Pendleton, T.A.. Pendleton, D.T. and Dale, M. "The size variation and age ratios found within the larval population of the Glow Worm (Lampyris noctiluca) during Autumn" www.eakringbirds.com; December 2012.

It was anticipated at the outset, that both projects would probably take several years to complete before enough data had been collected and from which any firm conclusions could be derived.

But our Project 2: Phase 1. 2012 research has already yielded some interesting results and we feel that we are already making progress towards finding the answers to some of our questions. This page details our current findings.

Project 2: Phase 1. 2012 was conducted by Trevor and Dilys Pendleton (TDP), based at Market Warsop, Nottinghamshire. We were also assisted by Martin Dale (MD) based in Nottingham, who assisted us with field survey work and also provided important comparison data based on observations of captive larvae. Martin is also co-author and researcher of Project 1: Phase 1. 2012 "Some notes and observations on the bioluminescence exhibited by the larva of the Glow Worm (Lampyris noctiluca) in captivity".

Our original focus for our Project 2: Phase 1. 2012 research, was to collate as much larval size range data from our study colony at Clipstone Old Quarter in Sherwood Forest during the late Summer and Autumn of 2012. But at a fairly early stage, it was decided that the research would be considerably more useful, if it was extended to cover at least one complete year and also researched other aspects of larval activity.

We decided that the primary aims of this research were to try and accurately determine the following aspects of Glow Worm larval activity.

• The number of active larvae present during the latter months of the year (post adult glowing season)
• The period of larval activity during the Autumn and early Winter months
• Peak time for greatest larval activity
• The range of larval sizes present on site
• The usual hunting range of larvae in relation to forest paths and tracks
• Activity of larvae in relation to ground temperature

1.1 ... Recording methodology and data collection

Recording larvae in the wild is time consuming, sometimes with little success for the number of hours spent in the field collecting data. The amount of success was largely dependent on the time of the year, with August and September providing the best results in terms of the amount of larvae recorded and data collected. Temperature also proved to be an important factor and site visits in late Autumn were confined to cloudy or moonless nights wherever possible and ideally when the temperature was 8°C or above. Clear nights with a near full moon provided very limited success.

The process of surveying for larvae was simple, with the observers walking slowly along forest paths and tracks until the bioluminescent glow of a larva was seen. The larva would be located with the aid of torchlight and its length measured, before being returned to the place of capture. Other similar larval measuring techniques adopted by other researchers, have involved measuring the width of the larva rather than length, but we are satisfied that our measuring techniques are accurate after measuring over 450 larvae during the course of four successive Springs and over 150 in Autumn 2012 and that the data is true.

1.2 ... A brief overview of the study colony history

The study colony at Clipstone Old Quarter is situated within Sherwood Forest in Nottinghamshire and although assigned SSSI status, the site does not fall within the Sherwood Forest National Nature Reserve boundary.

We have actively studied the number of adult female Glow Worms at this colony since 2008, but intensified our coverage in 2009 and have not missed a visit during the adult glowing season since. Coupled with our Spring larval surveying, we have accumulated well over 500 evenings worth of larval and adult data, probably making this the most intensely studied and recorded Glow Worm colony in the UK.

Despite being located in the middle of Nottinghamshire, this colony has proved to be one of the earliest UK sites for Glow Worms. We believe that this is down to observer coverage rather than females appearing genuinely early, as the site does not seem to have a particularly warm climate. Another interesting aspect of the site is the consistantly high numbers of female Glow Worms recorded each year (see Table 01) producing a total of 2,278 females between 2009 and 2012 and has yet to produce the occasional poor year often recorded at other UK sites.

Counts of over 200 females in 2008, have recently dropped down to just single females in 2010, 2011 and 2012, while three other nearby colonies are also struggling to survive with very small numbers of females annually.

1.3 ... Assigning ages to larvae

Ageing Glow Worm larvae is not a simple process and unless larvae are reared in captivity from egg to adult, then assigning the correct calendar year (cy) to a larva, is somewhat open to conjecture and recorder interpretation and the real difficulty is created by the fact that some Glow Worm larvae almost certainly reach adulthood within 12-14 months, while others could possibly take as long as three years before reaching the pupation stage.

Our data is based on the assumption that most Glow Worm larvae have more or less a two year period of development, or at least over-winter twice. This would mean that larvae found during both Spring and Autumn 2012, would generally originate from eggs laid by females in 2010 and 2011. Hence throughout 2012, 2010 larvae would be in their second calendar year (2cy) and 2011 larvae would be in their first calendar year (1cy).

Of course there are always exceptions to the rule and we have listed three possible examples of larval life cycles below, based over different lengths of larval development.

The charts in the Spring and Autumn larval size sections, show that we have categorised larvae into three groups or age ranges. These are 1cy, 2cy and intermediate. Intermediate larvae are those larvae whose length measurement places them in a grey area between both calendar year groupings. Effectively these larvae could be either well developed 1cy larvae or poorly developed 2cy larvae. Alternatively, they could be 2cy larvae with a third year of growth to complete before pupation. It is possible that intermediate larvae could make up a large proportion of late emerging females we record during the second half of July.

As the total number of females present at our Sherwood Forest study site has yet to be below 385 females in four years of comprehensive study, the evidence (based on female counts) for a two year larval stage for all larvae, has become indeterminable from the number of adult females recorded since 2009. Because larval development is obviously dependant on food, moisture and temperature, larvae that are successful (or fortunate) in prey location will develop at an increased rate, but if the percentage of larvae reaching adulthood within a year is very small, biennial differences in the number of females have clearly taken many decades to reduce.

