Nervous System of Insects. i \ 9 



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 tracheal tube 

 has a membranous wall strengthened by a coiled 

 spiral chitinous thread which keeps it open for con- 

 veying air from the surface through the body. Each 

 motion of the body by altering the tension of the 

 vessels promotes 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 

 together with the new blood from the intestines, and 

 propels it by the chief blood-vessels into lacunae or 

 interspaces between the tissues which are thus nour- 

 ished. The blood is colourless, or green, rarely red. 



Insects have a large and complex nerve ganglion 

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

 head backwards. The head ganglion sends branches 



