DEVELOPMENT 



135 



the oesophagus (Fig. 57) are three neuromeres : (i) protocerebrum, which 

 is to bear the compound eyes; (2) deutocerebrum, or antennal neuromere; 

 (3) tritocerebrum, which belongs to the segment which bears the rudi- 

 mentary intercalary appendages spoken of above. Behind the oesopha- 

 gus are, at most, four neuromeres, namely and in order, mandibular, 

 superlingual (found only in Collembola as yet), maxillary and labial. 



FIG. 202. Diagrammatic transverse sections to illustrate formation of dorsal wall in 

 the beetle, Leptinotarsa. a, amnion (breaking up in C); g, germ band; s, serosa. After 

 WHEELER, from the Journal of Morphology. 



Then follow the three thoracic ganglia and ten or eleven abdominal 

 ganglia. The first three neuromeres always unite to form the brain, 

 and the next four (always three; but four in Collembola and perhaps 

 other insects), to form the subcesophageal ganglion. Compound 

 ganglia are frequently formed also in the thorax and abdomen by the 

 union of primitive ganglia. 



Tracheae. The tracheae begin as paired invaginations of the ecto^ 

 derm (Fig. 204, /) ; these simple pockets elongate and unite to form the 

 main lateral trunks, from 

 which arise the countless 

 branches of the tracheal 

 system. 



Mesoderm. From the 

 inner layer which was derived 



from the germ band by gas- FIG. 203. Transverse section of germ layers 



, . /T ^. x of Clytra. c, coelom sac; n, neuro blasts (primi- 



trulatlOn (FlgS. 192-194) are tive nervous cells). After LECAILLON. 



formed the important germ 



layers known as mesoderm and entoderm. Most of the layer becomes 

 mesoderm, and thiiT~splits on either side into chambers, or ccelom 

 sacs (Fig7203, c), a pair to each segment. In Orthoptera these 

 ccelom sacs are large and extend into the embryonic appendages,_but in 

 Coleoptera, Lepidoptera and Hymenoptera they are small. These sacs 

 may share in the formation of the definite body-cavity, though the last 

 arises independently, from spaces that form between the yolk and the 



