Attack of the swimming saucers

Continuing with the theme of true bugs (Hemiptera) in ponds, not all are elongate, sluggish ambush-predators - others actively chase prey and one such species is the common Saucer Bug Ilyocoris cimicoides, the common name coming from its relatively broad, convex shape.

Saucer Bug Ilyocoris cimicoides

Unlike many aquatic bugs, the middle and hind legs are only slightly modified for swimming (there is some flattening, plus clear rows of swimming hairs), and can be used very effectively for walking on land. The front legs are however more strongly adapted, not for swimming but for seeking prey.

Ilyocoris cimicoides - ventral view showing hooked front legs

The hooked legs are used for sifting through sediment and detritus, and for grasping prey which is then pierced by the rostrum - humans should take care when looking through pond-nets as the bite can be very painful and includes toxic saliva causing a sensation equivalent to a bee or wasp sting (at up to 15mm long excluding appendages, it is moderately large insect). The front legs are shown clearly in the side view below, including the broad femur (for strong internal muscle attachments) which have long grooves on the underside which presumably help to grip prey and may form slots where the hooked tibia fits into it when the leg is folded shut.

Although it may be unfamiliar if you've never been pond-dipping, it is widespread though local in distribution and can be common in some waterbodies, especially in the weedy margins of large water-bodies such as marshy dykes, old canals and lakes/large ponds.

Ilyocoris cimicoides - side view showing hooked front legs and slightly modified mid- & hind legs

Note that there is now a second species of saucer bug of the family Naucoridae in Britain - Naucoris maculatus which has been found in England and may well spread being a strong flier (Denton 2007). Happy pond-dipping!

Reference

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. Surrey Wildlife Trust, Woking. [Covers a good range of species; lots of useful info and excellent photos even if you're not in Surrey].