Download Aspects of excretion of antlion larvae (Neuroptera: myrmeleontidae

Journal of Insect Physiology 44 (1998) 1225–1231
Aspects of excretion of antlion larvae (Neuroptera:
myrmeleontidae) during feeding and non-feeding periods
Amanda Van Zyl
*1
, M.C. Van Der Westhuizen, T.C. De K. Van Der Linde
Department of Zoology-Entomology, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South Africa
Received 20 March 1998; received in revised form 15 June 1998
Abstract
The main nitrogenous excretory products were determined for third instar Cueta sp. and Furgella intermedia larvae during periods
of food abundance and for F. intermedia during starvation periods. Biochemical analysis indicated that allantoin was the main
nitrogenous excretory product, with smaller quantities of ammonia, urea and uric acid. Respectively 9 and 13 amino acids of low
concentrations (0.005–0.329 g/l) were detected by high pressure liquid chromatography in the excreta of Cueta sp. and F. intermedia
larvae. The volume of urine produced and concentrations of the nitrogenous excretory products of fed Cueta sp. and fed F. intermedia
larvae did not differ. F. intermedia excreted smaller volumes of urine and smaller quantities of nitrogenous excretory products
during starvation than during periods of food abundance. Feeding conditions rather than the pitbuilding or non-pitbuilding lifestyles
seem to be the major influence on the excretory products of these antlion larvae.  1998 Elsevier Science Ltd. All rights reserved.
Keywords: Neuroptera; Myrmeleontidae; Excretion; Allantoin; Water
1. Introduction
A strong correlation usually exists between the major
nitrogenous excretory products and the nature of an
insect’s environment, where aquatic forms often excrete
ammonia and terrestrial forms uric acid (Cochran, 1985).
The antlion larvae, Cueta sp. and Furgella intermedia
(Markl), live in the semi-arid to arid Kalahari desert
(28°21⬙E, 21°16⬙S), where food resources are unpredictable for these larvae (Van Zyl et al., 1996). As surface
water is often absent in the natural environment of these
antlion larvae, it has been suggested that food is their
primary water resource during the dry season (Van Zyl,
1995). Antlion larvae are also unable to take up atmospheric water vapour as demonstrated for Myrmeleon
medialis Banks, Cueta lanceolatus Navas and Syngenes
longicornis (Rambur)(Youthed, 1973). The pitbuilder
Cueta sp. and non-pitbuilder F. intermedia larvae should
* Corresponding author. Fax: 00 27 12 3625242; e-mail: [email protected]
1
Present address: 244 Carinus Street, Meyerspark, 0184, South
Africa. Fax: (27 12) 3625242; E-mail: [email protected].
0022–1910/98/$19.00  1998 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 2 - 1 9 1 0 ( 9 8 ) 0 0 1 0 0 - 0
therefore be able to conserve water e.g. by excreting uric
acid, during prolonged periods of food shortage.
On the other hand, antlion larvae are extra-intestinal
digesters, i.e. they inject enzymes and probably ‘poison’
(Gaumont, 1976) into their prey, which dissolves the soft
internal tissue of the prey. The fluid is then drawn out
of the prey and into the alimentary canal of the antlion
larva. Van Zyl et al. (1997) demonstrated that third instar
Cueta sp. larvae (body weight of 31 mg) extracted 73%
of the total wet weight of sixth instar Hodotermes mossambicus (Hagen) larvae (body weight of 20 mg).
Antlion larvae ingest therefore large quantities of fluid
during extra-intestinal digestion and a need exists for the
storage or elimination of large quantities of excess fluid
after feeding. This was demonstrated in other fluid feeders, e.g. bloodsucking and plant-sucking insects where
an abundance of water is present after a meal and the
urine voided is a crystal clear fluid (Wigglesworth,
1965).
The study on the excretory products of antlion larvae
is to a large extent facilitated in that the alimentary canal
is discontinuous (Lozı́nski, 1911; Van Zyl, 1995). No
contamination by faeces occurs in the urine and the
excretory products are primarily of metabolic origin.
