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Harvestmen, of the class Arachnida, order Phalangida, include common species to be found during late summer and autumn months. Eleven species of these animals have been recorded on the island of Inchcailloch in the Loch Lomond National Nature Reserve (Curtis, 1973). Following completion of sampling and revision of species determination, the seasonal activity of these species is considered here, based on pitfall captures at four woodland sites during the two years August 1971 to August 1973, as described by Curtis (1973). The number of animals falling into the traps is dependent on both population density and the activity of individual animals; this number is, therefore, a useful indicator of their effective significance in the ecosystem to which they belong.
In considering seasonal activity, the following data are relevant: the numbers of animals taken, the time of year when most active and the degree to which this activity is restricted to a short time interval, i.e. stenochrone, or dispersed through the year, i.e. eurychrone. As an indication of this third aspect, in this paper an index for each species is calculated as the reciprocal of the sum of squared proportions (pi) of annual total captured in each month (i.e. l/Σ pi2). The value of this index can range from 1.0 for species totally occurring in only one month, i.e. completely stenochrone, to 12.0 for species occurring in equal proportions in all twelve months of the year, i.e. completely eurychrone. These data are considered below for a total of 11785 adults and sub-adults of the eleven species caught on Inchcailloch.
Nomenclature adopted here is that used by Sankey and Savory (1974). Where this differs from that used in the earlier paper (Curtis, 1973) the name previously used is given in brackets.
Oligolophus meadii Cambridge (= Odiellus meadei (Cambridge)) was taken mainly in November, within its normal seasonal range. Mitostoma chrysomelas (Hermann) (= Nemastoma chrysomelas (Hermann)) and the very infrequent Megabunus diadema (Fabricius) occurred at scattered intervals through the year, whilst Platybunus triangularis (Herbst) showed its typical pattern of nymphs taken in the winter and adults in summer months.
The most numerous species was Nemastoma bimaculatum (Fabricius) with 3270 captured in the first year (seasonality index = 5.00) and 5456 in the second year (index of 4.15). Peak time of capture was July-November and the seasonality indices for both years show restriction of activity to this time of year, in contrast to the continuous activity through the year indicated by Sankey and Savory (1974). In spite of the rather low seasonality index values, this species is still somewhat more eurychrone than the other abundant species. Captures of N. bimaculatum were noticeably higher in the second year of sampling than in the first (see Figures 1 and 2).
The next most abundant species, though far less numerous than
N. bimaculatum, was Oligolophus (Odiellus) palpinalis (Herbst) (= Odiellus palpinalis (Herbst)), with 617 captured in the first year and 580 in the second. Although the numbers in each year are similar, this species was much more stenochrone in 1972-73 with an index of 1.85, comparing with 3.17 in 1971-72. In both
years the peak activity in October-December is slightly later than the July-November period indicated by Sankey and Savory (1974).
Oligolophus tridens (C.L. Koch) was much more numerous (771 captures) and stenochrone (index of 2.18) in 1971-72 than in 1972-73 (288; 4.64), although in both years it occurred during its expected period of activity from August to November. This species, in fact, was taken at all four sites, not restricted to sites 1 and 2 as erroneously described by Curtis (1973). Somewhat less frequent was Lacinius ephippiatus (C.L. Koch) with 190 captures in 1971-72 (seasonality index of 2.98) and only 82 in 1972-73 (index of 3.99), a pattern similar to the previous species and also agreeing with its expected time of occurrence. Oligolophus agrestis (Meade) showed similar patterns in both years with two annual totals of 129 and 115 and corresponding indices of 3.66 and 4.15 respectively for 1971-72 and 1972-73; the observed activity was slightly later than the usual period for this species. The least numerous of these abundant species was Mitopus morio (Fabricius) which showed an increase in both numbers and eurychronicity from 1971-72 (62; 3.81) to 1972-73 (106; 5.41), occurring at the expected active period from August to December, but also during April-July in 1973.
