24 INSECT TRANSFORMATION 



might appropriately be termed kidney tubes, for their function 

 is to withdraw from the blood nitrogenous waste-matters 

 that are the end-products of the series of changes due to the 

 continual breaking-down of the insect's living tissues, whereof 

 the highly complex nitrogen-containing proteins are the 

 characteristic ingredients. 



The workings of the various sets of organs in the body are 

 correlated by means of the nervous system (Fig. 13), the cells 

 of which can be affected by outside influences and can pass on 

 the " nerve-impulses " that they receive so as to evoke suit- 

 able responsive action by the muscles or other tissues. Nerve- 

 cells are situated in special nerve-centres (or ganglia), which in 

 insects are usually small, white, rounded or oval bodies, and 

 these are connected with one another by nerves or nerve-cords, 

 which are essentially bundles of nerve-fibres, that is to say fine, 

 thread-like outgrowths of the nerve-cells ; these fibres, passing 

 from centre to centre, put the nerve-cells in touch with each 

 other. 



If the digestive tube of the grasshopper be carefully removed 

 to one side, a great part of the insect's central nervous system 

 will be exposed. Just above the ventral body- wall, a pair of 

 ganglia may be seen lying joined together in the central line in 

 each segment from the first thoracic to the sixth abdominal 

 inclusive, this series (Fig. 13, i, ii, iii, i 6) of nerve-centres being 

 linked up by a pair of longitudinal nerve-cords that run side 

 by side through the body, while from each ganglion finer nerve- 

 cords run out to muscles and organs belonging to its proper 

 segment. For example, the muscles of the three pairs of legs 

 receive nerves from the three sets of thoracic ganglia respec- 

 tively, the forewings from the mesothoracic, the hindwings 

 from the metathoracic ganglion. We notice that the hindmost 

 abdominal ganglia (Fig. 13, 6) are larger than the others, and 

 that the four or five hinder segments are innervated from them ; 

 hence we infer that these ganglia are complex, built up of 

 several nerve-centres fused together. The system is evidently 

 constructed on the plan of a pair of ganglia for each segment, 

 with the possibility of the ganglia of several adjacent seg- 

 ments becoming united. 



This possibility is realized in a high degree in^the insect's 

 head. From the prothoracic ganglia the main paired nerve- 



