Behavioral
Ecology
The official journal of the
ISBE
International Society for Behavioral Ecology
Behavioral Ecology (2014), 25(4), 975–983. doi:10.1093/beheco/aru080
Original Article
Caterpillar hair as a physical barrier against
invertebrate predators
Shinji Sugiuraa and Kazuo Yamazakib
aGraduate School of Agricultural Science, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan and
bOsaka City Institute of Public Health and Environmental Sciences, 8-34 Tojo-cho, Tennoji, Osaka
543-0026, Japan
Received 22 August 2013; revised 26 February 2014; accepted 11 March 2014; Advance Access publication 9 May 2014.
Predation has led to the evolution of defensive armor in prey species. The dense and long hairs of caterpillars (i.e., lepidopteran
larvae) are generally believed to play an important role as a physical defence against predators. However, few studies have been
undertaken to investigate how hairs protect caterpillars from a predator’s weapons. To determine the importance of caterpillar hairs
as a defensive armor, we observed adults of Calosoma maximowiczi (Carabidae) attacking 5 caterpillar species with different hairiness
under laboratory conditions. Carabids used their mandibles to catch caterpillars and thereafter fed on them. Almost all the larvae of
3 smooth species and a short-haired species were easily caught by carabids during their first attack. However, 53.2% of larvae in a
long-haired species Lemyra imparilis (Erebidae: Arctiinae) were able to escape from carabid attacks. Even when Lemyra larvae were
finally eaten, carabids required a larger number of attacks to catch Lemyra larvae. Dorsal hairs of Lemyra larvae were much longer
than the mandible length of carabid adults for any body size, suggesting that the dorsal hairs can function as a physical barrier against
carabid attacks. To test the hypothesis, we cut the dorsal hairs of Lemyra larvae shorter than the carabids’ mandibles. Cutting the
dorsal hairs of Lemyra larvae resulted in fewer carabid attacks with higher success rates. Therefore, we conclude that long hairs can
protect Lemyra larvae from carabid mandibles. This is the first study to clarify the adaptive significance of caterpillar hair length as a
morphological defence.
Key words: Arctiinae, carabid beetles, defence, predation, predator–prey interactions, tiger moth.
Introduction
When prey species encounter predators, they may use specialized
physical armor to defend themselves (Edmunds 1974). If predators
are injured by the armor or spend too much time breaking through
the armor, they may give up their attack. Therefore, prey species
with defensive armor are more likely to survive than are species
without amour (Edmunds 1974). Some armored prey species may
warn their predators using their defensive devices and body color,
thus influencing predators to avoid the prey. Therefore, various
types of armor have evolved independently purely for antipredator defence in many animal taxa, although no devices are effective against all potential predators (Edmunds 1974). Besides the
presence, the size of the armor may influence the effectiveness of
physical defences against predators. Long spines, for instance, can
increase the survivorship of prey species in some animal groups,
such as rotifers and sea urchins (Halbach 1971; Edmunds 1974).
Whether predators successfully attack such armored prey depends
Address correspondence to Shinji Sugiura. E-mail: [email protected].
© The Author 2014. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved. For
permissions, please e-mail: [email protected]
on the relative sizes of the predator and prey (Edmunds 1974).
However, such relationships have rarely been explored in terms of
prey armor size.
Caterpillars (i.e., lepidopteran larvae) possess various types of
defences against natural enemies (reviewed by Greeney et al. 2012).
The highly conspicuous spines or dense hairs of some caterpillar
species are believed to have evolved purely for antipredator defence
(Greeney et al. 2012; Battisti et al. 2013). For example, Dyer (1995,
1997) and Murphy et al. (2009) indicated that hairs and spines, respectively, could protect caterpillars from invertebrate predators. The
mechanism by which these armors protect caterpillars from predator’s weapons such as mandibles and proboscises is unclear. One can
hypothesise that the hairs are long and/or thick enough to prevent
penetration by predators’ weapons. However, the physical conflicts
between “arms” (predator’s weapons) and “armor” (the hairs or spines
of caterpillars) have remained unexplored. Furthermore, the defensive
success of prey could be influenced by factors other than the presence
of armor. For example, simply being larger could provide protection
through increased effectiveness of behavioral or physical defences
against predators because they are larger relative to their predators
(Remmel et al. 2011; Greeney et al. 2012). Although caterpillar body
Behavioral Ecology
976
size may influence the size of armor (e.g., hair length), the relationship
between caterpillar body size and amour size has not been well established in relation to the effectiveness of morphological defences.
