An insect larva with a ‘pig-snout’: structure and function of the nasale of Hyphydrus ovatus L. (1763) (Coleoptera: Dytiscidae)

Abstract

Hyphydrus ovatus beetles inhabit lakes, ponds and slow running streams. In central Europe the larvae develop at water temperatures of >10°C from May to July. All three instars live on or close to plants. The first instar prefers the upper areas of plants. Its prey are insect larvae and small stages of planktonic crustaceans. The second instar chooses deeper areas and catches larger prey, whereas the third instar prefers the root zone and feeds mainly on tubificids and chironomids that are caught in the detritus of the bottom layer. The elongated nasale of Hyphydrus ovatus is equipped with different sensilla. Despite the rather different shape of their outer parts, their ultrastructure indicates that they all have a mechanoreceptive function. Chaetiform sensilla are especially numerous and are especially long near the tip. Plate-like mechanosensilla are positioned in a half circle at the ventral edge of the rostral tip. The assumed function of these sensilla is the location of prey. As in other Hydroporinae, the axis connecting the dorsal and ventral points of articulation of the mandibles is sloped laterally. Thereby, the closing mandibles move upwards and press the prey against the underside of the nasale, which – like the upper jaw in vertebrates – serves as the counterpart of the mandibles. On its ventral side, the nasale is equipped with various cuticular teeth that serve to hold the prey when it is pressed against them by the mandibles.