(i26 



ZOOLOGY 



parts 



number of chambers ; its walls are perforated by a series of pairs 

 of valvular apertures or ostia. Running from the wall of the heart 

 to the terga are a series of segmentally-arranged fan-shaped 

 bundles of muscles — the alary muscles (Fig. 522, m.). 



Respiration takes place through the instrumentality of a 

 system of air-tubes or trarlicw (Figs. 497 and 498), opening on the 

 surface at the stigmata, to which reference has already been made. 

 These tracheas form a richly ramifying system extending to all 

 of the body. They possess a chitinous internal lining, 



supported by means of a spirally- 

 wound, fibre-like thickening. By 

 means of this system of air-tubes 

 air is conveyed throughout the 

 body to all parts, and there is 

 thus ensured the rapid and com- 

 plete oxygenation which the 

 functional activity of the Insect 

 requires. 



The nervous system consists 

 of a brain (Fig. 495, Irn., and 

 499, hr.), a sui-cesophageal pair 

 of ganglia {infr. gang.), three 

 thoracic (Fig. 499, thor. 1, 2, and 3), 

 and six ahdominal pairs of ganglia 

 (the members of each pair being 

 united), a system of connectives 

 uniting the ganglia together, and 

 a series of nerves given off to the 

 various parts of the body. The 

 brain consists of a bilobed mass 

 of nerve-matter situated in the 

 head, and divisible into two parts, 

 anterior and posterior. From the 

 anterior part is given off on each 

 side the optic nerve passing to 

 the eye to become expanded into an optic ganglion, and from 

 the posterior part the nerves to the antennas. It is supported by 

 a chitinous framewoi-k— the tentorium. From the brain there run 

 backwards a pair of o}sophageal connectives {conn.), passing, one on 

 each side of the oesophagus, downwards and backwards to the sub- 

 cesophageal ganglia. The latter, which are situated between the 

 submentum and oe.sDphagus, give off a pair of connectives, passing 

 backwards to the first thoracic ganglia. From the sub-cesophageal 

 ganglia are given off the nerves to the labi'um, the mandibles 

 and both pairs of maxillas. The three pairs of thoracic and six of 

 abdominal ganglia are connected together into a chain by a series 

 of double connectives ; the last pair of abdominal ganglia, situated 



Fig. 4fi7.— Portion of a trachea of a Cater- 

 pillar. B, C, D, branches ; a, cellular 

 layer ; b, nuclei. (From Gcgenbaur.) 



