628 



ZOOLOGY 



elaborate system of vessels being greatly diminished by the way in 

 which all the tissues and organs are supplied with oxygen through 

 the system of tracheae. The blood is colourless or faintly yellowish 

 or greenish, and contains colourless corpuscles. A contractile 

 dorsal vessel or heart (Fig. 533) extends through the abdomen and 

 sometimes the thorax immediately below the terga. Its cavity is 

 divided internally into a series of chambers by a system of valves. 

 In its walls are a series of slits or ostia, by which a communication 

 is effected between the internal cavity and a surrounding pericardial 

 sinus. Alary muscles, fan-shaped bundles of fibres, arise from the 

 terga and are in part inserted into the heart, causing or assisting 

 in causing its dilatation and the opening of the ostia. In front the 

 heart gives origin to a main vessel, or aorta (a). 



FIG. 532. Thorax and anterior abdominal segments 

 of a larval Ephemerid with tracheal gills. 

 HF, hind-wings ; tki, tkz, tk 3 , tracheal gills ; tl, 

 longitudinal tracheal trunks ; VF, fore-wings. 

 (From Lang's Comparative Anatomy.) 



FIG. 533. Heart of Cock- 

 chafer (Melolontha). a, 

 aorta ; m, m, alary muscles. 

 (From Gegenbaur.) 



The nervous system (Figs. 531 and 534) is on the same general 

 plan as in the Crustacea. There is a double supra-cesophageal 

 ganglion or brain, a sub-cesophageal ganglion, also double, and a series 

 of thoracic and abdominal pairs of ganglia, which are closely united 

 together in the middle line. The brain is relatively large in the 

 higher Insects, and is divided into several lobes. It gives off nerves 

 to the antennae, the ocelli and the labrum, and on each side arises 

 a large lobe the optic ganglion on which the compound eye 

 rests. A pair of cesophageal connectives pass backwards on either 

 side of the mouth from the brain to the sub-cesophageal ganglion. 

 These connectives are usually very short, and, as a consequence, the 

 brain and sub-oesophageal ganglia are closely approximated. From 

 the latter there originate nerves to the appendages of the mouth the 

 mandibles and the two pairs of maxillae. There are sometimes three 



