Behavioral Ecology Vol. 14 No. 5: 607–611
DOI: 10.1093/beheco/arg053
Accessory gland size influences time to sexual
maturity and mating frequency in the stalk-eyed
fly, Cyrtodiopsis dalmanni
Richard H. Baker, Matthew Denniff, Peter Futerman, Kevin Fowler, Andrew Pomiankowski,
and Tracey Chapman
The Galton Laboratory, Department of Biology, University College London, 4 Stephenson Way,
London, NW1 2HE, UK
Age at first reproduction is an extremely important life-history trait. Several factors such as nutritional state and age-specific
fecundity have been shown to influence time to sexual maturity; however, little work has been done in insects. We addressed this
in a stalk-eyed fly (Cyrtodiopsis dalmanni), by testing the hypothesis that time to sexual maturity is associated with the development
of male internal reproductive structures. We found that sexual maturity was attained after an increased rate of growth in the
accessory glands, several days after mature sperm bundles, and motile sperm were observed in the testes. Although testis
development is essential, the results suggest that accessory gland growth is more closely associated with the time taken to reach
sexual maturity than is testis growth. When we manipulated the growth of testes and accessory glands via a dietary manipulation,
we found that delayed growth rates increased the time taken to reach sexual maturity. Among the delayed individuals, sexually
mature males had larger accessory glands, but not testes, than did immature males. In adult males, mating frequency was
significantly positively correlated with accessory gland size, but not with testis length or body size. We conclude that accessory
gland size is a critical determinant of sexual maturity and male mating frequency in this species. Key words: accessory glands,
Cyrtodiopsis dalmanni, mating frequency, sexual maturity, sexual selection, stalk-eyed fly, testes. [Behav Ecol 14:607–611 (2003)]
ge at first reproduction is an important life-history trait
that varies hugely, ranging from minutes to hours, days,
weeks, and even years (Roff, 1992; Stearns, 1992). Several
factors have been identified that explain some of this interspecific variability, including growth rates, age specific
fecundity, and breeding system (Roff, 1992; Stearns, 1992).
However, the proximate mechanisms determining the time to
sexual maturity within nonvertebrates have not been subject
to extensive study. In one of the few such studies, Pitnick et al.
(1995) used a phylogenetic analysis of 42 Drosophila species to
show that the length of time required to reach sexual maturity
was positively correlated with testis size and sperm length. The
generality of these findings in other invertebrate groups
remains to be established. In addition, data on the causes of
intraspecific variation in time to sexual maturity in insects are
extremely scarce.
In this article, we examine the determinants of the time to
sexual maturity in the stalk-eyed fly, Cyrtodiopsis dalmanni.
Males of this species have highly exaggerated eyespan, and
females show strong mate preference for males with larger
eyespan (Burkhardt and de la Motte, 1988; Hingle et al., 2001;
Wilkinson et al., 1998). Under natural conditions, matings
occur at dusk and dawn, when groups of females gather
together on root hairs controlled by a single male; matings
also occur opportunistically during the day when females are
dispersed (Wilkinson and Reillo, 1994). In the laboratory,
males mate about four to eight times per hour during day
light (Reguera et al., in preparation). Copulation duration is
relatively short, usually lasting less than 60 s (Wilkinson and
Reillo, 1994), and sperm are transferred in a spermatophore
A
Address correspondence to T. Chapman. E-mail: t.chapman@
ucl.ac.uk.
Received 20 May 2002; revised 2 October 2002; accepted 5 October
2002.
Ó 2003 International Society for Behavioral Ecology
(Kotrba, 1996). The spermatophore is produced by secretions
from the accessory glands and constructed within the female’s
reproductive tract during mating (Kotrba, 1996).
Like other holometabolous insects, stalk-eyed flies are
largely postmitotic at adult emergence, with dividing cells
restricted to regions of the gut and gonads. Adult size is fixed
by the amount of resources assimilated during the larval stage
(David et al., 1998, 2000). However, adults must acquire
resources postemergence in order for the internal reproductive structures to grow before sexual maturity is attained. In
stalk-eyed flies, the length of this period has not previously
been determined accurately, although breeding experiments
have normally assigned between 3–6 weeks for males and
females to reach sexual maturity (Baker et al., 2001; Hingle
et al., 2001; Wilkinson and Reillo, 1994).
We started from the hypothesis that the growth and
development of the testes and accessory glands are the most
important factors in determining the time to sexual maturity.