To test the effectiveness of caterpillar hairs as a defence
against invertebrate predators, we observed adult Calosoma beetles
(Coleoptera: Carabidae) attacking the larvae of different moth species under laboratory conditions. Calosoma beetles, which exclusively
hunt lepidopteran larvae and pupae, are the natural enemies of forest
caterpillars such as gypsy moths Lymantria dispar (Linnaeus) (Forsythe
1982; Weseloh 1985). The larvae of tiger moths (Erebidae: Arctiinae),
also known as “woolly bears,” have dense and long hairs on their dorsal and lateral body (Wagner 2009), providing a good model organism for investigating the adaptive significance of caterpillar hairs (e.g.,
Lindstedt et al. 2008). In this study, we first compared the predation
success of Calosoma adults among 5 lepidopteran species (including a
tiger moth species) with different hairiness and body sizes. Second,
we cut the dorsal hairs of tiger moth larvae shorter than the Calosoma
weapons (i.e., mandibles). We then compared the predation success by
Calosoma adults between haircut and non-haircut larvae of the tiger
moth to clarify the effectiveness of dorsal hairs for defence against
predators. Finally, we investigated the relationship between “arms”
(Calosoma mandibles) and “armor” (tiger moth hairs) to clarify how
hairs could protect tiger moth larvae from Calosoma mandibles.
Materials and Methods
Study site and species
To test whether hairs can protect caterpillars from invertebrate predators, we used adults of Calosoma maximowiczi Morawitz (Carabidae)
and larvae of 5 lepidopteran species, Libythea celtis (Laicharting)
(Nymphalidae), Wilemania nitobei (Nitobe) (Geometridae), Orthosia limbata
(Butler) (Noctuidae), Lymantria dispar japonica (Motschulsky) (Erebidae:
Lymantriinae), and Lemyra imparilis (Butler) (Erebidae: Arctiinae). Calosoma
maximowiczi adults hunt caterpillars both on the ground and on the vegetation (Kamata and Igarashi 1995) and are important predators of lepidopteran larvae, such as the beech caterpillar Quadricalcarifera punctatella
(Motschulsky) (Notodontidae) in Japan (Kamata and Igarashi 1995). We
observed 8 Calosoma adults feeding on caterpillars during daytime in a
secondary forest, Hiraoka-kouen, Higashiosaka City, Osaka (34°40′N,
135°39′E, 160–290 m above sea level) in early/middle May 2013
(Figure 1a; Supplementary Table 1). Individuals used their mandibles to
catch and injure caterpillars and thereafter fed on them (Figure 1a).
Larvae of the 5 lepidopteran species that we used were abundant in this study area in May 2013, when Calosoma adults were
most active. When moving among host plants or searching
for pupation sites, the caterpillars (except the specialist species
L. celtis) were frequently found on the ground as well as on the
vegetation (Supplementary Table 2). Although Calosoma adults
were only observed feeding on Orthosia and Lymantria larvae in the
field (Supplementary Table 1), preliminary feeding experiments
indicated that Calosoma adults would potentially attack any species and size of lepidopteran larvae, including the other 3 species
studied here. Lymantria and Lemyra larvae have dense and inflexible hairs on their dorsal and lateral surfaces (i.e., secondary setae;
hairs developed in second- and/or later-instar larvae; Stehr 1987).
Although hairs of Lymantria larvae are known to cause urticarial
eruptions on human skin (Allen et al. 1991), Lemyra larvae do not
possess true urticating setae (threats to humans and other mammals). Libythea and Wilemania larvae have no visible secondary
setae, and O. limbata larvae have only a few flexible and long hairs,
Figure 1
Calosoma maximowiczi individuals attacking lepidopteran larvae. (a) An adult
feeding on a larva of Amphipyra monolitha surnia Felder and Rogenhofer
(Noctuidae) in the field. (b) An adult attacking a larva of Lemyra imparilis
under laboratory conditions. The larva coiled up to defend its ventral side
against Calosoma mandibles in a similar manner to hedgehogs. Scale: 5.0 mm.
which are extremely sparse (Sugi 1987). Therefore, we considered these 3 lepidopteran species as smooth larvae compared with
Lymantria and Lemyra larvae.
Lemyra larvae have longer dorsal hairs (mean 4.9 mm; see Results
for details) than Lymantria (mean 2.2 mm; see Results for details),
although Lymantria larvae have several long hairs (mean 7.2 mm)
on their lateral sides. Because the dorsal hair length of Lemyra and
Lymantria larvae is comparable with the mandible length (weapon
size) of their predator, Calosoma adults (mean 2.3 mm in males and
2.6 mm in females; Table 1), we used these 2 hairy caterpillar species to test the significance of the relationship between “arms”
(Calosoma mandibles) and “armor” (caterpillar hairs). For laboratory feeding experiments, all beetle adults and lepidopteran larvae
were collected from the Hiraoka-kouen secondary forest in early/
middle May 2013. No endangered or protected species were used
in this study.
