THE FOOD OF THE OTHER LAND ANIMALS 



We have seen that every part of a plant, and vegetable 

 and animal matter in every form, is utilized as food by insects, 

 and that all insects themselves serve as food for other insects, 

 spiders, and related types, as well as for various parasitic 



plants. 



Insects alone would dominate the world, consuming all the 

 surplus that could be spared by plants, were it not that they 

 are restricted in their sphere of action by three main con- 

 siderations. Their great muscular activity necessitates a 

 constant and a large supply of oxygen without which they 

 would soon become inactive and eventually perish, as well as 

 abundant food of a relatively high nutritive value, and their 

 external skeleton and method of breathing by slow diffusion 

 of oxygen wholly or chiefly through minute rigid tubes im- 

 poses upon them a relatively small maximum size. 



There is room on the land, therefore, for other animals with 

 a more perfect system of respiration and an internal skeleton, 

 admitting of a much larger size and greater activity; for less 

 active animals with a less consumption of oxygen; and for less 

 active animals capable of existing on food with less nutritive 

 value. The animals with a more perfect system of respiration 

 and with an internal skeleton are the vertebrates — the mam- 

 mals, birds, reptiles and amphibians; those with a less oxygen 

 consumption are the land planarians, nematodes or thread- 

 worms, most slugs and snails, and the land nemerteans; and 

 those capable of existing on food with a minimum of nutri- 

 ment are the earth-worms and some snails. 



The land leeches, land crustaceans, and onychophores would 

 seem to be direct competitors of the insects which have met 

 with limited success. The vertebrates, with a structure emi- 

 nently fitted for terrestrial life and which allows of a very 



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