Download Pitfall Trapping as a Method for Studying Populations of Carabidae

Pitfall Trapping as a Method for Studying Populations of Carabidae (Coleoptera)
Author(s): P. J. M. Greenslade
Source: Journal of Animal Ecology, Vol. 33, No. 2 (Jun., 1964), pp. 301-310
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/2632 .
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301
PITFALL TRAPPING AS A METHOD FOR STUDYING
POPULATIONS OF CARABIDAE (COLEOPTERA)
BY P. J. M. GREENSLADE
Department of Zoology, Imperial College, London*
Pitfall traps provide a convenient method of investigating the ecology of adult Carabidae
and have been used in studies on such topics as the seasonal incidence of adults, the spatial
pattern of distribution in populations, the relative numbers of a species in differentvegetation types, in describing the Carabid fauna of habitats, and in work on daily rhythms
of activity (Hikimiuk 1948; van der Drift 1951; Scherney 1955, 1960; Gilbert 1956;
1958; Dawson 1957; Kabacik 1957; Skuhravy 1956; Williams 1959b; Kirchner 1960;
Thiele 1960; and others). These traps suffer from the disadvantage that catches depend
both on the density of the population being sampled and the activity of individuals in it.
Briggs (1961) concluded that in the Carabid Harpalusrufipes(Deg.) the size of the population played a minor part in determining numbers trapped, and Morris (1960) has
stressed the desirability of obtaining estimates of absolute population in sampling programmes, that is the number of individuals per unit of habitat.
As pitfall catches vary with the amount of locomotor activity shown by Carabidae
they are influenced by weather, and Briggs (1961) and Greenslade (1961) found a correlation between catches and temperature. The problem of the relation between catch,
population size and activity is not dealt with here, but Taylor's (1963) recent discussion
of temperatureand insect flight and suction trap catches is relevant. There are, however,
other factors which may result in variation in pitfall catches of Carabidae, the two main
ones, which have not been considered by some workers, are the effect of ground vegetation in impeding Carabid movement and reducing catches, and the differential susceptibility of species to trapping according to their behaviour. Taking these factors into
account the cases in which it is legitimate to draw conclusions from pitfall trapping are
discussed. This is necessary for despite their drawbacks pitfall traps are often the only
method available in studies on Carabid populations. Although direct quadrat counts
are to be preferredthey may be precluded by low density or other factors.
On the Imperial College Field Station, Silwood Park, Berkshire, where the work
described in this paper was carried out, quadrat counts of Carabidae were made in three
habitats (Greenslade 1961), and the densities of adults were: beechwood, 1-33 per m2 (6
Carabidaein 4-46 m2sampled); grass heath, 28 per m2 (52 in 1 86 m2); arable land, 80 per
m2 (82 in 1 02 m2). With the sparse woodland population direct counts are not practicable
on account of the time and labour they involve, while in other habitats quadrat sampling
may result in excessive destruction of the site being studied, especially as Carabid distributions are often highly aggregated and centred on restricted favourable areas (Greenslade 1963a, 1964a, e).
TRAPPING METHODS
The traps used here simply consisted of 1 lb jam jars (11-5 cm high with a diameter at the
mouth of 6 5 cm), sunk in the ground with their rims level with the soil surface. More
*
Now at: Departmentof Agriculture,Honiara, British Solomon Islands Protectorate.
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Pitfall trapping
302
elaborate traps have been used by Williams (1958) incorporating a device to segregate
the catch according to time of capture, and by Fichter (1941) whose trap contained preservative and excluded rain water. Here such modifications were unnecessary as it was
convenient to visit traps daily or even more frequently.
Following observations by Walsh (1931, 1933), Kaufmann (1937) and others, experiments were made to determine whether baiting traps with carrion resulted in a marked
increase in catch of Carabidae, in addition the effect on catch of the density of traps in
a plot was investigated. Observations were also made on camouflaging traps in case
diurnally active species might see and avoid them; white-painted traps were compared
with black, and unpainted controls. In preliminaryexperiments it was found that neither
baiting nor camouflaging traps caused variation in catches, and no trap-density effects
were recorded. Therefore these topics were not pursued and the simple type of trap was
retained, although they were rarely spaced with a distance between them of less than
approximately 2 m, except in experiment II described here.
