504 The Outline of Science 



new areas and to migrate to fresh localities in times of stress. 

 Their bodies are extremely well adapted from the mechanical 

 point of view ; their sense-organs are highly developed sensitive 

 feelers, compound eyes, and so on and their mouth-parts are 

 remarkably adapted to suit different modes of feeding. Probably 

 much of their success in the struggle for existence is due to the 

 adaptations of their circulatory and respiratory systems, which 

 enable the nutrition of the organs of the body to go on with great 

 rapidity. The tissues are continually bathed in nutritive fluid, 

 while every part of the body is kept aerated by the extensive 

 system of air-tubes. These facts account for the abundant energy 

 and consequent activity which is so characteristic of the class. 

 It may be doubted if the insect's blood ever becomes impure. 

 Another factor tending towards success is the change of habit 

 due to the change of form which occurs during the course of 

 many life-histories. This implies changes in diet, and therefore 

 a lessening of the drain on any particular foodstuff. In other 

 ways, also, the changes of form and habit may lead to survival 

 in the struggle for life, for there is frequently a tiding over of 

 difficult times ; for instance, quiescence during periods when con- 

 ditions of temperature and food are unfavourable. Many insects 

 pass the winter in a lethargic state inside well-protected cocoons. 



Protective Adaptations 



Another factor which helps to give success to insects in 

 maintaining their hold in various habitats is the way in which 

 general form and colour are adapted to the environment. Pro- 

 tective colouring in animals has formed the subject of a special 

 article, but it may be noted that there are no clearer instances of 

 protective resemblance than among insects. Not only do they 

 very often closely resemble the general colour of their natural 

 surroundings, but form, as well as colour, may add still more 

 to this similarity, which gives security to the insect by concealing 

 it effectively from its enemies. We can thoroughly understand 