Unfortunately, only a limited number of studies have
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A. Van Zyl et al. / Journal of Insect Physiology 44 (1998) 1225–1231
been conducted on the excretion of neuropteran species.
Shaw (1955) and Staddon (1955) conducted an in depth
study on the ionic regulation of the larvae of the aquatic
neuropteran, Sialis lutaria L. Ammonia is the main
nitrogenous excretory product in these larvae. Razet
(1961, quoted by Bursell, 1967) demonstrated that uric
acid dominated in the excreta of the antlion larva, Uroleon nostras (Fourcroy), with smaller quantities of allantoin present. Spiegler (1962b) observed urate storage in
the larvae of the green lacewing, Chrysopa carnea
Steph. Of all these examples only U. nostras occurred
in a terrestrial environment comparable to that of Cueta
sp. and F. intermedia larvae.
In the present study the main nitrogenous excretory
products (uric acid, allantoin, urea, ammonia and amino
acids) of Cueta sp. and F. intermedia larvae were identified. Excretion during periods of food abundance and
food shortage was then related to survival in the semiarid to arid Kalahari desert.
2. Materials and methods
Third instar Cueta sp. and F. intermedia larvae were
collected in the Kalahari desert and transferred to glass
vials (120 mm in depth ⫻ 20 mm in diameter) filled to
a depth of 60 mm with sterilised sand from their natural
environment. Antlion larvae were acclimated to 28 ⫾
2°C with a diel cycle of 12:12 (L:D). Termites, sixth
instar H. mossambicus larvae, were used as their food
supply.
2.1. Uric acid, allantoin, ammonia and urea
determinations
Antlion larvae were starved for 42 days prior to the
experiments. Excretory products were collected from fed
and starved larvae in two treatments. In the first treatment nine Cueta sp. (body weight 29.8 ⫾ 7.0 mg) and
14 F. intermedia larvae (body weight 52.9 ⫾ 10.0 mg)
were each fed one termite (approx. 12 mg) every second
day over a 14 d period. These are referred to as ‘fed
larvae’. The excreta of the individual antlion larvae and
the termite carcasses were collected at the next feeding
and frozen at ⫺ 12°C. Presence of excreta and wet
weight of termites before feeding were recorded every
second day. Excreta was thus collected on seven of the
14 experimental days and pooled for each individual.
The liquid excreta forms pellets in the sterile sand. These
could be removed by sieving the sand through a 2 mm
mesh screen. Prey carcasses were removed before sieving. As antlion larvae eject the prey carcass above the
soil surface within an hour after feeding and excreted
the urine several hours later in the sand, at a depth of
20 mm below the soil surface where the antlion larvae
were situated, it is unlikely that the prey carcass could
have contaminated the excreta in any way.
In the second treatment in total 16 F. intermedia larvae were each fed one termite of approximately 19 mg
once. These larvae are referred to as ‘starved larvae’.
The excreta was collected 10 times (on day 4, 5, 6, 8,
10, 12, 14, 18, 25 and 36) over a 36 d period and pooled
for each individual. Presence of excreta was noted on
the collecting days.
The uric acid, allantoin, ammonia and urea concentrations were determined for the excreta of the fed and
starved antlion larvae, respectively. The volume of urine
excreted by the antlion larvae was estimated by
determining the quantity of water that saturated a known
weight of dry sterilised Kalahari sand viz. 0.1018 ⫾
0.017 ml of water/g dry sand (n ⫽ 9). The dry weight
of the collected pellets (urine mixed with sterile sand)
was then used to estimate the volume of urine excreted
by the antlion larvae.
For analysis the collected dry excreta of fed larvae
and of starved larvae were homogenised in 1.0 ml and
0.5 ml of 0.6% Li2CO3, (pH 11.53) respectively. All
nitrogenous products tested, are soluble in this substance. The antlion excretory extract was heated for
10 min at 100°C, centrifuged for 10 min at 4 000 g and
kept at 4°C. A sample volume of 0.1 ml of the supernatant was used in all the determinations.
