Figure 1. Structure of the insect complex eye. (a) Composite structure of the eye, which is composed of a large number (up to several thousand) of single hexagonal ommatidia. (b) Structure of the individual ommatidium. Light emanating from a small section of the environment (typical divergence angles of insect ommatidia are in the range of a few degrees) is focused by a lens on the photopigment of sensory cells, and shielded from entering neighbouring ommatidia by a sheath of pigment cells. The photopigment is contained in the membrane of microvilli-armed edges of the sensory cells, which are fused into a central rhabdom. (c) Cross-sections of an ommatidium at the levels indicated. After Wehner R and Gehring W (1995) Zoologie. Stuttgart: Georg Thieme Verlag.

Figure 2. Insect cuticular sensillae. (a) Olfactory hair; (b) mechanosensory hair; (c) campaniform sensilla; (d) scolopidial sensilla. The basic arrangement of cell types is similar in all these sensory structures. One or a few sensory cells extend their dendrites into the cuticular structures (red) which transport the external stimulus to the receptor proteins located in the dendritic membrane. These cuticular structures are specialized with regard to the particular type of stimulus transduction (arrows indicate direction of adequate stimulus for the mechanoreceptors; details in text). The sensory cells are surrounded by sheath cells (green), which secrete the receptor lymph surrounding the sensory dendrites (the receptor lymph area is sealed against the interstitial fluid of the body by tight junctions between sheath and sensory cells). The sensory cells extend their axons to the central nervous system (not shown). After Wehner R and Gehring W (1995) Zoologie. Stuttgart: Georg Thieme Verlag.

Figure 3. Circuit diagram of the pathway that processes mechanosensory signals from a leg of the locust. Large circles represent neurons, lines indicate major neurites or axons. Triangles represent excitatory synapses, and small filled circles indicate inhibitory synapses. After Burrows (1996).

Figure 4. Role of proprioceptive signals in leg motor control during (a) postural control and (b) locomotion. The diagram is based on the control system of the femur–tibia (knee) joint in the stick insect; however, it also represents the situation in other locomotor systems, for instance higher vertebrates.

(a) In the posture control mode (i.e. in the quiescent animal), resistance reflexes stabilize limb and body posture. Sensory input which signals knee flexion (indicated by a blue arrow) elicits activity in knee extensor muscles (indicated by a red arrow; recording trace labelled ‘extensor force’). At the same time, flexor activity is reduced (recording trace labelled ‘flexor force’). This motor output thus resists an imposed flexion of the joint, ensuring postural stability.

(b) In the movement control mode (i.e. during walking), the same sensory signals are employed for the control of joint movement. Sensory input that signals knee flexion (blue arrow) in this case elicits activity in flexor muscles (red arrow), while extensor activity is reduced. This assistance reflex thus reinforces the ongoing leg movement. The term ‘reflex reversal’ signifies this change in reflex response under different behavioural conditions.