402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



form a loop beneath the a3Sophagus (fig. 18 B, 3 Con), and the secon»l 

 nerve mass of the head also lies below the oesophagus, for which 

 reason it is known as the suboesophageal ganglion (figs. 18 B, 21, 

 SoeGng). The pair of ganglia in each body segment of the adult 

 condenses into one segmental ganglion. These segmental ganglia 

 may all remain separate, but usually some of them draAv together and 

 unite, though in different combinations in different insects. The 



>ThGna 



lAbGn^ 



Fig. is. — Diagrams showing the development of the central nervous 

 system of an ' insect 



A, the nervous system at a stage when it consists of paired seg- 

 mental ganglia {Gng) united by lengthwise commissures {Com) between 

 segments, and by crosswise connectives {Con) in each segment, with 

 lateral optic ganglia (OpGnu) in addition to median ganglia in flrst 

 head segment. (Other symbols as on Figure 14.) 



B, the mature nervous system, consisting of a brain {Dv) formed of 

 first three pairs of segmental ganglia and the optic ganglia (A), a 

 suboesophageal ganglion {SwGng) formed of the three pairs of raouth 

 part ganglia (A), and of a series of double ganglia in the Iwdy seg- 

 ments, the first three {ThGna) in the thorax, the rest In the abdomen. 

 The cross connective of the third brain ganglia (SCon) lies behind the 

 cesophagus {(E), because the stomodeum (tig. 20 B, 8toin) penetrates 

 between the second and third pairs of head ganglia. 



first three ganglia following the head belong to the thorax (fig. 18 B, 

 ThGnu)^ and the rest to the abdomen. There are seldom more than 

 eight separate ganglia in the abdomen, since the terminal ones unite 

 into a compound ganglion. 



A creature, now, with its nervous system buried within its body, 

 must find itself again in need of means of communication between 

 the nerve centers and the outlying parts of its system. To supply a 

 l^art of this need, nerves grow out from the ganglion cells to the 