Testes are clearly needed to produce mature sperm. Previous
work in Drosophila has shown that testis size is associated with
the time taken to reach sexual maturity (Pitnick et al., 1995).
Selection for increased testis length in Drosophila hydei also led
to correlated increases in egg-to-adult development time and
time to adult sexual maturity (Pitnick and Miller 2000). Testis
size is associated with reproductive success, with males from
species in which sperm competition is strong having relatively
larger testes (see Gage 1994; Simmons et al., 1999). In D.
melanogaster, a number of male ejaculate accessory gland
proteins (for review, see Chapman 2001; Wolfner 1997), and
accessory gland size itself (Bangham et al., 2002) affects male
reproductive success. Male accessory gland proteins are also
an essential component of male reproductive success in stalkeyed flies, as they form the spermatophore (Kotrba, 1996),
and without this structure, no sperm are transferred to the
female sperm storage organs. Accessory gland products are
therefore critical for male mating success, and it is likely that
608
accessory gland size has a strong effect on the rate at which
spermatophores are produced. We would expect accessory
gland size to be a determinant of male reproductive success in
stalk-eyed flies with high rates of mating, and males possessing
large accessory glands to be at an advantage.
In this study, we first determined the exact time taken to
reach sexual maturity in males and females of C. dalmanni
under our laboratory conditions. We then examined the
growth rates of testes and accessory glands and tested whether
these are correlated with the time taken to reach sexual
maturity. We then tested the hypotheses that delayed growth
of the testes and accessory glands leads to an extended period
of sexual immaturity by manipulating growth using dietary
restriction. Finally, we tested whether testis or accessory gland
size in adult males predicted male mating frequency.
METHODS
Study organism
Flies used in the experiments were from a large laboratory
colony collected in Gombak, Malaysia, in 1993. The flies have
been maintained in large population cages at high density
(more than 200 individuals per cage) with a 1:1 sex ratio. Flies
were fed ground corn medium and kept at 25 C on a 12-h/12-h
light/dark regime. The lighting regime includes a 15-min
‘‘dawn’’ period in which the room is illuminated by a single
60-W bulb. All observations of mating behavior commenced at
the beginning of this dawn period.
Time to sexual maturity
We investigated the length of time taken to reach sexual
maturity in males and females, and examined whether the
time taken to reach sexual maturity was correlated with body
size. Eggs were collected from the stock cages and were
assigned at random to Petri dishes containing 2–20 ml of corn
medium. This procedure resulted in the emergence of adults
with a wide range of body sizes. Newly emerged flies were
collected at 24-h intervals. All flies were measured for body
length, from the front of the face to the tip of the wing, by
using a monocular microscope connected to a computer with
the National Institutes of Health (NIH) Image software
package (version 1.55).
Sexually immature males and females were housed with two
sexually mature individuals (i.e., flies more than 7 weeks of
age) of the opposite sex in circular 500-ml plastic containers.
The base of each cage was lined with tissue paper, on which
females laid eggs. The tissue paper was collected daily and
stored on a moist cotton pad in a Petri dish for 5 days, and
eggs were then examined under a microscope to calculate the
percentage that had hatched. A test individual was defined as
sexually mature once hatched eggs were observed on three
consecutive days. The time to sexual maturity was measured
from the day of eclosion to the first day on which hatched
eggs appeared.
Mating behavior was not used as a indicator of sexual
maturity because it is difficult to continuously monitor male
behavior, and copulations may occur before sperm or seminal
fluids are completely mature. In contrast, all the eggs laid by
a female can be examined, and a hatched egg guarantees
male maturity. A total of 80 males and 80 females were
examined (13 females and five males died before reaching
sexual maturity).
Correlates of time to sexual maturity in males
We examined whether there was an association between testis
or accessory gland growth and the attainment of sexual
Behavioral Ecology Vol. 14 No. 5
maturity. After eclosion, male body length was recorded as
described above. To reduce variation owing to body size,
males below the mean were excluded; flies used were in the
range 6.85–7.50 mm. Experimental males were placed individually in circular 500-ml plastic containers. On the first
day after eclosion (day 0) and every 6 days until day 36,
a sample of 12 males were anesthetized on ice, and their testes
and accessory glands dissected in phosphate-buffered saline
solution. One randomly chosen testis was transferred to a glass
slide and uncoiled. Sometimes the testis broke apart during
uncoiling, and then the second testis was used instead. The
straightened testis was measured by using NIH Image
software. The length of the line that bisected the middle of
the testis was recorded. By using the same technique, we
measured both accessory glands and took their average. We
measured a total of 84 flies from seven time periods.
