518 CAROLINE BURLING THOMPSON 



THE ANT BRAIN 

 1. HISTORICAL ACCOUNT 



The older writers conceived of the insect brain as consisting of 

 six parts, corresponding to the six embryonic segments of the 

 head: I, the acron; II, the antennary segment; III, the inter- 

 calary segment, all preoral, and constituting the supraesophageal 

 part of the brain; IV, V, VI, the mandibular, maxillary and labial 

 segments, postoral, and forming the subesophageal mass. Owing 

 to the tendency toward a fusion of parts and to the changes in 

 position that have occurred the homologies of some of these seg- 

 ments are difficult to determine, and wide differences of opinion 

 have resulted,, but the later researches, especially those of Janet 

 ('05) and Jonescu ('09) tend to establish the older view on a » 

 firmer basis. 



Viallanes ('86) described the insect brain as consisting of the 

 following parts: Segment I, the protocerebrum, including the 

 protocerebral lobes, optic lobes, mushroom bodies, and ocelli; 

 segment II, the deutocerebrum, consisting of the antennary lobes; 

 segment III, the tritocerebrum, the nerves of the labrum. The 

 tritocerebral nerve of Viallanes was therefore the labral nerve. 

 Segments IV, V and VI formed the subesophageal part of the 

 brain. 



Haller ('04) believed in a very different homology of the head 

 region, based on a comparison of the insect, with the myriapod 

 head, but his view seems quite untenable. 



Janet ('05) although advocating the division of the brain into 

 six parts, differed from Viallanes in believing, first, that the labro- 

 frontal nerve, which now arises posterior to the antennary nerves, 

 should be considered as belonging to the protocerebrum and as 

 arising originally in a more anterior position; second, that the 

 tritocerebral nerve is represented by the nerve which supplies 

 the interior dilator muscle of the pharynx in Lasius niger. This 

 is a small unpaired nerve, running beneath the pharynx and aris- 

 ing from two roots posterior to the lateral nerves. The very 

 small lobes from which the paired roots arise represent all that 

 is left of the tritocerebrum or third head segment. 



