524 CAROLINE BURLING THOMPSON 



correlated with that of the compound eyes, which are much smal- 

 ler than in the queen and male. The ocelli are very minute, biit 

 may be distinguished in sections. 



To summarize: the workers of Camponotus and Lasius have 

 greatly reduced optic lobes, correlated with the smaller com- 

 pound eyes. Formica schaufussi with large compound eyes 

 has large optic lobes, but F. fusca with somewhat reduced com- 

 pound eyes has a reduction in the optic lobes. All these genera 

 have large mushroom bodies, equal or nearly equal to those of 

 the queen, Lasius, larger than those of the queen, Camponotus, 

 Formica. The ocelli are either much reduced, Formica, Lasius, 

 or absent, Camponotus. 



3. The male brain 



Camponotus, Formica, Lasius, figures 3, 6 and 9. The brains 

 of the males of these three genera are so similar that one general 

 description will apply to all. This is evidently the least variable, 

 conservative caste. 



The head of the male is the smallest with usually the largest 

 compound eyes and large ocelh. The brain occupies nearly the 

 whole width of the head and at least half of its interior. The 

 distinguishing characters of the male brain are the very highly 

 developed optic lobes, o.l., stout, and curving down toward the 

 large compound eyes, and the relatively well developed mush- 

 room bodies, m.h. These, although actually smaller than in 

 the other castes, are relatively as large, or nearly as large, in 

 proportion to the size of the heads and to the bulk of the brain. 



THE FINER STRUCTURE OF THE BRAIN 

 L THE BRAIN SHEATH 



The entire brain is surrounded by a delicate sheath composed 

 of small nerve cells placed side by side (fig. 30). These cells 

 are elongated and form a layer appearing like a columnar epithe- 

 lium. The cytoplasm of the upper or outer surface is dense and 

 takes a deeper stain, giving almost the appearance of a cuticle. 

 The proximal, inner, ends are prolonged into delicate fibers 

 which form a network, separating the sheath cells from the inner 

 ganglion cells which they cover. 



