74 THE PHYSIOLOGY OF INSECT SENSES 



air spaces ; hence, if the organs are to give meaningful information 

 regarding external pressure changes the internal air pressure must 

 be kept constant. Thorpe and Crisp (1947) have suggested that large 

 tracheal air sacs near these organs damp internal pressure changes. A 

 different interpretation of the function of these air sacs has already 

 been discussed (Larsen, 1955). 



Comparable, though simpler, so-called static organs have been 

 described in Ranatra, Lethocerus, and Belostoma, but little is known of 

 their physiology (MoUer, 1921). 



On the surface of water insects do not encounter the same sensory 

 problems as underwater, but, because of the great speed of swimming 

 that is possible, obstacle avoidance is a problem. Some imaginative 

 experiments by Eggers (1936 b, 1927) with the whirligig beetles Gyrinus 

 marinus and G. natator suggest that the antennae play a prominent 

 role. Beetles on the surface of water in a small container (diam. 35 cm.) 

 are able to avoid collisions with one another and with the walls of the 

 container. If the walls of the container are coated with paraffin so that 

 the meniscus is convex instead of concave or if the surface of the water 

 is carefully cleaned of all dust particles the frequency of collisions 

 against the walls increases markedly. When swimming beneath the 

 surface of the water the beetles also hit the wall. Neither vision, nor 

 detection of waves bouncing from the walls, nor air pressure produced 

 by the beetle moving towards the walls appear to be involved in 

 obstacle avoidance. Eggers postulated that detection of the meniscus 

 and detection of the resistance of the surface dust layer as it is com- 

 pressed between the beetles and the wall informs the beetle of the 

 proximity of the obstacle. It was presumed that the antennae, and 

 possibly their Johnston's organs, are the sensing elements involved. 

 Ordinarily the Johnston's organ is not highly developed in Coleoptera 

 (as compared with Diptera), but in Gyrinus it is fairly complex, and the 

 antennae have pecuhar structural modifications which, according to 

 Eggers, permit the pedicel to glide on the water surface while the 

 flagellum sticks up into the air. The pedicel would be very sensitive to 

 irregularities (menisci) in the water surface and to the resistance of 

 floating particles such as dust; when moved, it would cause counter- 

 movements of the flagella, with a consequent stimulation of John- 

 ston's organ. Amputation of the antennae results in an increased 

 number of collisions. 