We also examined the maturation of sperm over the same
period to determine the age at which individualized motile
sperm were first observed. The seminal vesicles of each testis
were put on a glass slide and ruptured, and the sperm gently
stirred. The two slides were examined at 3100 magnification
for the presence of mature sperm bundles and individualized
motile sperm. Samples of six males were measured at 3-day
intervals after eclosion, ending on day 24.
Manipulation of accessory gland and testis growth
We performed an experiment to manipulate accessory gland
and testis growth to examine how this altered the time taken
to reach sexual maturity. Adult flies were collected as above, in
the body size range of 6.85–7.50 mm, and separated into two
groups. Half were fed standard ground corn throughout
adulthood. The other half alternated between 24 h on corn
and 24 h on sugar water (250 mg sugar/ml water), which
restricted their protein intake. Both groups were housed in
1500-ml containers in single-sex groups of 10 males. The
treatment was continued for 21 days, at which point males
from both groups were placed individually in 500-ml containers with two mature females. After this time, all flies were fed
standard corn food. Time to sexual maturity was then assessed
for all males as described previously. After 36 days, all males
(n ¼ 69), regardless of whether they had reached sexual
maturity, were dissected, and their testes and accessory glands
were measured. Day 36 was chosen as a time at which all males
under normal conditions would be expected to have reached
sexual maturity. In addition, 20 males from each treatment
group were dissected and measured during the experiment
on day 24 to compare testes and accessory glands at a fixed
time point.
Correlates of male mating success
We investigated whether the size of male testes or accessory
glands influenced male mating frequency. Experimental flies
were collected as eggs from the stock population. Larvae were
randomly assigned to Petri dishes containing 2–20 ml of corn
medium to produce a wide range of body sizes. Emerging
adults were segregated by sex and allowed to reach sexual
maturity over 7–8 weeks. Body length and eyespan (from the
outer edge of each eye) of all individuals were measured. All
the females used in the experiment were provided food ad
libitum as larvae.
To measure male mating frequency, individual males were
housed with four virgin females in circular 1500-ml containers
that included a central roosting string, a moist tissue paper
base, and a food tray. To acclimatize them to the containers,
males were placed with females 3 days before the beginning of
the observations. We recorded the total number of matings in
Baker et al. Time to sexual maturity in a stalk-eyed fly
609
a 1.5-h period after and including dawn, as this is the period
in which most matings occur under natural and laboratory
conditions (Reguera et al., in preparation; Wilkinson and
Reillo, 1994). Copulations lasting less than 30 s were omitted
from the analysis as these do not normally result in
spermatophore transmission (Wilkinson and Reillo, 1994;
Wilkinson GS and Reillo PR, unpublished data). Observations
were conducted for five consecutive days, and the total
number of matings for each male was determined. The
experiment was divided into three blocks (sample sizes 40, 30,
and 40, respectively), each separated by approximately
a month. A total of 110 males were observed. After the
mating observations, males were isolated for 1 week and
anesthetized on ice, and their testes and accessory glands
measured as described above. A total of 71 individuals were
measured for both variables.
RESULTS
All statistical analyses were conducted by using JMP software,
version 3.6 (SAS Institute, 1997). Data are presented throughout as mean 6 SD unless specified.
Time to sexual maturity
The time to sexual maturity data could not be normalized;
therefore, a nonparametric analysis was used. Females (mean ¼
22.24 6 4.03 days after eclosion, n ¼ 67) reached sexual
maturity earlier than did males (mean ¼ 25.32 6 2.91 days
after eclosion, n ¼ 75; Mann Whitney U ¼ 953, p , .001).
Body length (female mean ¼ 6.33 6 0.40 mm, n ¼ 67; male
mean ¼ 6.67 6 0.83 mm, n ¼ 75) was not significantly
correlated with time to sexual maturity in either sex (Spearman
rank correlation coefficients: females, q ¼ 0.199, p ¼ .107;
males, q ¼ 0.097, p ¼ .406).
Correlates of time to sexual maturity in males
Both testes and accessory glands increased in size from after
eclosion until sexual maturity was reached (Figure 1a). Testis
growth was relatively greater earlier in development, whereas
accessory gland growth predominated later in development.
