Zonation, behaviour and morphology of the intertidal coral-treader Hermatobates (Hemiptera: Hermatobatidae) in the south-west Pacific

The coral-treaders (Hermatobatidae) are the only exclusively marine family of insects. Observations in the Cook Islands, Samoa, Tonga and Fiji show that the insects can be abundant on shores that provide rocks with cavities that remain air-filled during high tide. The zone of suitable habitats for the insects is restricted to a narrow (c. 20 cm) vertical band roughly centred around Mean Low Water Neaps. Transplant experiments show that the insects are unable to survive above or below this zone. The insects emerge from their rocky crevices about an hour before low tide, swim on the water surface during low tide, and return to their crevices about an hour after the tide has turned. There are only four larval instars, which can be readily separated by their morphology. Electron microscope studies show that the insects are clothed in a layer of micro-hairs of unique design: a short tapering shaft tipped with a sphere. These micro-hairs may act as a plastron: their distribution corresponds precisely to the pattern of the silvery ‘air-layer’ seen when the insects are submerged, their spherical tips provide a regular array that could support an air-water interface, they occur at high densities (up to 3.5 times 10⁶ mm^-2), and the layer makes connection with the thoracic spiracles. Half the insects were able to survive submergence in recirculated seawater for 12 hours. The adults appear to swim further from shore than the larvae, and they were observed copulating in tidal pools.