Uric acid was determined by the method of Liddle et
al. (1959) as summarised by Potrikus and Breznak
(1980). An assay cuvette contained 2 ml of 0.1 M glycine buffer (pH 9.4), 0.1 ml antlion excretory extract and
approximately 0.03 enzyme units (U) of purified uricase
(hog liver, type V, Sigma). Uric acid (free acid, Sigma)
was used as standard.
The allantoin content was determined by the method
of Borchers (1977). Allantoin was converted to allantoic
acid by dilute alkaline hydrolysis. This was done by
heating 0.1 ml antlion excretory extract with 0.25 ml of
0.6 M NaOH at 100°C for 12 min. The allantoic acid
was hydrolysed by the addition of 0.5 ml of 0.1% 2,4dinitrophenylhydrazine (crystalline, Sigma) dissolved in
2 M HCL. Heating was continued for 4 min and the
hydrazone of the resulting glyoxylic acid was formed.
The tubes were cooled and alkalified with 2.5 ml 2.5 M
NaOH. Absorbance was read at 520 nm after 11 min.
Standard solutions of 50 ␮M allantoin, 0.83 ␮M urea
and 1.2 ␮M uric acid were used. The small interference
due to uric acid or urea could then be corrected for.
Urea and ammonia were determined with the ultraviolet method described by Anonymous (1980). An
assay cuvette consisted of 1 ml of 0.5 M triethanolamine
buffer (pH 8.6), 0.1 ml of 6 mM NADH, 1.9 ml distilled
water and 0.1 ml antlion excretory extract. The enzymes
L-glutamate dehydrogenase (0.2 mg, bovine liver type
III, Sigma) and urease (0.05 mg powder, Sigma) were
added. Changes in NADH absorption were measured at
A. Van Zyl et al. / Journal of Insect Physiology 44 (1998) 1225–1231
340 nm. Urea (crystalline, Sigma) and ammonium sulphate (grade I, Sigma) were used as standards. An
extinction coefficient of 6.3 cm2/␮mole was used
(Anonymous, 1980). Samples of a standard solution of
ammonium sulphate, dissolved in 0.6% Li2CO3 were
either preheated at 100°C for 10 min (three replicates),
or kept at room temperature (three replicates) to determine the loss of ammonia during the preparation of the
antlion excretory extracts.
2.1.1. Data analysis
The cumulative body weight of termites used during
the feeding experiment for fed Cueta sp. and fed F.
intermedia larvae did not differ significantly (KruskalWallis test, N ⫽ 31, P > 0.05), and comparisons were
therefore possible. The quantity of dry weight extracted
from the termites during feeding was determined as
described earlier (Van Zyl et al., 1997). The nitrogen
quantity excreted was calculated as the sum of nitrogen
present in the quantities of uric acid, allantoin, urea and
ammonia excreted.
Statistical tests were applied according to Zar (1984).
The Kruskal-Wallis test and nonparametric Tukey-type
multiple comparisons were used to test for differences
between the feeding groups. The Mann-Whitney test was
used to test for differences between the amount of
nitrogenous excretory products within a feeding group.
2.2. Amino acids
The concentrations of the various amino acids in the
excreta were determined in a separate experiment. Seven
third instar Cueta sp. and 14 F. intermedia larvae were
starved for seven days and fed one termite (mean body
weight approx. 20 mg), after which excreta was collected
for two days. The excreta was homogenised in 0.5 ml of
10% iso-propanol (Merck).
