566 CAROLINE B. THOMPSON 



missure' of the hymenoptera and other insects. The lateral 

 portions of the protocerebral lobes are also connected by two 

 dorsal protocerebral comrhissures, the anterior, a delicate strand 

 of fibers, lying above the central body (figs. 15, 16, a.cm.) the 

 posterior, on the same level as the first, lying immediately pos- 

 terior to the central body, fig. 18, 'p.cm. These two commissures 

 are homologous with the two dorsal commissures found in ants, 

 and it is interesting to note that the posterior one, the 'Briicke' 

 of the older authors, has the same characteristic wide inverted 

 n shape. 



In figures 15 to 18, the connection of the optic lobes with the 

 protocerebral lobes may be seen. 



Figures 19 to 20 show the relation between the protocerebral 

 lobes, the deutocerebrum or antennary lobes, and the trito- 

 cerebrum. 



///. The mushroom bodies 



The two mushroom bodies, or the globuli, as Holmgren terms 

 them, are now generally considered the chief centers of intelli- 

 gence in insects. They appear, in frontal views of the head 

 (figs. 8 to 12, m.b.), as two slightly grooved projections from the 

 dorsal surface of the brain, the frontal gland occupying the 

 space between them. Each mushroom body consists typically 

 of two lobes, which in the termites are not distinctly separated 

 but form one continuous mass of tissue, the lobes being indi- 

 cated on the exterior of the brain merely by a slight groove or 

 depression of the surface. In sections (figs. 15 to 18) it may be 

 seen that the mushroom bodies are composed of an outer nerve 

 cell layer and an inner fibrous portion which forms the cups or 

 calyces, the stalks, and the three pairs of roots. 



a. The nerve cells of the mushroom bodies. The nerve cells of 

 the mushroom bodies are all of the same size and are among the 

 smallest cells of the brain (fig. 3, E). They have a round or 

 oval nucleus and so small an enveloping layer of cytoplasm that 

 it cannot be distinguished, even with the immersion lens, except 

 at the distal end where the axon is given off. The chromatin 

 masses of the nucleus are all about the same size, forming a 



