13 S.A.L. Hayward et al. Journal of Insect Physiology 47 2001 11–18
Fig. 1. a Linear humidity chamber arrangement. b Grid humidity chamber arrangement. c Schematic side view of experimental arena, to
assess the hygropreference of C. antarcticus and A. antarcticus. Chamber placed inside thermal containment system through which cooling fluid is circulated. Temperature regulated at 5, 10 or 20
° C by thermal circulating bath. Humidity conditions generated by different saturated salt solutions.
of individuals within each zone and, when an individual was between zones, the position of the head was con-
sidered indicative of its preference. A minimum of five replicates was carried out for each regime to permit stat-
istical analysis. The distribution of C. antarcticus and A. antarcticus was also observed under uniform humidity
conditions of 75 RH 5
° C within the linear arena.
These experiments were then repeated using 20 individ- uals to assess any influence of aggregative behaviour on
their orientation within the experimental apparatus. Within the linear humidity chamber, thigmotactic
behaviour may promote the collection of individuals at either end of the gradient. To determine if such behav-
iour influenced the distribution of either species, experi- ments were repeated using grid chambers that provided
equal body-surface contact in each compartment. During all experiments the chambers were also rotated 180
° to
compensate for any external asymmetries. All stock cultures were maintained under similar con-
ditions, but differences in the hydrated state of individ- uals prior to the investigation may have influenced the
observed results. The fresh weight of each group of four animals was recorded at the start of the experiment, then
re-weighed once the hygropreference assessment was completed. Finally, the samples were dried to constant
weight at 60
° C and the dry weight recorded Balance
type: Satorius M3P, accuracy ±
1 µ
g. The mean initial water content and percentage water loss of each sample
was then calculated for all temperature regimes. 2.1. Statistical analysis
Chi-squared tests were used to determine if the distri- bution of the two species differed significantly from ran-
dom at the end of the observation period. Because the number of individuals in each experiment was small, the
sum of the final frequencies from each regime was used in calculating the test statistic. The mean initial water
content and percentage water loss of samples for each species, was compared between temperatures using one-
way ANOVA Minitab software package.
3. Results
3.1. The influence of temperature on hygropreference The results of this investigation suggest that tempera-
ture does influence the humidity preference of both spec- ies.
Data obtained for C. antarcticus using the linear chamber are illustrated in Fig. 2a. At 5 and 10
° C, the
distribution after 2 h did not differ significantly from random
χ
2
= 7.71 and 6.25 respectively, 3 df; N.S.. At
20 °
C, however, this species demonstrated a clear prefer- ence for the most humid conditions, 98 RH
χ
2
= 36, 3
df; P ,0.001.
The distribution of A. antarcticus within the linear arena [Fig. 2b] differed significantly from random at
14 S.A.L. Hayward et al. Journal of Insect Physiology 47 2001 11–18
Fig. 2. a Distribution of C. antarcticus within a humidity gradient
at 5, 10 and 20 °
C linear chamber, 8, 8 and 6 replicates respectively. b Distribution of A. antarcticus within a humidity gradient at 5, 10
and 20 °
C linear chamber, 8, 5 and 5 replicates respectively. Mean and standard errors are presented for 12 observations, n
= 4.
5 °
C, and indicated a preference for the lowest humidity, 9 RH
χ
2
= 11.75, 3 df; P
,0.01. At 10 and 20 °
C, how- ever, no clear humidity preference was observed and the
distributions did not differ significantly from random χ
2
= 4.8 and 5.2 respectively, 3 df; N.S.
3.2. Orientation in the absence of an RH gradient In this experiment the distribution of both species was
assessed in the linear arena under uniform conditions of 75 RH 5
° C over 2 h. Both species distributed homo-
geneously, and data did not differ significantly from ran- dom C. antarcticus,
χ
2
= 6.4; A. antarcticus,
χ
2
= 3.2, 3
df; N.S. Fig. 3. 3.3. Assessing aggregative behaviour
To determine if aggregative behaviour would influ- ence the distribution of either species within a uniform
gradient, an experiment was undertaken using 20 indi- viduals Fig. 4. The distributions of both species dif-
fered significantly from random χ
2
= 71.6, and 66.4,
P ,0.001 for C. antarcticus and A. antarcticus respect-
ively. df =
3. 3.4. Excluding thigmotaxis
The distribution of C. antarcticus within grid arenas [Fig. 5a] did not differ significantly from random at 5
and 10 °
C χ
2
= 2.0 and 0.57 respectively, 3 df; N.S.. At
20 °
C, however, a clear preference for the highest humidity, 98 RH, was observed
χ
2
= 30, 3 df;
P ,0.001. The orientation of A. antarcticus within the
grid chamber [Fig 5b], indicates no clear humidity preference at 20
° C
χ
2
= 0.8, 3 df; N.S., but a preference
for the lowest humidity, 9 RH, at 10 and 5 °
C χ
2
= 12.75, P
,0.01 and χ
2
= 11.25, P
,0.025 respectively. df
= 3. For C.antarcticus, the results are consistent with
those from the linear chamber at all temperatures, and for A. antarcticus at 5 and 20
° C.
Fig. 3. Distribution of C. antarcticus and A. antarcticus within a uni-
form humidity gradient at 5 °
C linear chamber, five replicates for both species. Mean and standard errors are presented for 12 obser-
vations, n =
4.
15 S.A.L. Hayward et al. Journal of Insect Physiology 47 2001 11–18
Fig. 4. Distribution of 20 individuals of C. antarcticus and A. antarc-
ticus within a uniform humidity gradient at 5 °
C linear chamber, three replicates for both species. Mean and standard errors are
presented for 12 observations, n =
20.
3.5. Differences between A. antarcticus and C. antarcticus—the role of body water content
The mean initial water content of samples Table 1 did not differ significantly between temperatures one-
way ANOVA P =
0.126 and 0.567 for C. antarcticus and A. antarcticus respectively. Levels of water loss dif-
fered significantly between regimes, however, one-way ANOVA P
,0.001 for both C. antarcticus and A. antarcticus and were greatest at 20
° C Table 1. Con-
tinued desiccation of C. antarcticus samples, after each experiment, prevent accurate assessment of the water
loss responsible for inducing an aversion to low RH con- ditions. Desiccation of A. antarcticus beyond the period
of experimentation was less of a problem, however, and the greatest water loss of an A. antarcticus sample was
11 of its initial content at 20
° C.
4. Discussion