GRASSES
TRAP
SOIL
=7
.
B
A
FIG. 1.
C
Three types of simple pitfall trap.
Other work was carried out to determine the optimum position of the mouths of traps
in different types of ground cover, and for different species of Carabidae. The simplest
approach to this problem is comparison catches when a known number of Carabidae are
confined within an area containing one or more traps of given types. However, in such
experiments it was found that there was a decline in the incidence of marked Carabidae
which had been introduced into galvanized metal strip enclosures, and with recaptures
outside them, this showed that there was continual escape. Van der Drift (1951) also
reported difficulty in enclosing Carabid populations. Instead information on the relation
between the position of mouths of traps, and their catches of different Carabidae, and
ground vegetation types, was obtained from two other experiments and some supplementary observations.
Experiment I
A plot of nine traps was laid out in a uniform area of grass heath whose composition
is described in the second paper in this series (Greenslade 1964d). The traps were of three
types as shown in Fig. 1:
A. The mouth of the jar level with the upper surface of the vegetation.
B. The jar sunk in the ground with the lip level with the ground surface, within a
cleared area of 60 cm diameter from which all vegetation had been removed.
C. The jar sunk in the ground but without a cleared area.
The traps were arranged in a Latin square of total size 5 5 x 5 5 m, and were operated
for two periods in 1959: in the spring from 18 March until 17 May, and in the summer
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P. J. M.
GREENSLADE
303
from 14 July until 25 August. In the spring trapping the ground cover consisted of dead
vegetation from the previous growing season, mainly grasses, forming a mat 10-13 cm
deep, with occasional tussocks. By the summer this mass of dead grass had been reduced
to a depth of 6-9 cm and was bound together by fresh growth of grasses and ruderals,
many of the latter having a rosette growth form.
ExperimentII
A further four traps were situated near the grass heath plot, in woodland under
beach, where the ground was covered with loose leaf litter and a sparse recent growth of
Holcus mollis and Poa pratensis. Traps of types B and C were compared from 18 March
until 23 May 1959, and were arranged in a square of side I1 m, in which similar traps
were diagonally opposite each other.
Other observations
A transect of five traps of type B at intervals of 3-7 m was laid out in leaf litter under
beech, crossing a sandy track. The traps were operated from 13 May until 27 July 1959,
and litter depths were measuredat five points within a radius of 1-8 m of each trap.
Finally quadrat samples were taken in arable land in August 1960. A 30 cm square
quadrat was used and counts of Carabidae from a total of 1 m2 of soil were obtained. At
the same time, during August, three pitfall traps were also in use in the area of these
quadrat samples and their catches were recorded.
The results from the two experiments and other observations are shown here with
catches expressed for different categories of Carabid distinguished according to size and
habit. Detailed catches of the species in each trap are given by Greenslade (1961).
RESULTS
The distribution of the catch in the two experiments can be considered in terms of the
four variables: (1) Position of trap; (2) Type of trap; (3) Type of Carabid; (4) Vegetation.
Position of trap
In experimentI the plot was designed to avoid effects due to spatial trends in the density
of Carabidae. As studies of Nebria brevicollis(F.) and Pterostichusmadidus(F.) described
in the following paper (Greenslade 1964d) show that these species commonly travel
10 m in a 24 h period, the possibility that there is systematic variation in catch in terms
of the rows and columns of the plot need not be considered. There is, however, likely to be
variation in catches due to the position of traps in relation to near-by grass tussocks
and other minor irregularities of the ground surface, and it was these local differences
that traps of type B were intended to overcome.
Type of trap
It is convenient to add the catches in a plot for each type of trap, and to test the significance of the differencesbetween them by calculating x2. The total number of Carabidae
taken in the three types of trap in the two experiments are compared in Table 1.
