Clearwing moths are fairly closely related to the more familiar burnet moths. There are about 1,000 species worldwide, of which 14 are resident in the British Isles, and one is of uncertain status. They mimic wasps, which is thought to confer protection from vertebrate predators, but they can in most cases be distinguished from wasps by the dark bar or blotch across the forewings, from which numerous veins eradiate. Also, the head and eyes are much smaller. They have very narrow forewings and quite narrow hindwings, both with large transparent areas, and black bodies banded with yellow or red. Species in the genera Sesia, Bembecia and Pyropteron hold their wings quite close to the body when settled. Synanthedon species settle with their wings extending at a greater angle from the body.
The adults are active by day, particularly in sunny weather, but they are very elusive and until recently those of many species were rarely encountered in the wild. They may be found when freshly emerged by searching the trunks of the larval foodplants from early to mid-morning, but soon disperse when warmed by the sun. Some visit flowers and are occasionally caught when sweep-nets or malaise-traps are used for general invertebrate sampling. The males fly around the larval foodplants, seeking out unmated females. Recently, the use of lures containing synthetic sex pheromones came into use asd a recording technique. This has greatly improved the speed of detection and some species are proving to be more widespread and frequent than previously indicated.
Locating the actual breeding sites requires patience, persistence, and specialised, yet simple techniques. Eggs are usually laid singly, often on freshly cut tree-stumps and on callouses and other damaged bark, or on the leaves of herbaceous foodplants. The larvae feed in stems, trunks or roots, boring out tunnels which usually bear traces of silk, distinguishing them from those of other plant-boring insects.
Some betray their presence by issuing brown sawdust-like droppings (frass) from holes in the plant, or by causing a swelling (gall). The life cycle may take up to three years for some species. The pupa is formed in a fibrous cocoon just under the outer layer of the occupied trunk or root, behind a lidded exit hole prepared by the larva, which can be found by careful searching or scraping, and the section containing the insect carefully cut, chiselled or dug out for rearing to adult. In species feeding below ground, a silken tube may be formed leading to the surface of the earth, Breeding sites are best located by searching for empty pupal cases, which are left protruding from exit holes in stems and trunks at the start of the flight season and may remain in place for some weeks.