152 



ENTOMOLOGY 



200, ri) which have been derived by direct proliferation from 

 those of the germ band, and are therefore ectodermal in origin. 

 This primitive double nerve cord becomes constricted at inter- 

 vals into segments, or neuromeres, which correspond to the 

 segments of the germ band. Each neuromere consists of a 



FIG. 196. 



fir' 



pair of primitive ganglia, and these 

 are connected together by paired 

 nerve cords, which later may or 

 may not unite into single cords; 

 moreover, some of the ganglia 

 finally unite to form compound 

 ganglia, such as the brain and the 

 subcesophageal ganglion. In front 

 of the oesophagus (Fig. 55) are 

 three neuromeres : (i) proto * cere- 

 brum, which is to bear the com- 

 pound eyes; (2) deuto cerebrum, or 

 antennal neuromere; (3) tritocere- 

 brum, which belongs to the seg- 

 ment which bears the rudimentary 

 intercalary appendages spoken of 

 above. Behind the oesophagus are, 

 at most, four neuromeres, namely 

 and in order, mandibular ', super- 

 lingual (found only in Collembola 

 as yet), maxillary and labial. 

 Then follow the three thoracic gan- 

 glia and ten (usually) abdominal 



Embryo of CEcanthus, ventral 

 aspect. a, antenna; al-a?, ab- 

 dominal appendages; e, end of 

 abdomen; /, labrum; li, left 



fundament of labium; ip, labial ganglia. The first three neuro- 



meres always unite together to 

 form the brain, and the next four 

 (always three; but four in Col- 

 lembola and perhaps other insects), to form the subceso- 

 phageal ganglion. Compound ganglia are frequently formed 

 also in the thorax and abdomen by the union of primitive 

 ganglia. 



palpus; P-l s , thoracic legs; m, 

 mandible; mp, maxillary palpus; 

 mx, maxilla; p, procephalic lobe; 

 pr, proctodaeum. After AYERS. 



