MECHANORECEPTION 69 



of Stable equilibrium is altered they are still able to orient with respect 

 to gravity (Oevermann, 1936). No special localized static organ has 

 been found; however, in Notonecta, Plea, Naucoris, and Corixa the 

 Johnston's organ is employed in conjunction with a bubble of air to 

 mediate position sense (Rabe, 1953). Notonecta and Plea swim ventral 

 side uppermost. When they are properly oriented an air bubble 

 trapped between the antennae and the surface of the head causes by its 

 buoyancy the antennae to be deflected away from the head. If the 

 insect is turned upside down the antennae are then deflected towards 

 the head. In each instance the deflexion activates the Johnston's organ. 

 If the bubble is removed, Notonecta placed in darkness will swim dorsal 

 side up (that is, reversed) because the antennae by their own weight 

 lean away from the head. For Corixa, which swims dorsal side up, the 

 mechanism is the same but acts in reverse (Rabe, 1953). 



Lack of a speciahzed static organ is characteristic of many aquatic 

 species, and it appears that much assistance is derived from extero- 

 ceptors. BHnded insects, while not losing their ability to retain their 

 normal attitude and position with respect to gravity entirely, do tend 

 to show locomotory aberrations (Oevermann, 1936; Tonner, 1938; 

 Hughes, 1958; Dingle, 1961). Compensatory movements to rotation 

 cease when eyes are blackened, and some species show a tendency to 

 swim in spirals when unilaterally bhnded. In larvae o^ Acilius sulcatus 

 and Dytiscus marginalis, which possess twelve simple eyes (stemmata), 

 Schone (1950) has shown that the structure and arrangement of these 

 eyes are especially suitable to mediate optical orientation in space by 

 sensing the distribution of brightness in the visual space. In rotations 

 around the longitudinal axis a displacement of the ratio of excitation 

 between the right and left groups of eyes occurs ; in rotations around 

 the transverse axis a change in the quotient front : back forms the 

 basis of regulation. 



Exteroceptive information concerning water movement is received 

 via the antennae. It is assumed that Johnston's organ is involved. A 

 swimming Dytiscus holds its antennae at a slight angle to the median 

 line. Any deviation from the axis of swimming causes the antennae to 

 bend (Hughes, 1958). Some proprioceptive information is necessary 

 in any case. Giant water bugs possess hair plates on the trochanters at 

 the coxo-trochanteral joints which are important for the co-ordination 

 of swimming (Dingle, 1961). When the legs are flexed, as when grasping 

 a stem or when flying, the hair plates are stimulated by a covering fold 

 of cuticle. When the legs float freely in the water in an extended 

 position the hair plates are no longer stimulated by the fold, and 



