124 



ENTOMOLOGY 



as the brain and the subcesophageal ganglion. In front of the oesophagus 

 (Fig. 55) are three neuromeres: (i) protocerebrum, which is to bear the 

 compound eyes; (2) deutocerebrum, or antennal neuromere; (3) tritocere- 

 brum, which belongs to the segment which bears the rudimentary inter- 

 calary appendages spoken of above. Behind the oesophagus are, at 

 most, four neuromeres, namely and in order, mandibular, superlingual 



FIG. 200. Diagrammatic transverse sections to illustrate formation of dorsal wall in 

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

 WHEELER, from the Journal of Morphology. 



(found only in Collembola as yet) , maxillary and labial. Then follow the 

 three thoracic ganglia and ten (usually) 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. 202, /); 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 gastrulation (Figs. 



190-192) are formed the impor- 



tant 



FIG. 201. Transverse section of germ layers 



(primi - 



derm and entoderm. Most of the 



layer becomes mesoderm, and this splits on either side into chambers, 

 or ccelom sacs (Fig. 201, 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 