The amino acids in the antlion excretory extract were
then derivatised according to Bidlingmeyer et al. (1984)
and Cohen et al. (1984) before further analysis. The
samples were derivatised as follows. Subsamples
(0.25 ml) of the antlion excretory extracts were transferred to 50 mm ⫻ 6 mm tubes. These subsamples were
freeze-dried after the addition of 0.03 ml ethanol: water:
triethanolamine (2:2:1, by volume). They were freezedried again after the addition of 0.03 ml ethanol: water:
triethanolamine: phenylisothio-cyanate (PITC, protein
sequencing grade, Sigma) (7:1:1:1, by volume). After
derivatisation, the samples were left at room temperature
for 20 min. They were then diluted in 0.16 M sodium
acetate buffer (2:1 by volume), dissolved in an ultrasonic
bath and centrifuged for 5 min at 4000 g before a subsample (1–4 ␮l) was used in the analysis. The two
mobile phases of the high pressure liquid chromatography (HPLC) system consisted of 0.16 M sodium acetate
buffer containing 0.05% triethanolamine (99%,
1227
Merck)(pH 6.35) and 61% acetonitrile (ACS reagent,
Sigma). Amino acid concentrations of 25 nM
(Boehringer Mannheim) were used for standardisation.
3. Results
Allantoin was the main nitrogenous product in the
excreta of Cueta sp. and F. intermedia larvae and
occurred in significantly larger concentrations (Fig. 1)
and quantities (N ⫽ 13–17, P ⬍ 0.05) than ammonia,
urea and uric acid in each species. Ammonia was present
in similar concentrations (Fig. 1) and quantities (N ⫽
16–17, P > 0.05) than urea in the excreta of fed Cueta
sp. and fed F. intermedia larvae. The concentrations of
the standard ammonium solutions preheated at 100°C or
kept at room temperature were identical to within
measurement uncertainty, indicating that significant
amounts of ammonia were not lost during the preparation of the antlion excretory extracts. Uric acid
occurred in low concentrations of only 0.35–1.05% of
the allantoin concentrations.
Nine amino acids were detected by high pressure
liquid chromatography in the excreta of Cueta sp. larvae
(Table 1), with lysine and methionine having the highest
concentrations. In the excreta of F. intermedia larvae 13
amino acids occurred in relatively small concentrations,
with lysine and tyrosine exhibiting the highest concentrations (Table 1). Not all the amino acids were present
in the excreta of the individual antlions. For example
valine and alanine were present in the excreta of all the
F. intermedia larvae (100%, Table 1), while alanine was
present in the excreta of all the Cueta sp. larvae, but
Fig. 1. Mean ⫾ standard deviation of the concentrations of the main
nitrogenous excretory products of fed Cueta sp. (n ⫽ 4), fed F.
intermedia (n ⫽ 8) and starved (n ⫽ 8) F. intermedia larvae. Means
with different symbols (*# ⫹) are significantly different at P ⬍ 0.05
between excretory products and between feeding groups.
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A. Van Zyl et al. / Journal of Insect Physiology 44 (1998) 1225–1231
Table 1
Amino acid concentrations (g/l)† identified in the excreta of Cueta sp. (n ⫽ 7) and F. intermedia (n ⫽ 14) larvae
Amino acid
Lysine
Methionine
Tyrosine
Phenylalanine
Valine
Serine
Aspartic acid
Isoleucine
Glutamic acid
Threonine
Alanine
Glysine
Leucine
Total‡
†
‡
Cueta sp.
0.329 ⫾ 0.04
0.319 ⫾ 0.11
0.101 ⫾ 0.06
0.017 ⫾ 0.01
0.062 ⫾ 0.05
0.085 ⫾ 0.08
0
0
0
0.009
0.007 ⫾ 0.004
0
present
0.511
F. intermedia
0.135 ⫾ 0.1
0.030 ⫾ 0.009
0.133 ⫾ 0.262
0.036
0.032 ⫾ 0.019
0.006
0.031 ⫾ 0.001
0.016 ⫾ 0.009
0.011 ⫾ 0.004
0.006 ⫾ 0.003
0.006 ⫾ 0.002
0.005 ⫾ 0.002
present
0.269
(57%)
(57%)
(57%)
(43%)
(36%)
(29%)
0
0
0
(14%)
(100%)
0
(79%)
(14%)
(43%)
(7%)
(100%)
(7%)
(21%)
(71%)
(14%)
(43%)
(100%)
(21%)
Mean ⫾ standard deviation, with in brackets the percentage larvae where the amino acid was detected in the excreta.
