THE ARTHROPODA 105 



of post-oral ganglia supplies the first maxillae, but the nerves 

 going to the second maxillae have no visible ganglion correspond- 

 ing to them : they seem to be given off from the connectives 

 between the third and fourth pairs. The fourth pair of post-oral 

 ganglia supplies the first thoracic limbs, and for every succeed- 

 ing pair of limbs there is a pair of ganglia united by two trans- 

 verse commissures and joined to the pairs before and behind it 

 by longitudinal connectives. There are no ganglia behind the 

 last pair of limbs, thus the nervous system, though segmentally 

 arranged, does not correspond with the annulations of the body 

 but with the limbs. 



Turning now to the nerves supplying the antennae, we find 

 that the first pair of antennary nerves is given off from the 

 pericesophageal connectives about half-way between the cerebral 

 and first post-oral ganglia. The fibres of the nerves can be 

 traced forward in the pericesophageal connectives as far as the 

 cerebral ganglion where they end in two groups of ganglion 

 cells situated on the external posterior margins of the brain. 

 These groups of ganglion cells are quite distinct and separate 

 from the larger mass from which the optic nerves originate and 

 appear to represent the ganglia of the first antennae. There 

 is reason to believe that the antennae were originally post-oral 

 appendages which have been shifted to a pre-oral position, and 

 that their ganglia have been still further shifted forward till they 

 have coalesced with the prestomial ganglion, the two forming the 

 compound cerebral ganglion or syncerebrum of the adult Apus. 



The second pair of antennae, as we shall see, are post-oral in 

 the larval Apus, and are shifted forward in the course of de- 

 velopment. Their nerves spring from the perioesophageal 

 connectives just in front of the stomatogastric or first post-oral 

 ganglia, and originate from a little lateral group of nerve 

 ganglion cells on either side, which undoubtedly represent the 

 ganglia of the second antennas. ' The nerves of the second 

 maxillae, though they seem to spring from the ventral nerve 

 cords, are found to originate from two little groups of ganglion 

 cells imbedded in the substance of the 'cords, these groups 

 representing the rudimentary ganglia of a rudimentary pair of 

 appendages. 



It has recently been stated that some of the northern species 

 of Apus viz. A. glacialis and A. spitzbergensis are herma- 

 phrodite, but the writer has not been able to find any evidence 



