ORIGIN OF CASTES OF TERMITES 121 



highly differentiated and the increase in the size of the worker- 

 soldier mushroom bodies is very marked. Practically no differ- 

 ence is to be noted in the compound eyes of the worker-soldier 

 type since the last phase, but those of the reproductive castes 

 are growing in size and complexity. The sex organs are growing 

 larger in both types, markedly in the reproductive castes, and 

 slightly in the worker-soldier type. 



Phase 6. Nymphs with twelve antennary segments, the third 

 entire and either bare or hairy; body length 2 mm. (figs. 16, 17). 

 The large head of the worker-soldier type is now recognizable 

 without a lens. Nearly half of the space within it is occupied 

 by the large radiating bundles of the mandibular muscles. The 

 chitinous bars of the tentorium are stouter, the cuticula covering 

 the head and body is much thicker, and the mandibles and 

 maxillae have a heavy yellow margin. In order to obtain con- 

 secutive sections of individuals of this age, the specimens must 

 be kept longer (three to four hours) in paraffin. 



The differences between the heads of the two types (figs. 16, 

 17), are now very marked, the reproductive castes being plainly 

 'small headed' and the worker-soldier type 'large headed.' The 

 brain of the reproductive castes is larger and more highly differ- 

 entiated than in the preceding phases, but the brain of the worker- 

 soldier type, although it has increased in general bulk and in 

 the size of the mushroom bodies, is still inferior to that of the 

 reproductive castes (fig. 2 c), and shows the narrow pointed optic 

 lobes which characterize the adult workers and soldiers. The 

 space within the head of the worker-soldier type is filled chiefly 

 by the large and stout mandibular muscles, which are larger 

 than in the reproductive castes. The chitinous cuticula is 

 thicker on the outside of the head and body, and within the 

 head in the tentorial bars. 



Except for a very slight increase in diameter, the compound 

 eyes of the worker-soldier type of 2 mm. in length (fig. 23), 

 are almost as simple as at the time of hatching. The eye con- 

 tains more cells and these are arranged in two ill defined layers, 

 but no differentiation into large and small cells and no grouping 