Examination of the fit of the accessory gland versus testis
length data (Figure 1b) shows that the data are better
explained by an exponential (R 2 ¼ .86) than by a linear (R 2 ¼
.80) regression model (F1,81 ¼ 9.857, p , .01; Zar, 1999). In all
six individuals, mature sperm bundles were present in the
testis 12 days after eclosion. Motile sperm were not evident in
any of the seminal vesicles examined between days 12 and 18.
At day 21, five out of the six individuals had motile sperm in
their seminal vesicles, and at day 24, all flies had motile sperm.
Manipulation of accessory gland and testis growth
Restricting the amount of protein in the adult diet increased
the length of time before sexual maturity was reached.
Treating males that were not sexually mature on day 36 as
missing data, we found that corn-fed males matured earlier
than sugar-fed males (corn-fed mean ¼ 26.41 6 3.07 days
after eclosion, n ¼ 32; sugar-fed mean ¼ 29.67 6 3.09 days
after eclosion, n ¼ 15; Mann Whitney U ¼ 104.5, p ¼ .002).
This difference was also significant when sugar-fed males that
were not mature on day 36 were given a score of 37 days (cornfed mean ¼ 26.73 6 3.54 days after eclosion, n ¼ 33; sugar-fed
mean ¼ 33.44 6 3.90 days after eclosion, n ¼ 36; MannWhitney U ¼ 137.5, p , .001).
Restricting the amount of protein in the adult diet also
reduced the growth of the testes and accessory glands. In flies
sampled on day 24 after eclosion, sugar-fed males had
Figure 1
(a). Accessory gland and testes lengths against time in days from
adult eclosion. Data presented are mean 6 SD. Top line indicates
testis growth; bottom line, accessory gland growth; and vertical dotted
line, mean time to sexual maturity (days). (b) Growth of testes
relative to accessory glands. Mean accessory gland length is plotted
against mean testis length for each sample. Twelve males were
dissected and measured at each time point. An exponential line was
fitted to the data (y ¼ 0.1971 e0.4477x, R 2 ¼ .856).
significantly smaller accessory glands and testes than did
control corn-fed males (sugar-fed accessory gland mean ¼
0.99 6 0.25 mm, n ¼ 20; corn-fed accessory gland mean ¼
1.37 6 0.22 mm, n ¼ 21, t ¼ 5.349, p , .001; sugar-fed testis
mean ¼ 3.82 6 0.42 mm, n ¼ 21; corn-fed testis mean 4.19 6
0.52 mm, n ¼ 21, t ¼ 2.305, p ¼ .026). The same comparisons
on day 36 after eclosion again revealed that accessory glands
and testes were smaller in sugar-fed males (sugar-fed accessory
gland mean ¼ 1.78 6 0.37 mm, n ¼ 35; corn-fed accessory
gland mean ¼ 2.33 6 0.35 mm, n ¼ 26, t ¼ 5.848, p , .001;
sugar-fed testis mean ¼ 3.92 6 0.39 mm, n ¼ 35; corn-fed
testis mean 4.34 6 0.47 mm, n ¼ 26, t ¼ 3.842, p , .001).
By day 36 after eclosion, all but one of the corn-fed males
were sexually mature, whereas only 14 of the 35 sugar-fed
males were sexually mature. This allowed us to test whether
the delay in time to sexual maturity was associated with
reduced size of the testes or accessory glands in day 36 sugarfed males. Sexually mature sugar-fed males possessed significantly larger accessory glands than did immature males
(mature mean ¼ 2.07 6 0.33 mm, n ¼ 14; immature mean ¼
1.60 6 0.26 mm, n ¼ 21, t ¼ 4.722, p , .001) (Figure 2);
however, there were no differences between the two groups in
testis size (mature mean ¼ 4.06 6 0.38 mm, n ¼ 14; immature
mean ¼ 3.82 6 0.37 mm, n ¼ 21, t ¼ 1.779, p ¼ .084) (Figure
2). A logistic regression analysis, including both accessory
gland and testes as effects, also indicated accessory gland size,
610
Figure 2
For the sugar-fed males, the difference in reproductive gland size
between males that reached maturity within 36 days (solid bars)
and the males that did not reach maturity within 36 days (hatched
bars). There was a significant difference in gland length for the
accessory glands, but not the testes, between these groups. Bars
represent 95% confidence intervals.
but not testis size, was significantly correlated with sexual
maturity (R 2 ¼ .352; accessory gland, p ¼ .004; testes, p ¼ .926).