N. bimaculatum occurs most frequently when the climate temperature is steady above the annual mean and its numbers decline with falling temperature. L. ephippiatus increases in
abundance whilst temperature is rising from annual mean and declines when tenperatures are maximal, whereas M. morio activity increases with rising temperature and then diminishes with falling temperatures. The latter two species are thus essentially summer species. The remaining three species could be regarded as autumn species: 0. tridens increasing while temperature is near maximal levels and decreasing when temperature drops to near minimal; or winter species, i.e. 0. palpinalis occurring when temperature falls to annual mean levels and 0. agrestis which was taken mostly while temperatures were largely below the annual mean.
In comparing the two years, there is a similar pattern of succession shown by the harvestmen. However, in 1972-73, this seems to start somewhat later; for example in 0. palpinalis, M. morio, notably 0. agrestis and to some extent 0. tridens and N. bimaculatum. In general, the activity peaks for these species in 1972-73 are about 1-2 months later than in 1971-72. This may be related to differences in climatic conditions between the two years. These climatic conditions are, however, modified by the habitat structure, vegetation etc., in which the harvestmen live. Thus, for example, in both year's sampling later peaks are seen at
station 3 than at the other sites for all species (apart from
L. ephippiatus which is absent from this site). This might be explained by the nature of the ground vegetation cover which at this site includes many mosses, Calluna and Vaccinium; during the winter months these persistent vegetative structures could
provide some degree of shelter and insulation, allowing later
activity of the phalangids.
The picture presented here is a somewhat simplified consideration
of the effect of climate on invertebrate activity. More thorough
analysis should include factors other than temperature, such as humidity, precipitation, wind speed and direction. Consideration
of temperature as a meteorological factor can also include aspects
other than gross fluctuations. These other aspects, e.g. occurrence
of warm, sunny days, can have a direct bearing on the activity
of harvestmen and spiders in quite complex ways as indicated by
Pearson and White (1964). It is hoped to examine these aspects.
References
CURTIS, D.J. (1973). Spiders and phalangids of Inchcailloch,
Loch Lomond. I. General considerations. Western Nat. 2: 29-39.
PEARSON, R.G. and WHITE, E. (1964). Factors contributing to the
annual cycles of surface-active arthropods in moorland country.
Ent. Mon. Mag., 100: 201-206.
SANKEY, J.H.P. and SAVORY, T.H. (1974). British Harvestmen.
Synopses of the British Fauna. No. 4: Linnean Soc., London.Frequent Species
The remaining six species were taken in sufficient numbers to
allow examination of seasonality and comparison of the two 12-month
periods August 1971-1972 and August 1972-1973.
Explanation of Figures
These data for the frequent species are summarised in Figures 1 and 2, which also indicate the variation in temperature over each respective period 1971-72 and 1972-73. The maximum, minimum and grass minimum temperatures are those recorded at the weather station at Arrochymore, only about 1 km from the island. The annual means for the respective 12-month periods are indicated by dotted lines (max - maximum; min - minimum; gm - grass mimimum).
Similarity between the two years' temperature patterns is apparent and there is also similarity in the phalangid occurrences. The time of peak activity of the various harvestmen is suggestive of a pattern of succession by the species, particularly the oligolophines (L. ephippiatus down to 0. agrestis), with N. bimaculatum being slightly more spread through the year. The relative abundance of the species is shown for the four sites in each month. Note the difference in scales between the two figures, especially for
N. bimaculatum, 0. tridens and 0. palpinalis.
Acknowledgements
I am grateful to colleagues in the Department of Biology, Paisley
College of Technology, especially Professor J.C. Smyth, for advice
and encouragement; to the Nature Conservancy Council, Mr. E.T.
Idle in particular, for permission to work on the Reserve;
and to the Clyde River Purification Board for meteorological
data from their weather station at Arrochymore.
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Figures 1 and 2: Monthly abundances of opilionids on Inchcailloch
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