Nervous System of Insects. 119 



Body. The head is joined to the thorax by a nar- 

 row neck, and this region is generally strong, and the 

 limbs are attached to the under part of the side of 

 each of its three rings. Each limb is composed of 

 five joints : hip (coxa ), a ring segment (trochanter), thigh 

 (femur, fig. 63, k], a shin (tibia], and a tarsus of seve- 

 ral joints ending in the claws to which sucking cushions 

 or pads may be appended. The wings are jointed to 

 the middle and hinder rings of the thorax ; these are 

 modified lateral flaps of the body wall, such as exist in 

 some crustaceans ; the thin skin folds of which they 

 consist are supported by chitinous ribs (costa) con- 

 taining branches of the tracheae. 



Internal Structure. On the sides of each abdo- 

 minal ring are the apertures of the long, finely branch- 

 ing tracheae, which sink into the body and are distri- 

 buted widely among the tissues. Each tube has a 

 membranous wall strengthened by a coiled spiral 

 chitinous thread which keeps it open for conveying 

 air from the surface through the body. Each motion 

 of the body by altering the tension of the vessels pro- 

 motes this method of respiration. The dorsal tubular 

 heart placed in the abdomen, consists of a chain of 

 chambers separated the one from the other by valves. 

 This receives the impure blood and the new blood 

 from the intestines, and propels it by the chief blood- 

 vessels into lacunae or interspaces between the tissues 

 which are thus nourished. The blood is colourless, 

 or green, rarely red. 



Insects have two large and complex nerve ganglia 

 in the head, and ganglia in all the segments from the 

 head backwards. The head ganglia send branches 



