Form-function relationships of the compound eyes and sensory sensilla of a tiny arboreal hemipteran herbivore: Adaptations for close encounters with leaves

Herbivorous insects experience diverse plant stimuli, the relative influence of which depends upon the scale of the interface between both organisms and the insect's life history. Using microCT and SEM, we conducted a whole insect study of the sensory structures of Glycaspis brimblecombei (Hemiptera: Psylloidea; commonly called psyllids or jumping plant lice) to understand this tiny insect's utilisation of the leaves of their tree hosts – especially to reconcile rapid host assessment versus protracted, sinuous searching behaviours. Each compound eye comprises 360 ommatidia of relatively uniform density and facet diameter indicating limited spatial resolution and sensitivity. The areas of highest relative sampling resolution are not directed ventrally towards the surface of leaves but laterally and dorsally. There is a high abundance of chemo- and mechanosensory sensilla on the genal cones (216–240) and fewer on the terminalia (120–150), i.e. body parts regularly in contact with leaf surfaces. There are even fewer such sensilla on the basitarsi (10–16) and only putative olfactory sensilla on the antennae. Leaf surface conformation probably guides females to veins while contact chemoreception likely stimulates probing; the number of eggs deposited is likely determined by the flow and quality of nutrients experienced during feeding. For this psyllid, vision aids movements among leaves and relocation of hosts if dislodged by wind or escaping predators. Walking, as opposed to flying, maintains continuity of exposure to plant stimuli essential to maximising reproductive success. Such a life history is possible on large, evergreen hosts and is facilitated by rapid accept/reject discrimination of ingesta.