Groups III and IV form the sides of 

 the cup and are composed respectively of small and very small cells. 

 Group I is found in worker ants as well as in queens and males. 



h. Fiber tracts. The fibers from Groups I, II, and some from 

 III, can be traced into the stalks and form the chief efferent fiber 

 tracts of the mushroom bodies. 



The fibers are combined into bundles or tracts that have a 

 definite and constant arrangement. Some tracts are present in 

 all castes of the three genera, some are confined to the queen 

 caste, and others vary in occurrence. The queen brain has more 

 fiber tracts than the other castes, and the queen of Lasius more 

 than either Formica or Camponotus. 



c. Roots. The anterior roots of the mushroom bodies end as 

 described by other writers: on the anterior surface of the pro- 

 tocerebral lobes. The stalks or 'inner roots' of the mushroom 

 bodies do not end abruptly beneath the central body as hereto- 

 fore described, but each stalk divides there into two bundles or 

 roots, making two dorsal and two ventral masses. The fibers of 

 the two dorsal bundles enter the central body, and may be termed 

 the central body roots, the two ventral bundles, which are the 

 posterior roots of the mushroom bodies, continue backward and 

 enter the posterior part of the protocerebral lobes on their dorsal 

 surface. These posterior roots of the mushroom bodies are 

 identical with the structures described by other writers as the 

 "tubercles of the central body" and the "ocellar glomeruli," neither 

 of which terms can be correctly applied to the three ant genera 

 in question. 