Correlates of male mating frequency
In the sample analyzed, body length (5.4–7.4 mm) and
eyespan (5.2–9.6 mm) varied considerably and were strongly
correlated (r ¼ .982). Therefore, our model examined the
effect of both body length and variation in eyespan independent of body length. For the latter variable, the
residuals from a regression analysis of eyespan on body length
were used. Males mated an average of 18.76 6 8.02 times over
the 5-day observation period. (range, 1–40; n ¼ 71). Given
that each male was exposed to four females, this works out at
a mean mating rate of 0.94 matings per female per day during
the 1.5-h morning observation period. A general linear model
was used to test for an association among accessory gland,
testis size, body length, and male mating rate, with experimental block as a random effect. Body length and mating rate
were log-transformed before analysis, and the adequacy of the
model was verified by examining normality with Shapiro-Wilk
tests. Accessory gland length (F1,65 ¼ 15.51, p , .001) (Figure
3), but not body length (F1,65 ¼ 0.95, p ¼ .33), residual
eyespan (F1,65 ¼ 0.08, p ¼ .78), or testis length (F1,65 ¼ 0.66,
p ¼ 0.42), showed a significant positive association with mating
rate. There was also a significant effect owing to experimental
block (F2,65 ¼ 6.31, p , .003), reflecting the variable nature of
behavioral traits such as male mating frequency.
DISCUSSION
In this study, we found that male C. dalmanni stalk-eyed flies
reached sexual maturity about 25 days after eclosion (slightly
longer than females, which take about 22 days). Delayed male
maturity has also been found in Drosophila; a survey of 42
species found 29 had males maturing at the same time as or
after females (Pitnick et al., 1995). These findings emphasize
that male and female sexual maturity is time-consuming and
energetically costly. We studied the time taken to reach sexual
maturity by analyzing the growth rates of the testes, where
sperm are produced, and the accessory glands, where a variety
of proteins are produced that go to make up the spermatophore (Kotrba, 1996). Several lines of evidence support the
Behavioral Ecology Vol. 14 No. 5
Figure 3
The relationship between accessory gland length and the total
number of matings achieved by a male during 90-min observation
periods over five consecutive days. Each male was housed individually
with four females. Data shown are the mean length of both accessory
glands. Linear fit (y ¼ 6.716 þ 9.757x, R 2 ¼ .203).
view that the size of the male accessory glands is critical to the
timing of sexual maturity.
At emergence, both testes and accessory glands were very
small. These structures then grew rapidly. The initial growth
phase consisted largely of increases in testes length, whereas
accessory gland growth was more prominent in the latter
phase of maturation (Figure 1). Sexual maturity is attained
during this second phase. Mature sperm bundles (12 days)
and motile sperm (21 days) appeared in the testes before the
flies reached sexual maturity, suggesting that it is the late
burst of accessory gland growth that completes the process of
sexual maturation. This conclusion is further supported by
the results of the diet manipulation experiment. As expected,
flies given restricted access to protein as adults took longer to
reach sexual maturity. Measurements of sugar-fed males 36
days after eclosion (when about half were sexually mature)
showed that sexually mature males had significantly larger
accessory glands than did sexually immature males. Testis size
did not differ between these two groups.
Taken together, these results suggest that the timing of
sexual maturity is critically associated with accessory gland,
but less so for testis growth. Testis growth and the production
of motile sperm are necessary for male fertility. There was also
no evidence in our study that time to sexual maturity was
dependent on body size in males or in females.
In addition to studying time to sexual maturity, we also
studied how reproductive gland size affected male mating
frequency. In adult males, accessory gland size was found to
affect adult male mating frequency. It is not clear how such
a relationship occurs. Probably accessory gland size limits the
number of spermatophores, or some essential spermatophore
component, that a male can produce. It will be important to
investigate the mechanism by which mating frequency and
accessory gland size are linked in future work.
There was no relationship between body or testis size and
mating frequency. Results for testis size indicate that sperm
production is not a limiting step for mating frequency in this
species, or if there is a relationship between some aspect of
sperm production and mating frequency, testis length is not
the correct way to measure it. Our results are consistent with
those of a recent study using D. melanogaster, in which a positive
correlation was found between mating frequency and
accessory gland size but not between mating frequency and
Baker et al. Time to sexual maturity in a stalk-eyed fly
testis size, suggesting that this is a more general phenomenon
in insects (Bangham et al., 2002).
