466 



COMPARATIVE ANATOMY 



CHAr. 



nervous system, consisting of the brain, oesophageal commissures, infra-resophageal 

 ganglion, 3 ganglia of the 3 thoracic segments and 8 (in Oampodea 7) ganglia of the 

 abdomen (Fig. 324). The finer structure of the last and largest abdominal ganglion 

 and the number of nerves proceeding from it show that it consists of 3 fused ganglia. 

 The number of abdominal ganglia would according to this be 10, corresponding with 

 the number of segments. In the CoUemiola the number of abdominal segments is 

 reduced, and in accordance with this reduction of the body there is, as it appears, 

 ■only 1 abdominal ganglion. Smiiithurus is said to have only 1 thoracic ganglion. 

 The 2 longitudinal commissures remain distinctly separate in the Thysanura. 

 From each ganghon 2 nerves are given off on each side, and the same number proceed 

 from the oesophageal commissures. In front of the infra-cesophageal ganglion and 

 behind the oesophagus a transverse commissure connects the o3Sophageal commissures. 

 Pterygota. — The nervous system of the winged Inseda shows very great 

 variety in its arrangement : it is impossible here to go into details — the Diptera 



FIG. 325.— ^-B, Tlie nervous systems of 4 species of Diptera, to deinonstrate their various 

 degrees of concentration. A^ Non-concentrated nervous system of Chrlonomus plumosus, with 3 

 thoracic and 6 abdominal ganglionic masses. J5, Nervous system of Empis stercorea, -with 2 thoracic 

 and 5 abdominal ganglionic masses. C, Nervous system of Tabanus bovlnus, with one thoracic 

 ganglionic mass and the abdominal ganglia moved towards each other. Z>, Nervous system of 

 Sarcopliaga camaria. All the ganglia of the ventral chord except the infra-oasophageal ganglion, 

 which always remains separate, are here united into one single tlioracic ganglion mass (after E. 

 Brand). 



(Fig. 325, A-D) are particularly instructive. In no other natural order of insects 

 are the extremes so great, and yet connected by such numerous intermediate stages. 

 The series begins with the suborder of the Nemocera, the Ciilicidai, Culiciformes, 

 Tijiididcc, Fungicolce [e.g. Chironomus, A), which have very slightly concentrated 

 nervous systems. The ventral chord here consists of an infra-cesophageal ganglion, 

 3 thoracic ganglia, and 5-6 abdominal ganglia. The last thoracic ganglion is not 

 simple, but at least 1 of the anterior abdominal ganglia is fused with it. The last 

 and largest abdominal ganglion is also not simple ; it consists of several (in Chirono- 

 mus probably 2) fused ganglia. The concentration of the nervous system among the 