1.5 ... The effects of annual rainfall and temperature on larval size and over-wintering success

In Table 02 we have looked at the period of April 1st - 7th of each year, taken the average larval sizes found along one stretch of cycle track (sections A, B, C and D on the map) and compared them with the recorded rainfall between the months of April to August & September to March.

But larvae found in the Spring of subsequent years, will have had to contend with slightly drier conditions for much of their larval life and that may have contributed to the lower average larval length shown since 2009.

But two other factors to consider are food availabilty (which should be as much affected by rainfall as larval growth) and temperature. Wet weather usually means milder temperatures, so we have generated the graph in Figure 01 to show the monthly minimum and maximum daytime temperatures over the same number of years. Again the data is from Waddington, Lincolnshire.

If the average larval size for the period April 1st - 7th 2009 is true, then the mean temperature over Winter could also have had some effect on the over-wintering success of larvae. This could mean that larvae were able to hunt and find food on mild Winter nights. However, the large number of larvae found at Clipstone Old Quarter in Spring 2011, would indicate that larvae are also quite capable of surviving several weeks of almost constant sub-zero temperatures, following the very cold November and December 2010.

This image of the Clipstone Old Quarter survey route above, measures approximately 1,300 metres from north to south (A-D).The two yellow dots marked T1 and T2, indicate the on-site locations of ground temperature measurements, with T2 being the highest of the two points. The central grid reference for the site is SK 60549 67012.

Although Project 2: Phase 1. 2012 officially began in August 2012, we felt it important that the opportunity was taken to collate and publish our larval size data from previous years.

Our most detailed recording of larval size ranges has always come during Spring (late February to early May) when larval activity resumes after Winter dormancy/hibernation and so the first part of this research looks at the size data from Spring 2009, 2010 and 2011, before examining the data collected in Spring 2012.

Surveying for larvae takes place during the early evening, once the afternoon and evening temperatures generally improves to above 10°C and usually takes just over an hour to complete.

Figure 02 shows the larval sizes grouped into four size categories of 12-16mm, 17-21mm, 22-24mm and 25-28mm.

A total of 103 larvae were recorded from sections A, B, C and D. The first of the year were two on 04/04/10, found in sections L (not used in the data calculations) and in D. Larvae were then found regularly from 06/04/10, including notable counts of ten on both 23/04/10 and 25/04/10.

The first adult females appeared on 24/05/10, a few evenings earlier than in 2009.

With the extremely cold period of weather in late November and throughout the whole of December 2010, it was always uncertain how successful larval over-wintering would be.

A good amount of valuable larval size data was collected and an accurate picture of the increases in larval size between March and May is apparent in Tables 12, 13 and 14. Note the increase in intermediate size larvae during April in Table 13. 2011 proved to be exceptional for numbers of Spring larvae, eventually providing a total of 214 larvae.

Notable in 2011 is the high number of 17-21mm range larvae, compared to both previous years.

Very mild weather at the end of February produced two larvae on our first visit on 28/02/12. Hopes were then high, but several further surveys along sections A, B, C and D in early March, all proved negative.

There was a return to peak larvae numbers being more in line with that of 2009 and 2010 (the last week of April) and in many ways, the activity profile is quite similar to that produced in 2009, although the number of larvae is less.

Length measurements for 70 larvae were collected, which includes the two February larvae. Adult females appeared from 30/05/12.

Nationally and historically, little study seems to have been devoted to the activity of Glow Worm larvae after the end of the adult glowing season in early August. Certainly we had very little data ourselves, so even if just for our own reasons, we decided to make a concerted effort to expand our knowledge of larval activity during the Autumn.

Project 2: Phase 1. 2012, was designed to run in conjunction with Project 1: Phase 1. 2012, which is looking more into larval bioluminescence. Despite having gained a great deal of knowledge about the glowing habits of female Glow Worms, we had very little knowledge of the post-adult season. Some aspects we wanted to find more about were larval distribution in relation to forest paths and tracks, their typical hunting range and activity during the latter months of the year. These, coupled with certain aspects of larval bioluminescence and behaviour in the wild, compared to that exhibited by captive larvae, all go to form the basis for Project 1: Phase 1. 2012.

3.1 ... Autumn 2012 larval size data

Since 2009, we had made casual notes and records of any larvae found during surveys for adults at Clipstone Old Quarter and at the start of 2012, we only held size data for 30 larvae out of a total of 43 larvae found in the months June -August since 2009, so concerted research commenced in August 2012. This section details and breaks down the available data.

By the end of December 2012, we had accumulated larval size and age ratio data for 154 larvae out of a total of 180 found on 35 evenings/nights. This at least provides us with a baseline for further research. After several evening surveys, we had already gathered enough data to realise that most larvae fell quite clearly into two distinct groups. These groups were based on larval size ranges of 9-15mm (1cy) and 19-28mm (2cy). As we mentioned earlier, some larvae fall into a size range between the two main groups and are referred to as 'intermediate'.

Table 19 shows the average larval size ranges recorded each month during Autumn 2012.

The averages have been calculated where data is available for at least five larvae, so we were able to include larval size data for July, but have omitted two larvae recorded in November.