The lack of a relationship in our study between male mating
frequency and body size is puzzling. It is known that males
possessing large eyespan, a trait that is highly correlated with
body size, are preferred by females (Hingle et al., 2001;
Wilkinson and Reillo, 1994;) and succeed better in male-male
competition (Panhuis and Wilkinson, 1999). However, in the
current experiment, we measured male mating frequency in
a noncompetitive environment. This may have biased the measures away from any detection of eyespan effects and instead
recorded a measure of maximum male mating frequency
under noncompetitive conditions. Our results suggest that the
increased mating success of large eyespan males found in
other studies is related primarily to competitive interactions
and female choice rather than to a physiological capacity of
large-eyespan males to mate more often.
In conclusion, our study reports a relationship between
accessory gland growth rates and the development of sexual
maturity, as well as the subsequent importance of accessory
gland size in determining adult male mating frequency.
Further work will focus on identifying the underlying mechanisms involved and in untangling the relationship of these
traits with eyespan and adult feeding conditions.
We thank the National Environment Research Council (research
grant to T.C., K.F., and A.P.), the Royal Society (university research
fellowship to T.C.), and the Department of Biology (University
College London) for financial support, as well as two anonymous
referees for helpful comments.
REFERENCES
Baker RH, Ashwell RIS, Richards T, Fowler K, Chapman T,
Pomiankowski A, 2001. Male effects on female reproductive output
in the stalk eyed fly, Cyrtodiopsis dalmanni. Behav Ecol 12:732–739.
Bangham J, Chapman T, Partridge L, 2002. Effects of body size,
accessory gland and testis size on pre- and postcopulatory success in
Drosophila melanogaster. Anim Behav 64:915–921.
Burkhardt D, de la Motte I, 1988. Big ‘‘antlers’’ are favoured: female
choice in stalk-eyed flies (Diptera, Insecta), field collected harems
and laboratory experiments. J Comp Physiol A 162:649–652.
611
Chapman T, 2001. Seminal fluid-mediated fitness traits in Drosophila.
Heredity 87:511–521.
David P, Bjorksten T, Fowler K, Pomiankowski A, 2000. Conditiondependent signalling of genetic variation in stalk-eyes flies. Nature
4:186–188.
David P, Hingle A, Greig D, Rutherford A, Pomiankowski A, Fowler K,
1998. Male sexual ornament size but not asymmetry reflects
condition in stalk-eyed flies. Proc R Soc Lond B 265:2211–2216.
Gage MJG, 1994. Associations between body-size, mating pattern,
testis size and sperm lengths across butterflies. Proc R Soc Lond B
258:47–254.
Hingle A, Fowler K, Pomiankowski A, 2001. Size-dependent female
mate preference in the stalk-eyed fly, Cyrtodiopsis dalmanni. Anim
Behav 61:589–595.
Kotrba M, 1996. Sperm transfer by spermatophore in Diptera: new
results from the Diopsidae. Zool J Linn Soc 117:305–323.
Panhuis TM, Wilkinson GS, 1999. Exaggerated male eye span
influences contest outcome in stalk-eyed flies (Diopsidae). Behav
Ecol Sociobiol 46:221–227.
Pitnick S, Markow TA, Spicer GS, 1995. Delayed male maturity is a cost
of producing large sperm in Drosophila. Proc Natl Acad Sci USA
92:10614–10618.
Pitnick S, Miller G, 2000. Correlated response in reproductive and
life history traits to selection on testis length in Drosophila hydei.
Heredity 84:416–426.
Reguera P, Pomiankowski A, Fowler K, Chapman T, 2002. Costs of
reproduction in females of two species of stalk eyed flies. J Insect
Physiol (in review).
Roff DA, 1992. The evolution of life histories: theory and analysis.
New York: Chapman and Hall.
Simmons LW, Tomkins JL, Hunt J, 1999. Sperm competition games
played by dimorphic male beetles. Proc R Soc Lond B 266:145–150.
Stearns SC, 1992. The evolution of life histories. Oxford: Oxford
University Press.
Wilkinson GS, Kahler H, Baker RH, 1998. Evolution of female mating
preference in stalk-eyed flies. Behav Ecol 9:525–533.
Wilkinson GS, Reillo PR, 1994. Female preference response to
artificial selection on an exaggerated male trait in a stalk-eyed fly.
Proc R Soc Lond B 255:1–6.
Wolfner MF, 1997. Tokens of love: functions and regulation of
Drosophila male accessory gland products. Insect Biochem Mol Biol
27:179–192.
Zar JH, 1999. Biostatistical analysis, 4th ed. Upper Saddle River, New
Jersey: Prentice Hall.