50 WILLIAM PATTEN. 



of the larval ganglion, there is rapid multiplication of cells, 

 but it is the middle lobe which increases most rapidly, and 

 which forms the main part of the adult ganglion; traces of the 

 other two bands may be seen even in the adult (see my paper on 

 the '^ Eyes of Vespa"). Now it is a remarkable fact that we 

 find these three identical bands of the pupse ofAcilius in the 

 embryos of Vespa, but they are there formed by a 

 direct invagination of the ectoderm of the cephalic 

 lobes. Either the same kind of an invagination or a solid in- 

 growth is found in a variety of other forms, as in the Hymen- 

 optera,Orthoptera, and Hemiptera — that is, in forms that do 

 not pass through a free ocellate larval stage. This 

 proves that the Acilius type of cephalic lobes is the most 

 primitive, and that in such forms as the Hymenoptera and 

 Orthoptera, &c., important embryological processes are omitted; 

 it also shows that the optic ganglion of the convex eye of 

 insects is formed by the fusion of the three larval ganglia. 



As to whether the three frontal ocelli found in many adult 

 insects are derived from the larval ocelli, or are new for- 

 mations, like the compound eye, is a question of great 

 morphological importance, but we have as yet no evidence upon 

 which to found an opinion concerning them. Their position 

 and number suggest their identity with the three-lobed median 

 eye of Limulus and Crustaceans. Careful investigation of 

 some forms, the Sialidse, for example, which pass through 

 an ocellate larval stage and possess frontal ocelli in the adult, 

 would probably settle this question. It is certain that early 

 in the pupal stage of Acilius and Cecropia all the larval ocelli 

 break away from the ectoderm and take up their position on 

 the under side of the optic ganglion, where they seem to 

 undergo complete degeneration. 



II. The Brain of Arachnids. 



A. The cephalic lobes of Arachnids have at first the 

 same shape and appearance as in insects. They soon divide 

 into three segments, which can be identified with even greater 



