Worcestershire Record No. 23 November 2007 p. 50-51

John Clarke – conservation advisor and naturalist
Sue Chandler – conservation volunteer

Glow-worms belong to the family of beetles called Lampyridae which also includes Fire-flies (which do not occur Britain). They get their English name from their extraordinary ability to emit light and from the shape of the female insect, which has a worm-like appearance.

Female Glow-worms cannot fly and rely on producing light to attract males. At dusk the female moves into a prominent position in the vegetation and ‘switches on’ her light by increasing the air/oxygen supply to the last three sections of her abdomen, producing a pale green light. Once they have mated the female ‘switches off’ her light, lays her eggs and dies. The eggs hatch into larvae and develop over a two or three year period before pupating and emerging as adults. Larvae feed on small slugs and snails, attacking them by injecting a digestive fluid that in effect liquefies the victim, before eating it.

Glow-worm habitat is likely to be unimproved grassland along track-sides, railway embankments, field margins, heaths and open scrubland – particularly on chalk and limestone (presumably because their prey - small snails – are more common there).

Because female Glow-worms are flightless it would appear that individual populations couldn’t move further than the larvae might crawl (although the winged males can ensure genetic variation). Together with the fact that the insects are found where there is an ample supply of small molluscs in ‘rough’ or unimproved grassland there appears to be a possibility that the species could be used as a simple method of finding areas that might therefore contain a diverse range of flora. If so, simply assessing the presence/absence/numbers of Glow-worms could be a quick way of covering large areas of potential ‘richer’ habitat using surveyors who need not be botanists.

JC is familiar with all sections of the survey area and indeed Kemerton Conservation Trust (on whose behalf this survey took place) holds records of the flora and fauna found there.

1. To assess presence or absence of Glow-worm along a fixed transect.

2. To assess whether this information can be compared simply with existing knowledge of floristic values within various sections of the transect.

3. To encourage a wider survey of presence or absence of Glow-worm in other areas around Bredon Hill.

4. To contribute to longer-term national surveys of Glow-worm.

SC surveyed along a fixed transect, measuring approximately 1.25 kilometres identified by JC, at approximately weekly intervals – in suitable calm, dry weather conditions. The survey began at dusk when the females had begun to ‘glow’ and continued for approximately 1-1.5 hours. Using a sketch map of the area the observer walked quietly and slowly, looking for small pinpricks of light at or just above ground level – bearing in mind that in thicker vegetation the glow may be visible from only a comparatively small angle. The position and number of insects were marked on the map. No attempt was made to assess whether larvae rather than female adults were emitting paler lights.

Surveys took place between the end of June and mid-October, continuing until two weeks had passed without any sightings. Occasionally, notes were made about weather conditions, position of the insects and behaviour.

The data was later transferred onto a spreadsheet and sections of the transect were identified and marked on a computer-generated map of the area.

For reasons of safety it is advisable not to carry out surveys on your own. SC took along friends and also used the opportunity to train others who might take part in future surveys.

For the first two weeks of the survey the observer did not map the location of Glow-worm lights but merely totalled them. (see spreadsheet)

The highest numbers of Glow-worm sightings occurred on transect sections known to be the richest botanically. The top three Glow-worm sites (Sections 5, 6 and 7) are probably the top three botanical sites respectively. Similarly the lowest numbers of Glow-worm were found on the two sections known to be least rich botanically.

The period for peak numbers appeared to be between 28th June and 30th July although another slight peak occurred on 20th September.

The glowing insects were located low down in fairly long grassy vegetation (40-60 cm high) although a few were occasionally found in the open.

The verges of Westmancote Track Upper West were excavated/modified about 18 months ago and this year is the first time since then that SC has seen glow-worms there.

2007 was the first year that SC had seen Glow-worm on Westmancote Track Lower.

The Glow-worms were usually located strung-out along a section. However on one occasion SC found a concentration of 11 in vegetation adjacent to a barn. A casual observer reported counting approximately 30 lights at the same site – no date provided. Interestingly there appear to be no other records throughout the season for that particular site.

On one occasion SC inspected the source of a glowing light, which was extinguished as she got close. It was found to be a larva eating a snail.

On 20th September 12 of the 14 sightings were of small pinpricks of light and two were described as ‘large glow-worms with 2 bars and 2 dots very visible’ (probably female adult) – one was in the middle of the road and one was in the open.

On 26th September the survey was conducted in full moonlight. This may be the reason why no Glow-worms were found – either their lights was not visible in these conditions or the Glow-worms were not active.

Weather conditions did not appear to be related to numbers of Glow-worms recorded.

This survey will continue in 2008 with SC attempting to distinguish the sources of light as larval or adult.

Information (including Ordnance Survey references) from this year’s survey to be passed to official Glow-worm recorders at county and national level.

Attempts to be made to recruit other volunteers to survey a wider area of Bredon Hill – and perhaps beyond.