Preliminary Defi?iitions and Illustrations. 5 



heard instinctive activities defined as those on which you 

 could safely bet; but, if the element of chance is that 

 which gives zest to the wager, one must add that such 

 betting would be dull work. You might almost as well 

 lay a wager that the sun will rise to-morrow morning, as 

 bet, for example, that the larva of the great water-beetle 

 (Hydrophilus piceus) will, as the period of pupation draws 

 nigh when the insect is soon to pass into the chrysalis 

 stage, leave the pool and bury itself in the damp earth, 

 so constant, under the appropriate circumstances, is the 

 performance of an instinctive activity. 



It need hardly be said that there is the closest possible 

 connection between the structure and organization of any 

 given animal and its instinctive activities. The spinnerets 

 of the spider are associated with its web-making instinct ; 

 the activities of flight are rendered possible by the 

 possession of wings ; the burrowing habits of the mole 

 are correlated with many peculiarities and adaptations of 

 structure. The specific way in which any essential life- 

 function — say, for example, that of respiration — is per- 

 formed, and the activities which are rendered necessary for 

 its performance, have been developed in direct associa- 

 tion with the organic mechanism which is subservient to 

 the process and the activities in question. In insects 

 respiration is effected by means of a delicate system of 

 ramifying tubes (the tracheal system) which open to the 

 outer air by orifices or spiracles on either side both of 

 the middle or thoracic and of the posterior or abdominal 

 region. Of the two large aquatic beetles that are found 

 in our English ponds, each must come from time to time 

 to the surface to renew its supply of air ; * but the manner 

 in which they take in and store the air is quite different. 

 Dytiscus floats slowly to the surface with its hinder end 



* See Professor MialPs "Natural History of Aquatic Insects" (1895). 



