522 CAROLINE BURLING THOMPSON 



separated from each other and from the median line. The optic 

 lobes, O.I., are the most highly developed part of this brain, and 

 extend far out in each lateral direction. These two points of 

 structure, namely, the slight development of the mushroom bodies 

 and the lateral extension of the optic lobes, give this brain a 

 flattened, laterally drawn out appearance which is very charac- 

 teristic. The compound eyes and the ocelli are very large, the 

 former slightly smaller, the latter larger than those of the male. 



b. Formica queen: figure 4- The brain occupies more than 

 one-half the width of the head, and is nearer to the apex than in 

 Camponotus. The optic lobes, o.L, are large, but are relatively 

 less extended in the lateral direction and more extended in the 

 dorso-ventral direction. The mushroom bodies, m.b., are not 

 highly developed, although they project farther on the dorsal 

 surface and extend nearer to one another. Both compound eyes 

 and ocelli are smaller than those of the male Formica (fig. 6). 



c. Lasius queen: figure 7. The brain occupies more than three- 

 fourths of the width of the head. Taking the mushroom bodies 

 and the optic lobes as a standard, this brain is more highly de- 

 veloped than that of any other queen or of any other caste. By 

 measurement the mushroom bodies of the Lasius queen are 

 larger in proportion to the total size of the brain than those of 

 Camponotus or Formica. By comparing figures 1, 4 and 7 it 

 will be noted that these bodies extend farther in a dorsal direction 

 and nearer to one another in this genus. Furthermore, the mush- 

 room bodies of the Lasius queen are almost equal in size, if not 

 actually equal, to those of the Lasius worker (fig. 8). Measured 

 from side to side the queen mushroom bodies are wider than 

 those of the worker, measured in the dorso-ventral direction the 

 worker has a slight advantage over the queen. The optic lobes, 

 o.L, have the same lateral extension as in Camponotus, but the 

 dorso-ventral thickness is less than in Formica. The compound 

 eyes are larger than those of the male, the ocelli smaller. 



To summarize: in these three genera the queen has the largest 

 head and the largest brain. The brain is usually extended in 

 the lateral direction owing to the size and the lateral position of 

 the optic lobes. The mushroom bodies may be equal, or nearly 