Experiments
To test whether caterpillar hairs function as a physical defence
against attacks by Calosoma adults, we compared their rates of
attack and predation among 5 lepidopteran species with different
Sugiura and Yamazaki • Caterpillar defence against predators
977
Table 1
Developmental stage and body size of lepidopteran larvae and their predators used in this study
Body weight (mg)
Family
Speciesa
Sexes or instars
N
Mean ± SD
Carabidae
Geometridae
Calosoma maximowiczi
Wilemania nitobei
Nymphalidae
Noctuidae
Erebidae
Erebidae
Libythea celtis
Orthosia limbata
Lymantria dispar japonica
Lemyra imparilis
Lemyra imparilis (haircut)
Male
Female
Middle–late instar
Late instar
Middle–late instar
Middle–late instar
Middle–late instar
Middle–late instar
25
22
47
50
47
48
52
47
482.5 ± 82.2 b (354.7–622.9)
740.4 ± 151.6 a (471.7–1004.9)
142.2 ± 56.8 f (50.2–280.7)
254.3 ± 37.6 cd (148.2–384.8)
227.8 ± 136.4 de (46.3–566.7)
192.0 ± 100.1 ef (33.3–461.0)
255.3 ± 115.0 cd (65.6–514.1)
242.0 ± 90.0 cde (78.6–431.3)
(range)b
Body length (mm)
Mandible length (mm)
Mean ± SD (range)
Mean ± SD (range)
25.5 ± 1.3 (23.0–27.7)
28.2 ± 1.5 (25.9–32.1)
23.1 ± 3.6 (15.7–30.4)
20.9 ± 3.1 (16.0–30.47)
23.8 ± 4.8 (12.6–34.3)
23.8 ± 3.9 (13.8–34.6)
25.1 ± 4.1 (17.4–35.3)
25.2 ± 3.5 (18.4–32.3)
2.3 ± 0.2 (2.0–2.8)
2. 6 ± 0.2 (2.2–2.8)
—
—
—
—
—
—
aLibythea
celtis larvae exclusively feed on leaves of Celtis sinensis Pers. (i.e., specialist), whereas larvae of other species feed on leaves of various plant species (i.e.,
generalist).
bBody weight differed among the species studied (one-way analysis of variance, F
7,330 = 104.8, P < 0.0001). Different letters indicate significant differences
(Tukey’s Honestly Significant Difference, P < 0.05).
hairiness and body sizes. We used 22 females and 25 males of
Calosoma. Before the experiment, we measured the body weights of
Calosoma adults and lepidopteran larvae to the nearest 0.1 mg using
an electronic balance (CPA64, Sartorius Mechatronics Japan K.K.,
Tokyo, Japan). Females were larger than males (Table 1). Ninetyeight percent of Orthosia larvae and all those of the other 4 species
were smaller than Calosoma adults (Table 1).
We conducted a feeding experiment in a plastic case (diameter
200 mm, height 120 mm) under laboratory conditions (22–24 °C).
We placed a Calosoma adult and a lepidopteran larva on the soil
in the plastic case (20 mm soil depth). Although we preliminarily placed some leaf litter on the soil, it reduced the chances that
Calosoma adults encountered caterpillars. Our aim was to compare
the difference in predation rates by Calosoma adults among prey caterpillars, and not detection rates. Therefore, although this experiment may not have completely reflected a natural setting, we simply
placed a predator beetle and a caterpillar on the soil.
During a 10-min period, we observed 1) whether a Calosoma
adult attacked a lepidopteran larva and 2) whether the adult finally
consumed that larva. The larval body parts (ventral/dorsal and
thorax/A1–A5 abdomen/A6–A10 abdomen) initially injured by
Calosoma mandibles were also recorded. When a Calosoma adult
gave up attacking a larva without injuring it, we considered that
the beetle failed to catch (consume) the larva. Furthermore, we
counted the number of attack attempts that Calosoma adults
needed to catch prey using their mandibles when Calosoma finally
consumed the prey. When Calosoma individuals successfully caught
prey during their first attack, the speed of attack was too fast to
measure. Therefore, we did not use the time to attack but the
number of attacks by Calosoma to compare the costs of attack for
the different prey types. Because the experimental space (diameter
200 mm) was very limited, the rate of predation by Calosoma was
probably inflated or exaggerated compared with what would occur
in a more natural setting. For example, under field conditions, caterpillars may escape from Calosoma by dropping from the trees or
taking refuge under litter or herbaceous plants on the forest floor.
In consideration of this factor, we also used the numbers of attacks
necessary to Calosoma adults to catch the prey. Therefore, we investigated both overall predation success and the number of predation attempts necessary to catch prey to determine the foraging
patterns of Calosoma adults.
