Thursday, 29 June 2006

Shieldbugs raise a stink

Many readers will be familiar with the shieldbugs, a group of large and distinctive insects encountered during early summer and autumn. Most shieldbugs can be identified on sight but, with only a handful of the largest and most common species having English names, we know rather less about their distribution and status than we might. Adult shieldbugs will often remain still when approached, only flying off with a loud buzz of wings as a last resort. Such bold behaviour, unusual in an insect, comes from an ability to produce a pungent smell from strategically positioned stink glands.

In shieldbugs, the forewings are hardened in their front half and membranous in their back half. This characteristic feature gives rise to the name Heteroptera (different wings), used to describe these and a number of related insects. The forewings overlap to form a diagonal cross at the front of which a shield-shaped triangle is formed, which gives these bugs their name. A second characteristic feature is the four-jointed beak, known as a rostrum, with which the bugs can ingest fluids (either from plants or other insects). When not in use the beak is folded away underneath the body. Most shieldbugs spend the winter as adults, emerging in spring to mate and lay eggs. Mating is a somewhat protracted affair, with male and female paired end to end for up to three and a half days. The eggs are laid in clusters and, in all but one species, left alone to develop. The barrel-shaped eggs change colour as they mature and as hatching approaches you can see the eyes through the shell. A temporary projection at the back of the head allows the young nymph to push its way out of the shell and into the wider world. In one species, the female sits over her eggs and newly hatched nymphs to protect them from predators. This behaviour has been known about for more than 200 years and gives the species its name ­– parent bug. 

Other shieldbugs with English names tend to be named after the plant species upon which their young feed. Hence the hawthorn shieldbug is most commonly associated with hawthorn, the birch shieldbug with birch and the gorse shieldbug with gorse. Named after its appearance rather than its choice of foodplant, the green shieldbug is probably the most familiar species, though it is not our only green coloured shieldbug. This species can sometimes be found in gardens on ornamental shrubs or runner beans. As a group, shieldbugs are well worth study. Large, approachable and easy to identify, ideally with the help of Roger Hawkins’ Shieldbugs of Surrey (published by Surrey Wildlife Trust), they are well worth the effort.

No comments:

Post a Comment