Graphoderus bilineatus (De Geer, 1774)
This generally scarce species occurs very sporadically from eastern France and northern Italy to the southern provinces of Sweden and Finland and east through Ukraine and Russia to western Siberia, it was common in many areas as recently as the 1950’s but due to changes in water quality has suffered a general and drastic decline, it is now considered to be critically endangered in many areas and is protected under the Bern Convention and the EU Habitats directive although new populations have recently been discovered in several areas. The species inclusion in the British list is based on several early twentieth century records from Norfolk and it is now presumed to be extinct. On the continent it occurs in a range of permanent and stagnant water bodies e.g. ponds, drains, fens and peat bogs, usually with dense marginal vegetation and little shade and often in shallower parts of deep ponds. Breeding occurs in the spring and evidence suggests that mating may occur very early in the year, eggs are laid in plant stems out of the water from mid May, or possibly earlier, and larvae develop through the summer; the cycle from egg to adult takes between two and three months and new generation adults appear from mid-summer. Larvae pass through three instars and pupate out of water among marginal moss or debris. There is probably a single generation each year with adults overwintering; they are known to fly but whether they overwinter in water or move to overwintering sites on land is not known. Both adults and larvae are predatory and it is thought that larvae feed on cladocerans in open water or, when these are not available, predate other organisms among submerged vegetation. This species has been the subject of much research and there are many interesting papers outlining its European status etc. for more information and some good pictures see e.g. HERE or HERE.
Adults may be identified from the rather broadly oval shape and distinctively coloured pronotum. 14.0-15.7mm. Body outline broadly oval and widest behind the middle. Head extensively pale with a variable transverse Y-shaped darker marking across the vertex and frons, antennae pale with each segment darkened apically, palps pale with darker apices. Pronotum pale with the posterior and anterior margins dark; the basal dark margin much narrower than the central pale area, the apical dark margin a little wider. Elytral ground colour yellow, extensively and rather finely mottled with interconnected dark brown marks so that the overall colour is often dark brown-they appear much lighter when submerged-the margins more extensively yellow.