39 



dition of insects gives them a great freedom of movement, and we 

 may safely assume that the need of a greater freedom of movement 

 has led to their segmentation. The crawling movement of an 

 unsegmented worm-like body would tend to lateral movement, and 

 such lateral strain, acting intermittently or alternately from side to 

 side, would tend to keep certain parts of the body flexible, whilst 

 the more prominent portions to which the muscles were attached 

 would require to be specially protected from injury. This necessity 

 has been met by deposits of chitin, and thus has probably arisen 

 the separation of the more prominent indurated portions of the 

 body-wall from the flexible intersegmental areas, and, as a result, 

 segmentation, as we understand it, has probably taken place. 



We have already stated that the overwhelming superiority in 

 numbers of insects over all other terrestrial animals is probably due 

 to the fact that their wings enable them to escape from many 

 animals that would otherwise prey on them. True, many insects 

 have no wings at all, and others only have them in their adult or 

 imaginal state, yet the possession of wings is the important structural 

 feature by which insects differ from all other Arthropoda, and it is 

 well known among entomologists that it is in the wingless larval and 

 pupal stages that the greatest amount of destruction of insect life 

 takes place. 



The wings of insects are thin, broad, leaf-like folds of the integu- 

 ment, which are, in reality, outgrowths of the lateral parts of the 

 mesothorax and metathorax respectively. They are moved by 

 powerful muscles, which occupy the greater part of the thoracic 

 cavity. The size of the mesothorax and metathorax depends largely 

 upon the size of the wings they carry. In certain Orthoptera the 

 hind wings are larger than the front pair, and we then find the meta- 

 thorax larger than the mesothorax. In the Odonata, or dragon-flies, 

 in which there is little difference in the size of the two pairs of 

 wings, there is little difference in size between the meso- and meta- 

 thorax, whilst in the Diptera, which have only rudimentary hind 

 wings (called halteres), the metathorax is correspondingly decreased 

 in size. 



The wings of insects are simple, very thin, chitinous lamellae, 

 consisting of an upper and lower layer united round the edges, so 

 that in reality each wing forms a closed sac. B : etween these two 

 thin layers is a fine network of hollow chitinous tubes called nervures. 

 These latter are somewhat complicated in structure, each consisting 

 of a central hollow tracheal vessel encircled by a blood-vessel. The 

 latter in turn is surrounded by the hypodermis of the wing, upon 

 which the outer chitinous wall (consisting of two layers) is spread. 

 The structure of the wings of insects themselves still leaves much 

 room for original study. 



I will now invite your attention for a few r minutes to the way in 

 which the wings of insects are formed, and for this purpose we 

 will consider them in relation to the Diptera and Lepidoptera, in 