Summed amino acid concentrations.
the excreta of only 36% of Cueta sp. contained valine
(Table 1).
3.1. Excretion during starvation and feeding periods
The comparison between the nitrogenous products
excreted during starvation and feeding periods was
restricted to F. intermedia, as both starved and fed larvae
of this antlion species were used in these experiments.
Starved F. intermedia larvae ingested significantly less
dry weight and excreted a significantly smaller volume
of urine during the 36 day starvation period compared
to that of fed F. intermedia larvae during the 14 day
feeding period (Table 2). Starved F. intermedia larvae
also produced excreta less often, namely they did not
produce excreta on eight days (average) during the first
14 days after feeding. Fed F. intermedia larvae did not
produce excreta on five days (average) during the 14 day
feeding period. Starved F. intermedia excreted also significantly smaller quantities of each nitrogenous excretory product than fed F. intermedia larvae (N ⫽ 17–27,
P ⬍ 0.05).
The concentrations of each nitrogenous excretory product did not, however, differ significantly between fed
and starved F. intermedia larvae (Fig. 1). The excreta of
starved F. intermedia larvae comprised both the urine
excreted directly after feeding one termite as well as the
urine excreted during the 36 day starvation period. Comparisons between the concentrations of the excretory
products of starved and fed F. intermedia larvae could
therefore not be directly related to excretion during feeding and non-feeding periods.
There was, however, a tendency as shown by the fact
that the concentrations of the water soluble excretory
products were higher in the excreta of fed larvae i.e.
ammonia was 1.08 times and urea 3.34 times the concentrations of that obtained for starved larvae. The water
insoluble products i.e. uric acid concentration was higher
for starved larvae i.e. 1.41 times the concentration of that
obtained for fed larvae (Fig. 1). The average allantoin
concentration in the excreta of starved larvae was 78.5%
of that in fed larvae.
Van Zyl et al. (1997) demonstrated that when third
instar F. intermedia larvae were offered one sixth instar
H. mossambicus larvae, 44% of the dry weight extracted
from the termites consisted of protein. Starved F.
intermedia larvae ingested 3.4 mg dry weight (Table 2)
and thus approximately 1.51 mg protein and 0.242 mg
Table 2
The volume of urine produced, dry weight ingested and nitrogen excreted by third instar fed Cueta sp. and fed and starved F. intermedia larvae
Fed Cueta sp.
Experimental period (days)
Dry weight ingested (mg)†
Nitrogen excreted (mg) ‡
Volume excreted (␮l)
14
9.3b ⫾ 2.9 (9)
0.272
29.3 b ⫾ 6.8 (9)
Fed F. intermedia
14
9.4b ⫾ 5.0 (14)
0.204
17.6b ⫾ 7.1 (13)
Starved F. intermedia
36
3.4a ⫾ 1.0 (16)
0.080
9.0a ⫾ 3.2 (16)
Mean ⫾ standard deviation, with sample size in brackets. Means with different symbols (a,b) are significantly different at the P ⬍ 0.05 significance level.
‡
Accounted nitrogen calculated as the sum of nitrogen present in the quantities of allantoin, uric acid, urea and ammonia excreted.
†
A. Van Zyl et al. / Journal of Insect Physiology 44 (1998) 1225–1231
nitrogen. When the accounted nitrogen excreted is considered (Table 2), starved F. intermedia larvae excreted
approx. one third of the nitrogen initially fed to them
over the 36 day starvation period.
Sixth instar H. mossambicus larvae consisted 71.8 ⫾
4.7% (n ⫽ 91) of water and third instar F. intermedia
larvae extracted 56–62% of the available water in the
prey (Van Zyl et al., 1997). In the present study starved
F. intermedia larvae extracted therefore approx. 8.5 mg
water from their prey (body weight 19.1 mg). The volume excreted by these antlion larvae (9.0 mg or ␮l,
Table 2) over the 36 days was close to the estimated
volume of water extracted from their prey.