Traps of type B consistently took larger numbers of Carabidaethan the others although
in experiment IIthe differencebetween B and C was not significant.It was noticed that the
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Pitfall trapping
304
cleared areas around these, if kept clean, were not affected by seasonal changes in vegetation, or its wearing when the traps were visited frequently over a long period. In experiment I, in the spring observations, traps of type C were more efficient than A (X2- 13'33,
P<0-001), while in the summer the situation was reversed, A showing significantly
higher catches than C (X2= 16-0,P<0 001). This suggests that in the spring some Carabid movement occurred within the tangle of vegetation, while in the summer the more
compact mass of dead grasses permitted greater activity on its surface, and less between
it and the soil.
Table 1. The catches of Carabidaein three types of pitfall trap
Type of trap
r
A
f Spring
Expt
Expt.II
5
219
XSummer
_
A
B
57
300
C
25
173
-~---~
B
71
Expt. II
C
53
P
X22
47-4
35-4
<0 001
<0-001
x22
p
1P7
>010
For explanationsee text and Fig. 1.
Type of Carabid
The Carabidaewhose habits are known and which occurred in numbers in experiment I
in the spring, can be divided into two groups: the larger ground-living, mainly predacious
species (Davies 1959), Pterostichus madidus and P. caerulescens (L.), and those species
which are commonly associated with plants and are often collected by sweeping, Amara
species (A. lunicollis Schdt., A. communis (Pz.), A. aenea (Deg.), A. familiaris (Duft.)
and A. tibialis(Payk.) occurred), and Bradycellusharpalinus(Serv.).
Table 2. The catches of two types of Carabidae in experiment I in
the spring (excluding traps of type A)
Type of trap
Type of Carabid
Largerground-livingspecies
Speciesassociatedwith plants
B
31
17
C
5
16
Xi2
P
18 4
01
<0 001
>0-7
As very small numbers were taken in traps A in this period comparison is restricted
to B and C in Table 2 which shows that type B traps took significantly more of the
Pterostichus species, while those associated with plants occurred equally in both kinds
of trap. This indicates that the larger species were moving within and under the vegetation, and the cleared patches in B traps may have presented a larger catching area than
the simple sunken traps of type C. Ibbotson (1958) recorded high catches of the Frit fly
(Oscinellafrit L.) on sticky traps in cleared areas of 1-5 m in diameter in oat fields
which he attributed to klinokinetic movements due to abrupt temperature gradients at
the periphery of the cleared circles. Here the higher catches of the Pterostichus species in
B traps may result from rapid turning movements after entry into the unfavourable,
or at least strange, environment of the cleared areas. In contrast the Bradycellus and
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305
P. J. M. GREENSLADE
Amara species move on or in the vegetation to a greater extent, and may tend not to
leave it when encountering a cleared area.
The same comparison cannot be made for experiment I in the summer or for experiment II, as Amara and Bradycellus species were infrequent. But in the former other
species occurred which are known to be phytophagous (Davies 1959), and their distribution can be compared with that of Pterostichus madidus, the only abundant large
predatory species which was active in the plot then. The phytophagous Carabidae fall
into two groups, Harpalus rufipes, and H. affinis (Schrank), and Pterostichus melanarius
(Ill.), all larger species which seem to feed to some extent on fallen plant seed (Briggs
1957; Davies 1959), and the smaller species, Bradycellus harpalinus(a single specimen)
and the more agile Calathusfuscipes (Gz.) and C. melanocephalus(L.). Catches of these
groups are shown in Table 3.
Table 3. Catches of different types of Carabidae in experiment I in the summer
Type of trap
Type of Carabid
_
Pterostichus madidus
flarger
Phytophagous species ><smaller
P-
p
X22
A
155
B
263
C
136
64-66
<0 001
8
19
15
12
14
29
2.33
7.3
>0 3
<0 05
Here again for the non-phytophagous Pterostichus madidustype B traps are the most
efficient, while for the larger phytophagous species the distribution of the catch can be
attributed to chance, suggesting that they move more within the vegetation. For the
smaller species traps of type C are the most efficient and these can be regardedas moving
mainly on the surface of the vegetation.
Here these trapping differencesbetween types of Carabidae are attributed to the strata
in which they are active within the ground vegetation, but whether or not this is correct,
the point demonstrated is that, within one type of ground cover, differenttypes and species
of Carabidae are trapped at different rates in any given trap.
