Habit and Instinct. 



at a slightly higher level than its head, and this end 

 breaks the surface film and rises into the air. The last 

 pair of spiracles, or breathing orifices, near the hinder 

 end of the body, are large and take in a direct supply, 

 while air is also stored in the space between the wing- 

 cases and the body, and is thus supplied to the smaller 

 abdominal spiracles. In the great water-beetle, Hydro- 

 philus, however, the front pair of abdominal spiracles are 

 the largest, and the air is stored not only beneath the wing- 

 cases, but along the hairy ventral surface of the beetle. 

 When the insect comes up to breathe, it is the head 

 and not the hinder end that reaches the surface ; the 

 feeler, or antenna, is specially modified so as to effect 

 a communication between the whole storage area and the 

 atmosphere ; and the air is taken in at the junction of 

 head and thorax. Thus the activities which minister to 

 the process of respiration are seen in each case to have 

 a close dependence on the general structural organization 

 of the insect. And throughout the whole animal kingdom 

 we have abundant illustration, not only of the intimate 

 connection between the structure of a given organ and 

 its particular function, but also between such specific 

 performance and the activities which lead up to it. In 

 other words, not only does an organ respond functionally 

 under the appropriate conditions, but the whole organism 

 co-operates by carrying out a sometimes complicated set 

 of activities subservient to what we see to be the end 

 in view. 



This leads us on to consider the relation which an 

 instinctive activity bears to what is termed a reflex action. 

 If the foot of a sleeping child be lightly tickled, the foot 

 and limb will be withdrawn from the source of irritation. 

 This is a case of reflex action. The effects of the stimulus 

 (tickling) are carried inwards by nerves to the spinal cord ; 



