NO. 3 INSECT HEAD SNODGRASS QI 



4. The protocephalon carried the labrum, the mouth, the eyes, the 

 preantennae, and the antennae, also the postantennae when it included 

 the third body segment. 



5. During the protocephalic stage of insects, as shown by the em- 

 bryo, the thorax was differentiated as a locomotor center of the body, 

 and the region between the head and the thorax, consisting of the 

 fourth, fifth, and sixth body segments, became a distinct gnathal 

 region. 



6. The gnathal region was eventually added to the protocephalon to 

 form the definitve head, or telocephalon. In the Crustacea, in which 

 there was no thoracic region corresponding with that of the insects, 

 the gnathal region was not definitely limited posteriorly, and the 

 definitive head in this group may include as many as five segments 

 following the protocephalon. In some of the crustaceans the gnathal 

 segments have united with segments following to form a gnatho- 

 thorax, leaving the protocephalon as a separate anterior head piece. 

 In the Arachnida the protocephalon included the prostomium and two 

 postoral metameres, and it has combined with the following six seg- 

 ments to form the cephalothorax. 



7. In the definitive insect head, the prostomium, according to some 

 embryologists, contril)utes the clypeus and frons and the region of 

 the compound eyes ; according to others it forms the clypeus and 

 frons only. The labrum is a median preoral lobe of the prostomium. 



8. The arthropod brain probably always includes the median pro- 

 stomial ganglion, combined with the ganglia of the preantennal segment 

 to form the protocerebral lobes. It may still be questioned whether 

 the optic lobes are derived from the prostomium or from the prean- 

 tennal segment. The ganglia of the antennal segment form the deuto- 

 cerebrum. The commissures of the protocerebrum and the deutocere- 

 Ijrum are formed above the stomodeum, and unite with the archicere- 

 bral rudiment to form the median part of the brain. The ganglia of 

 the postantennal segment, when united to the preceding ganglia, 

 become the tritocerebral brain lobes, but they, remain separate from 

 the brain in some crustaceans, and their uniting commissure always 

 preserves its sub-stomodeal position. 



9. The prostomial region of the adult arthropod is innervated from 

 the postantennal ganglia, but this is probably a secondary condition 

 owing to the loss of the true prostomial nerves. 



10. The appendages of the definitive insect and myriapod head are 

 the preantennae, the antennae, the postantennae, the mandibles, the 

 first maxillae, and the second maxillae. Rudimentary, evanescent 

 preantennae have been reported only in the embryo of Scolopcndra 