Vegetation
P. madidusformed a high proportion of the total catch throughout these experiments
and the relative numbers in differenttraps are compared in Table 4.
Table 4. Catches of Pterostichus madidus in experimentsI and II
Type of trap
_
A____
Expt I
Expt.I
f Spring
Xj,Summer
A
3
155
I_
_
B
26
263
C
5
136
B
47
C
27
X22
28 66
64-66
X2
Expt. II
54
P
<0 001
<0.001
p
<0 05
In each case B type traps were most effective for this species. Their superiority was
least marked, but still significant, in experimentII and this can be attributed to the sparser
vegetation allowing more freedom of movement over the surface of the ground, so reducing the difference between the traps. In experiment I B traps took a lower proportion
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Pitfall trapping
306
of the total catch in the summer observations, 470% as opposed to 760% in the spring.
As the differencesbetween A and C are slight in both seasons, the data for this species in
experiment I in Table 4, can be treated as a contingency table, and x2 calculated applying
Yate's correction: X22 = 11X17,P<0001. Traps of type B were therefore, compared
with A and C, significantly more efficient at trapping P. madidusin the spring than in the
summer.
80-
w
6040-
020-
O
1
3
2
LITTERDEPTH(cm)
FIG. 2.
Pitfall catches of Nebria brevicollis,and litter depth in a woodland transect.
As observation showed no evidence for major changes in behaviour in P. madidus in
the two seasons, the differencein the effectiveness of B type traps can be related to changes
in the ground vegetation. It is suggested that in the summer compaction of the dead
plant material and the effect of new growth reduced the amount of activity which was
possible within and beneath it, and so also reduced the differences between the three
trap types.
Other observations
Catches of Nebria brevicollis which comprised 72% of the Carabidae taken in the
woodland transect traps, are shown in Fig. 2, where they are plotted against litter depth.
There is an inverse relation between the depth of the litter around a trap and its catch of
N. brevicollis.
Table 5. Carabidaeof three types of daily activity cycle taken
in soil quadratsand pitfall traps in an arablefield, August 1960
Activity rhythm
-`
Soil quadrats
Pitfall traps
Diurnal
36
26
Plastic
13
16
Nocturnal
33
70
Total
82
112
Carabidae taken in quadrat samples in an arable field are compared with those occurring in pitfalls in the same area at the same time in Table 5. In the table the Carabidae are
grouped according to their daily periodicity of locomotor activity, where the plastic
category distinguishes those which may be active in the day or night in response to variation in their environment, especially climatic factors (Greenslade 1963b). The direct
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P. J. M. GREENSLADE
307
population counts in the quadrats show that diurnal and nocturnal species are present in
similar numbers while in the pitfall traps the catch of nocturnal species is significantly
higher than that of diurnal. If the plastic species are omitted the data can be treated as a
2 x2 contingency table: X12= 1078, P<0-01.
DISCUSSION
The type of trap with a cleared area, B, was selected for further studies of Carabidae, as it
consistently made higher catches, the only exception being Calathusspecies in experiment
1, and can be used in most sorts of ground cover; it was also noticed that in comparison
with C, B traps were less frequently fouled by dead leaves and other debris. B traps also
eliminate variation in catch due to grass tussocks and similar obstacles close to traps.
Other variation in pitfall catches, apart from that which the traps are commonly
used to investigate, comes from two main sources, the type of Carabid, and the nature of
the ground cover. Experiment I showed that the traps are selective in that different Carabidae are taken at different rates. It was suggested that this was due to various species
and life-forms being active in different strata in thick grass heath. Kabacik (1957) found
that Carabus arcensis Herbst moved only on or near the surface of leaf litter, while
Pterostichus niger (Schall.) may be active both on and within it. If large Carabus and
small Bembidionare at risk among traps for a given time, the larger species, travelling
further, will tend to be trapped in higher numbers. While this is clear for an extreme
example it also holds when species are compared which are apparently similar in size
and activity as in Pterostichusmadidus,P. melanariusand Calathusfuscipesin experiment
I. In addition to this the daily time of activity of species affects trap catches. Van der
Drift (1951) commented on the ability of diurnal Notiophilusspecies to evade capture by
traps; they are small and often recover balance and escape when on the lip of a pitfall,
but may also see and avoid traps. This is supported by the comparison of pitfall catches
and direct counts of population densities in Table 5, which shows that the diurnal Carabidae are under-representedin the former.