3.2. Excretion by the pitbuilder and non-pitbuilder
When the excretory products of the fed larvae of the
pitbuilder, Cueta sp., and fed larvae of the non-pitbuilder, F. intermedia, were compared, no significant
differences were detected in the dry weight quantities
ingested or the volume of urine produced after feeding
(Table 2). No statistically significant differences exist
between the concentrations (Fig. 1) or the quantities (N
⫽ 17–23, P > 0.05) of ammonia, allantoin and urea in
the urine of fed Cueta sp. and fed F. intermedia larvae
(Fig. 1).
4. Discussion
The present study demonstrated that allantoin was the
main nitrogenous product in the excreta of Cueta sp. and
F. intermedia larvae. The small quantities of uric acid
in the excreta were in contrast to the expectation that
uric acid is the main nitrogenous excretory product of
terrestrial insects (Wigglesworth, 1965; Chapman,
1983). The findings of the present study are also in contrast to the observations that the antlion larva U. nostras
produced ten times more uric acid than allantoin in its
excreta (Razet, 1961 quoted by Bursell, 1967). Urea,
ammonia and amino acids were not determined in the
excreta of U. nostras. A broader comparison of the
excretory products of different antlion species may clarify this discrepancy between U. nostras and Cueta sp.
and F. intermedia larvae.
As uric acid is less soluble than allantoin, its excretion
is usually regarded as an important mechanism to conserve water (Bursell, 1970). It is hypothesised that the
pattern of nitrogen excretion, observed in these antlion
larvae, can be linked to their discontinuous alimentary
canal and the need to eliminate excess water, salts and
nitrogen after feeding rather than the conservation of
water during starvation. The question arises therefore,
which mechanisms are used by Cueta sp. and F. intermedia larvae to conserve water. This will be explained
1229
below, where we find in the larval cotton stainer, Dysdercus fasciatus Signoret a basis for comparison.
D. fasciatus also has a discontinuous alimentary canal
(Berridge, 1965a) and allantoin is the dominant nitrogenous excretory product with a concentration of 0.7–2.0 g/l
in its liquid excreta after feeding (Berridge, 1965b). This
is lower than the estimated 21.1–26.9 g/l in the excreta
of Cueta sp. and F. intermedia larvae. Some of the allantoin in the antlion’s excreta was probably not in solution,
but was excreted, as a salt as Bursell (1967) reported the
solubility of allantoin as 0.6 g/l in water.
Berridge (1965b) argued that allantoin, rather than
uric acid, is the dominant excretory product of D. fasciatus as allantoin is ten times more soluble in water than
uric acid (Bursell, 1967). Allantoin, due to its solubility,
can attain high concentrations in the haemolymph of D.
fasciatus; this ensures rapid filtration of allantoin into
the malpighian tubules. Consequently filtration can occur
without the active transport of allantoin across the malpighian tubule membrane. Filtration can also occur without the associated movement of large volumes of water,
due to the large allantoin concentration gradient between
the haemolymph and the malpighian tubules (Berridge,
1965b). However, D. fasciatus excreted large quantities
of fluid due to the inability of the rectum to reabsorb
water (Berridge, 1965b). In D. fasciatus these large
quantities of fluid are needed to eliminate excess ions,
while there is little recycling of water within the insect
(Berridge, 1965b).
For Cueta sp. and F. intermedia larvae this argument
of Berridge (1965b) may be valid during periods of food
abundance when large quantities of fluid must be eliminated, especially as Cueta sp. and F. intermedia larvae
ingested large quantities of nitrogen i.e. nucleic acids
and proteins (Van Zyl et al., 1997). However, during
starvation periods the elimination of nitrogen as allantoin
rather than uric acid would seem to be disadvantageous.
Under these circumstances urine is retained in the rectal
pouch where reabsorption takes place through the modified epithelium of the rectal fold and the six cryptonephric malpighian tubules which are laterally displaced on the rectal pouch (Van Zyl, 1995).
Wigglesworth (1965) demonstrated this experimentally
by injecting dyes into Myrmeleontidae larvae. As fluids
entered the perinephric space, some were absorbed by
the modified epithelium and returned to the haemocoel.
