THE NERVOUS SYSTEM 147 



appendages. These, however, have become fused into a single more 

 or less compact mass termed the brain. Similarly in the thorax, the 

 three ganglia are usually, and always in the Diptera, fused to one mass. 

 The abdominal ganglia may either remain distinct, or they may fuse 

 with one another and even with the thoracic ganglion. In all cases 

 the two bilateral ganglia of each segment are found to be fused with 

 one another, the double ganglion lying in the middle line. 



The brain, then, consists of six fused ganglia which belong to the head 

 segments and supply the head appendages. As one might expect, 

 the distinction between the several parts is not evident in the highly 

 specialized Diptera, as far as the external appearance is concerned, and 

 we find a rounded mass, generally considerably longer in the transverse 

 diameter than in the antero-posterior, from which are given off nerves 

 to the antennae, tne palps, and other appendages. The lateral portions 

 send out many radiating fibres for the supply of the compound eyes, 

 and are distinguished as the optic lobes, while the middle portion, 

 which may appear to consist of a single mass or two bilateral halves, 

 is known as the central body. But of the six segments in the head 

 three, or two where the third segment is wanting, as it is in the 

 Diptera, are preoral, while the last three are postoral. The mouth 

 lies, therefore, between the anterior and posterior parts of the brain, 

 while the alimentary tract lies dorsal to the nerve cord and ganglia in the 

 rest of the body. The tract has thus to pass through the brain to gain 

 its subsequent position, and its anterior part is consequently encircled 

 by a ring of nervous tissue. The particular part of the tract which pierces 

 the brain differs in the different genera. In Tabanus, as already pointed 

 out, the pharynx is anterior to the brain, which is pierced by the 

 oesophagus, as is also the case in the Muscidae, whereas in the 

 mosquitoes the pharynx is mainly behind the brain ; the portion of the 

 tract which pierces its substance is the chitinous tube connecting the 

 buccal cavity with the pharynx. 



The brain is thus divided into two parts, one which lies above the 

 alimentary tract (strictly speaking, anterior to it), and is known as the 

 supra-oesophageal ganglion, and one which lies below it, and is known as 

 the sub-oesophageal ganglion. From the latter are given off two stout 

 nerve trunks which soon unite with one another and pass backwards to 

 the thorax, where they join the compound thoracic ganglion. From this 

 emerge the nerves which supply the muscles of the thorax, and at its end 

 a nerve trunk which passes to the first abdominal ganglion. Each of the 

 abdominal ganglia gives off a pair of nerves for the supply of the muscles 



