Functional aspects of flight in belostomatid bugs (Heteroptera)

Abstract

1. There are four pairs of fibrillar muscles in the mesothorax of the Belostomatidae. The dorsal longitudinal muscles provide power for the downstroke and automatic pronation of the wings. The dorso-ventral muscles provide upstroke power and automatic supination. The oblique dorsal muscles act mainly as wing supinators; they are also important in the wing unlocking process. The fourth pair of fibrillar flight muscles are basalars which act indirectly via an insertion on the pre-episterna; their action is that of an accessory wing depressor and pronator. The only direct flight muscles in the mesothorax are the tonic wing-folding muscles which insert on the third axillary sclerites. There are no fibrillar flight muscles in the metathorax. 2. The pterothorax contains a fused meso- and metathoracic ganglion. The most anterior nerve trunk from this ganglion provides the motor supply to the dorsal longitudinal and oblique dorsal muscles. There are no recurrent nerves between pro- and pterothoracic ganglia, yet some of the motor neurons of the dorsal longitudinal and oblique dorsal muscles are located anterior to the pterothoracic ganglion. This is not true of the motor neurons of any of the other pterothoracic muscles. There are at least three motor units in each oblique dorsal muscle and five or more in each dorsal longitudinal muscle. The anterior nerve trunk of the pterothoracic ganglion also supplies a sensory nerve to the wings and a small nerve which sup­plies the mesothoracic scolopophorous organ which probably monitors the flight rhythm. The second nerve trunk of the pterothoracic ganglion supplies all of the other mesothoracic muscles and sends one nerve to the mesothoracic legs. 3. Wing-beat frequency for a specimen ofL. maximus105 mm long and weighing 23·4 g was 21-25/s at 23-24°C. ForHydrocyrius57 mm long and weighing 2·9 g wing beat was 30/s. ForL. uhleritypical values are 42 mm long, 1·7 g weight and wing-beat frequency of 38/s. 4. The fibrillar muscles all display strong spike activity coincident with wing opening. The wings may be held open indefinitely without flight and fibrillar muscle activity then subsides to a lower level within a few seconds. Once open, the wings may be held open in the absence of any muscle activity. When flight is initiated directly from closed wings a phasic burst of spikes is recorded initially from the fibrillar muscles but this subsides quickly to a lower level characteristic of steady flight. When flight is initiated from open wings and these muscles are already active electrically there is no change in pattern of spike activity signalling start of flight. In steady flight the pattern of spike activity is irregular and bears no temporal rela­tionship to the regular wing beat. The activity of motor units from each muscle of a pair or from different fibrillar muscles also show random temporal relationships.