NO. 3 



INSECT HEAI 



-SNODGRASS 



writers hold that the crustacean eye-stalks are secondary outgrowths ; 

 and, as for their innervation from the protocerehral lobes, it might 

 be claimed that the roots of the oculo-motor nerves come from a part 

 of the protocerebrum derived from the prostomial archicerebrum. A 

 definite opinion on these matters must await the results of further re- 

 search. Since, however, in the Annelida, the prostomium is the seat 

 of primary sensory development, and of the principal sense organs 



AntNv 

 ^- MdNv 



LbNv 



Fir,. 15. — Evolution of the insect brain as it must be conceived if it includes 

 an archicerebral rudiment, and ;/ the compound eyes and the optic lobes are 

 derived from the prostomial region, as claimed by Heymons. 



A, theoretical generalized condition in which the ganglia of the prostoinium 

 (Pst), preantennal segment (/), antennal segment (//), and postantennal 

 segment (///) are yet distinct, and in which the prostomial archicerebrum (Arc) 

 is the brain. 



B, the prostomium and the first three postoral segments united into a proto- 

 cephalon ; the brain composed of protocerehral lobes (iBr) formed of the 

 archicerebrum (Air) and ganglia of preantennal segment (/), and of deuto- 

 cerebral lobes (jBr) representing ganglia of antennal segment ( // ) ; ganglia of 

 postantennal segment (///) distinct and connected by postoral commissure. This 

 condition retained in some lower crustaceans. 



C, the definitive condition in all insects : the ganglia of postantennal segment 

 (III) are added to the brain to form the tritocerebral lobes (sBr) of the 

 latter; the ganglia of the gnathal segments (/F, V, VI) united in a compound 

 suboesophageal ganglion (SocGnc;). 



Aiit, antenna; AntNv, antennal nerve; Arc, archicerebrum; iBr, protocere- 

 brum; 2Br, deutocerebrum ; sBr. tritocerebrum ; 3C0111. tritocerebral com- 

 missure ; E, compound eye ; LbNv, labial nerve ; Md. mandible ; MdNv, mandib- 

 ular nerve ; iMx, first maxilla ; 2Mx, second maxilla ; MxNv, maxillary nerve ; 

 O, ocellus, OpL, optic lobe; Put, postantennal appendage; Pre, protocephalon ; 

 Pst, prostomium; SocGng. suboesophageal ganglion; Stom. stomodeum. 



(fig. 10), it is at least in harmony with the assttmed annelid ancestry 

 of the Arthropoda to suppose that the arthropod eyes had their origin 

 on the prostomial region of the head, and that their definitive posterior, 

 dorsal location has resulted from the backward revolution of the 

 anterior part of the head, a transformation that actually takes place 

 in the growth of the embryo. 



We may conclude, without going farther into matters of contro- 

 versy, that the immediate ancestors of the arthropods possessed a 



