graphic

We tend to think of predators as moving quickly and striking hard. But for many animals, moving quickly and sticking fast is a preferred tactic. Rove beetles (Stenus spp.) are a good example of this. They have evolved mouthparts (labia) tipped with a set of sticky pads. When prey wander by, the beetles protrude their labia and reel in a meal stuck to their ‘lips’. Rove beetles feed on a wide array of invertebrates, so their prey-capture device must presumably be able to stick to surfaces with a broad range of topologies and moisture levels. What is it about rove beetle lips that allows them to stick to varying surfaces? Lars Koerner, Stanislav Gorb and Oliver Betz at the University of Tubingen and Christian-Albrecht University, Germany, explored this question in a pair of recent papers, one published in the Journal of Insect Physiology, the other published in the journal Zoology.

First, the team used a combination of force measurements and high-speed video to characterize predatory strikes in two species of rove beetle. In the lab, they lured the animals into launching strikes at insect pins with attached force sensors. They found that in both species, the force of adhesion was much higher than the force of impact. When the team watched high-speed videos of strikes, they could also see that the process is a very wet affair. The sticky pads of both species are slathered in a layer of low viscosity mucus and each strike leaves behind a film of slime. These results show that the prey-capture device is optimized for adhesion and that mucus secretions are involved.

At the same time, the team tested how well beetle labia stick to different types of surfaces. They coaxed beetles into launching strikes at hydrophobic and hydrophilic surfaces and at surfaces with varying levels of roughness. Neither hydrophobicity nor surface roughness influenced the attachment ability of the prey-capture apparatus. This shows that rove beetle lips are indeed well adapted to stick to a diverse array of different surfaces.

Koerner and colleagues then examined labial morphology using electron microscopy. In both species, the labia are tipped with a pair of oval pads. Each pad in turn is composed of hundreds of tiny outgrowths with terminal ramifications (think little ropes with frayed ends). All of these outgrowths are deeply immersed in mucus. This arrangement combines features of dry adhesive systems (multiple contact points, branched morphology) with wet ones (widespread secretions), and probably explains why beetle lips are so good at sticking to different types of surfaces.

The team did a very careful and thorough job of describing both the functional properties and morphology of the prey-capture device. And although they didn’t test the idea directly, they provide very strong circumstantial evidence that a mix of features common to dry and wet adhesive systems is what gives rove beetle lips their ‘universal stickiness’. Overall, this work is important because it highlights the value of studying species that have evolved unique prey-capture devices. It also shows how much we can learn as biologists by measuring how small animals do seemingly simple things like stick to stuff.

References

Koerner
L.
,
Gorb
S. N.
,
Betz
O.
(
2012
).
Adhesive performance of the stick–capture apparatus of rove beetles of the genus Stenus (Coleoptera, Staphylinidae) toward various surfaces
.
J. Insect Physiol.
58
,
155
163
.
Koerner
L.
,
Gorb
S. N.
,
Betz
O.
(
2012
).
Functional morphology and adhesive performance of the stick-capture apparatus of the rove beetles Stenus spp. (Coleoptera, Staphylinidae)
.
Zoology
doi: 10.1016/j.zool.2011.09.006