As the excreta of starved F. intermedia larvae consisted
of urine produced directly after feeding as well as during
the 36 day starvation period, the present study didn’t
demonstrate the effect that reabsorption of water from
the rectal pouch would have on the concentrations of
the excretory products. The smaller volumes excreted by
starved larvae over 36 days compared to the urine produced by fed F. intermedia larvae over 14 days can be
in part the result of reabsorption, but in part also the
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A. Van Zyl et al. / Journal of Insect Physiology 44 (1998) 1225–1231
result of the smaller quantities of water ingested by
starved larvae.
However, during starvation F. intermedia larvae can
diminish water loss, not only through adjustments made
in excretion, but also by closing the spiracles to minimise evaporation. It has been previously reported that a
low relative humidity of 22% had no significant influence on the body weight of F. intermedia (Van Zyl,
1995). This is an important feature as low relative
humidities (20–65%, Van Zyl, 1995) are characteristic
of their natural environment. Youthed (1973) proposed
also that Myrmeleon obscurus Rambur and Cueta lanceolatus larvae actively control water loss by closure of
the spiracles, as the rate of water loss at 0% relative
humidity increased after death in these larvae.
It is suggested that in F. intermedia larvae the water
loss through excretion and evaporation during starvation
is replaced by water derived from lipid catabolism and
through reabsorption from the stored urine. Water
derived from lipid catabolism also contributes to the
water gain of these larvae, because F. intermedia
depends on lipids as an energy resource during starvation
(Van Zyl, 1995). It is therefore suggested that the elimination of excess ions, nitrogen and water during periods
of food abundance, rather than the conservation of water
during starvation, is the critical factor for Cueta sp. and
F. intermedia larva. This is probably an important reason
why allantoin, and not uric acid, is the dominant
nitrogenous excretory product.
The quantities of urea in the excreta of Cueta sp. and
F. intermedia can probably be regarded as metabolic byproducts, as suggested by Cochran (1985) for other
insects. The presence of ammonia is not such an uncommon phenomenon, as Cochran (1975) stated that
ammonia excretion should pose no problem for terrestrial insects voiding a wet excreta. Ammonia contributes
only 3.6% and 4.0% of the nitrogen in the excreta of fed
Cueta sp. and fed F. intermedia larvae, respectively.
This in contrast to the aquatic neuropteran larva, S. lutaria, which can afford to excrete 90% of its nitrogen as
ammonia (Staddon, 1955). When water is limited, as in
the terrestrial adult stage, S. lutaria excretes predominantly uric acid (Staddon, 1955). The availability of
water is thus crucial for ammonia excretion.
The wide range of amino acids in the excreta of Cueta
sp. and F. intermedia larvae is not unusual for insects.
For example fifteen amino acids were observed in the
excreta of the crane-fly, Tipula paludosa Meigen
(Griffiths and Cheshire, 1987), seven in the excreta of
Rhodnius prolixus Stal (Wigglesworth, 1965) and six in
the excreta of D. fasciatus (Berridge, 1965b). C. carnea,
Chrysopa cubana and Chrysopa rufilabris Burmeister
eliminated an adhesive substance during the larval stage
(Spiegler 1962b) containing ␣-amino acids which is an
end product of nitrogen metabolism (Spiegler, 1962a).
Finally, as the non-pitbuilder, F. intermedia was twice
as large as the pitbuilder Cueta sp. it can be concluded
that neither the pitbuilding and non-pitbuilding lifestyles
nor the differences in body weight influence excretion to
a large extent. Feeding conditions seem to be the major
influence on the excretory products.
Acknowledgements
We thank the Department of Microbiology of the University of the Free State for technical assistance and the
use of their facilities; J. Grimbeek of the University of
Pretoria for his statistical advice, J.D. Mitchell, L.H. van
Zyl and F. D. Duncan for comments on earlier drafts
and the Department of Zoology and Entomology of the
University of Pretoria for the use of their facilities in
preparing the manuscript. The Foundation of Research
Development provided financial support.
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