The antennal setal trap of the Ground beetle Loricera pilicornis: a specialization for feeding on Collembola

Abstract

Loricera pilicornis Fabricius 1775 has long strong setae on its proximal antennal segments which are used to form a trap for capturing the collembolans on which it feeds. The structure of this prey‐capture apparatus was investigated with the scanning electron microscope and its function with mechanical experiments using a microbalance. Long setae which do not belong to the trap were included for comparison.The positions of the setae on the antennal segments and their shapes change from proximal to distal along the antennae. Typical trap setae are found on the four proximal antennal segments. They are characterized by their extraordinary strength and by the shape of their sockets, which always have their highest point on the outside of the trap.The restoring forces for single setae were measured by bending the setae against the highest point of the socket rim. The resistance to bending of the trap setae is approximately 10 times that of similar setae which are not part of the trap. The trap setae were bent by up to 60° with their resistance increasing linearly, whereas the other setae reached a range at about40–50° in which the resistance increased more sharply; no seta was damaged by such bending. The behaviour of the trap setae is discussed with respect to their function in hunting and prey‐capture.To calculate the escape chance of a medium‐sized springtail, specimens of Heteromurus nitidus (Arthropleona) were pulled upwards out of the trap while measuring the holding force, which amounted to at least79–99 × 10‐5, depending on the position of the antennae and of the prey between them. The setae were bent up to50–90° without damage during these experiments.By calculating the kinetic energy of a jumping Heteromurus nitidus, we show that this species is unable to escape from the setal trap, whereas another species, Sminthurus viridis (Symphypleona), develops enough kinetic energy to bend the setae up to 109°, which is enough to open the trap.