The less marked superiority of B traps over C in experiment II as compared with I
implies that a sparse ground cover allows more movement over the surface. This is also
noticed in the spring and summer trapping in experiment I when surface movement was
easier on the compressed mat of vegetation in the summer. Although this variation in the
efficiencyof traps caused by differencesin the physical charactersof various types of grassland is slight, its presence implies that there will be a greater discrepancy between
the proportions of populations of the same species trapped when more diverse types of
ground cover are compared, for example a thin layer of leaf litter, and rank pasture. This
is supported by the woodland transect which shows that catch varies inversely with the
depth of the ground cover. In the small area of the transect the high catches near the track,
where low litter depths were measured, cannot be attributed to a locally dense population of Nebria brevicollis,for hand collections did not show any concentration of individuals there; but the distribution of catches can be explained by the absence of litter
allowing in places a greater freedom of movement, and hence a greater likelihood of any
individual Carabid being trapped.
These observations demonstrate that pitfall trapping cannot properly be used for the
quantitative assessment of the Carabid fauna of any habitat; nor should it be employed
to comparethe numbers of one species in differenthabitats. There are also indications that
seasonal changes within a type of vegetation cover may affect the trapping of Carabidae.
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308
Pitfall trapping
However, Fichter (1941), discussing pitfall traps, states that they may be used to compare the 'composition of soil surface arthropod populations in two or more habitats'.
But for quantitative work this is evidently untrue, especially if it embraces groups more
diverse than a single family of Coleoptera.
Boyd (1960) used pitfall traps to compare the surface active invertebrate fauna of
grazed and ungrazed maritime grassland on the premise that 'pitfall catches probably
give a fairly accurate indication of the surface activity of the soil fauna'. The comparison
was between closely cropped grassland and an ungrazed sward which reached ankle
depth, and while the results showed distinct qualitative differences in the fauna of the
two areas, it is questionable whether any quantitative comparison is permissible.
Williams (1959a) used pitfall traps to measure the activity of ground-living invertebrates in different habitats interpreting the catches of traps as measures of the level of
activity in their respective vegetation types. However, catch representslocomotor activity
expressed as movement in a horizontal plane, and for any given energy outlay the amount
of such movement will be greater in an essentially two-dimensional habitat, for example
a thin layer of leaf litter, than in a three-dimensional one such as grassland.
After using pitfalls to investigate the Carabid fauna of arable land, Scherney (1960)
concluded that Carabidae were associated with cultivation; the numbers taken in pitfall
traps in differenthabitats were in the order wheatfields>barley>potatoes>clover>grass
meadow>densely weed-coveredwaste ground. His conclusion may be valid, for the quadrat counts mentioned in the present work showed the highest Carabid density on arable
land, but it is noticeable that the sequence of increasing Carabid abundance in traps,
from a dense weed cover to cereal fields, is also a sequence of decreasing resistance to the
passage of Carabidaeover the ground surface.
These examples illustrate the fact that pitfall traps must be used with discretion,
especially for comparative purposes. This does not affect their value as a sampling
method in marking and recapture work, or for studies on life histories. In the latter case
the chance of variation in catches due to seasonal changes in vegetation must be noted,
and results from pitfalls are best combined with observations on larvae, and gonad
dissections, as in studies on Calathus species (Gilbert 1956), and Pterostichus madidus
(Greenslade 1964b). They can be used also in comparing the fauna of different habitats
in conjunction with marking and recapture or quadrat counts, and for investigating the
distribution of one species in a single vegetation type.
Apart from these cases pitfall results are of limited value but three exceptions may be
noted. In conjunction with hand collections they can be employed for the qualitative
assessment of different Carabid faunas, especially when many species are restricted to
one habitat or another, and they yield some information on the frequency of species;
but it is desirable that observations should be made also on the distribution of larvae.
If two vegetation types are sampled with pitfalls and a species is trapped in higher numbers in the one which presents the most resistance to Carabidae movement, for example
in long grass when it is compared with a thin litter layer, it can be concluded that traps
indicate the species' true distribution. Examples of pitfalls used in these ways are given
by Greenslade (1964c).
Finally in the second paper in this series (Greenslade 1964d) it was found that where a
plot of traps extended over more than one type of ground cover, two species might occur
each associated with one type, usually being absent when the other was present. It was
concluded that in inverse patterns of this kind, pitfall catches did show the true distributions of the species involved.
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P. J. M.
GREENSLADE
309
SUMMARY
Pitfall trapping is reviewed as a sampling method for Carabidae. Catches are determined
primarily by the size of the population at risk and the level of locomotor activity but
species may show differential susceptibility to trapping according to size, behaviour, and
the strata in which they are active in the ground vegetation. Catches of a single species
may vary in different types of ground cover depending on the resistance they present to
horizontal movement. These observations severely restrict the number of cases in which
it is permissible to make quantitative comparisons between the catches of differentpitfall
traps.
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310
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APPENDIX
A recentpaper by Mitchell(1963: J. Anim. Ecol. 32, 377-92) raisestwo furtherpoints concerningpitfall
trappingon which some observationswere also made duringthe work at Silwood (Greenslade1961 and
unpublishedresults). They are predation in traps, and the use of pitfall catches in conjunctionwith
populationestimatesto investigatevariationin locomotor activity.
Mitchellfound that small Carabidaewere preyedupon in traps both by largerspecies and by birds;
although looked for in the work describedhere neither type of predationoccurred.Briggs (1961) encounteredbird predationand placed guardsabove traps but these affectedcatches, and Mitchell'ssolution, crumpledwire netting within traps, appearsthe more satisfactory.When predation by the larger
Carabidaeoccurs it is possible that it might be avoided by supportinga false floor of coarsewire gauze
(mesh size 4-5 mm) in traps a few centimetresabove the bottom; this would allow small Carabidaeto
pass through but retain the large species. In this case a guard above the trap or a funnel in the mouth
might be necessaryto preventbird interference.At Silwood Trechus species,which are soft-bodied and
presumablysusceptibleto predation,were infrequentin traps and were not studied, but other trapped
arthropods,especiallyspidersand Collembola,were usually attackedby some Pterostichus species.
As in Mitchell's work attempts were made to use pitfall catches and population estimatesto study
activity,and in particularto segregatethe effectsof climaticand other factors,using the partialregression
technique.It was concludedthat the method was of limitedpracticalvalue although Mitchellshows that
it can be useful in detailed studies on one or two species in a uniformhabitat. There are three major
difficulties:
(a) The complexity of the factors determininginsect activity. Mitchell deals with time-trends,the
averagelevel of activityvaryingwith the age structureof the adult population,and simpleclimateeffects;
amongotherproblemsto be consideredare conditioningof Carabidaeto temperature,and the interaction
of temperatureand humidity.It should be noted also that there may be time-trendsin the sex ratio with
associateddifferencesin the activity of the sexes.
(b) The problemof describingthe microclimate.Although activitycan be relatedat times to standard
meteorologicalrecordsit may be necessaryto make preciseobservationsin a species'habitat;here it has
been shown that differentCarabidaemay move in differentstratawheresteep temperatureand humidity
gradientsoccur. In additionsome speciesare active in responseto weatherand show no consistentdaily
rhythm(Greenslade1963b),so that it may not be possibleto representthe microclimateby observations
made at the same time each day.
(c) The inadequacyof populationestimates.Any analysisof activitywill dependon the accuracyof these
estimates,and if derivedfrom markingand recapturethey will usuallyhave high standarderrors.At the
beginningand end of a species'seasonal activity,when it maybe most importantto distinguishbetween
variationin activityand populationdensity,pitfall catches tend to be so low that captureand recapture
is inapplicable.In directquadratsampling,on the other hand, it is often difficultto differentiatebetween
membersof the active populationrestingbetweenbursts of activity,and individualsstill quiescentafter
hibernationor aestivationor before emergence.
I am gratefulto Dr B. Mitchellfor kindlysendingme a copy of his paper,and to the editorsfor allowing
the inclusion of an appendix.
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