To test whether long dorsal hairs can protect Lemyra larvae from
Calosoma adults, we trimmed the dorsal hairs of Lemyra shorter than
the Calosoma mandibles using a small razor (Nose Hair Trimmer,
ER-GN30-K, Panasonic Corporation, Osaka, Japan). Although it
was difficult to remove all the hairs of Lemyra larvae, we were able
to cut the dorsal hairs to 1.5 mm. The hairs cut by the razor were
shorter (<1.5 mm) than Calosoma mandibles (range 2.0–2.8 mm;
Table 1). The haircut treatment did not injure the Lemyra larvae. To
test whether the cut hair affected larval activity and/or behavior,
we compared the distance moved and behavior during movement
between haircut and non-haircut larvae. We individually placed
Lemyra larvae on flat ground (i.e., laboratory floor), observed their
behavior, and measured the distance travelled over a 60-s period.
The behavior of Lemyra larvae did not differ between haircut and
non-haircut larvae. The distance travelled after the hair cut (mean
± standard deviation [SD], 569.5 ± 322.6 mm/min, N = 20) was
not significantly different from that of non-haircut larvae (mean
± SD, 575.0 ± 33.5 mm/min, N = 20; t-test, t = 0.053, P = 0.96).
Therefore, we were able to compare the rates of attack and predation by Calosoma adults between haircut and non-haircut larvae in
Lemyra. The body weight of the haircut larvae was not significantly
different from that of non-haircut larvae in Lemyra (Table 1).
We gave 6 types of prey larvae (5 lepidopteran species and haircut Lemyra) to each adult of Calosoma during daytime. One larva
per day was provided. We then conducted the feeding experiment
everyday for a week. To avoid any potential systematic effects of a
prey species used on the previous day on the attack and predation
of another prey by Calosoma adults, we gave each lepidopteran larva
to each adult of Calosoma in random order. When a Calosoma adult
did not attack a lepidopteran larva within the 10-min period, we
conducted the same experiment on a different day.
To investigate the relationship between “arms” (predators’
weapon size) and “armor” (caterpillar hair length), we also measured the hair/mandible length as well as the body weight in
Lemyra, Lymantria, and Calosoma. Because it was difficult to measure the length of mandibles and hairs in live individuals, we used
dead beetles and caterpillars. Calosoma adults (N = 47) were killed
by freezing (< −18 °C) after the feeding experiment and we measured the mandible length to the closest 0.1 mm using slide callipers. Lymantria and Lemyra larvae, which were sampled from the
study site (N = 30/species), were killed by freezing (< −18 °C).
After being weighed on an electronic balance, we randomly sampled 10 hairs from the dorsal hairs of each larva using tweezers.
The length of the dorsal hairs was measured to the closest 0.1 mm
using slide callipers. We also observed the surface of hairs under a
978
stereomicroscope to investigate the defensive structure of hairs in
Lymantria and Lemyra larvae.
The experiments were approved by the Institutional Animal
Care and Use Committee and undertaken according to the Kobe
University Animal Experimentation Regulations. The experiments
also comply with the current laws of Japan.
Data analysis
To examine the effects of prey types on predation success by
Calosoma adults, we used generalised linear mixed models (GLMMs)
with a binomial error distribution and a logit link. Predation success
or failure (1/0) by Calosoma adults was used as the binary response
variable. Prey types (5 lepidopteran species and shorn Lemyra) were
treated as fixed factors. Calosoma individuals were treated as a random effect because the same individuals (N = 47) were used repeatedly for each caterpillar type. When all Calosoma adults consumed
at least one type of prey, a GLMM including all prey species could
not be used because parameters extend to infinity in cases when
all values in a category are 0 or 1. Therefore, we conducted pairwise comparisons using GLMMs only when both prey types were
not entirely consumed, which included the following comparisons:
Lemyra versus Lymantria, Lemyra versus haircut Lemyra, and haircut Lemyra versus Lymantria. GLMMs with a Poisson error distribution and a log link were also used to clarify the effects of prey
type on the number of attacks by Calosoma adults. The number of
attacks that Calosoma adults needed to capture prey was used as
the response variable. Prey type was treated as a fixed factor and
Calosoma individuals were treated as a random effect.
A generalised linear model (GLM) with a binomial error distribution and a logit link was used to clarify which factors determined predation success by Calosoma in the hairy caterpillar Lemyra.
Predation success or failure (1/0) was used as the response variable. Caterpillar (Lemyra) weight, predator (Calosoma) sex, predator
weight, and predator weapon size (Calosoma mandible length) were
treated as fixed factors. To investigate the effects of the fixed factors, a model comparison was conducted using backward stepwise
selection of the fixed factors and Akaike’s information criterion
(AIC; Crawley 2005); the lowest AIC value was used to select the
model that was most appropriate for determining predation success
by Calosoma adults on Lemyra larvae. The difference in AIC between
the model with the lowest AIC value and each other model was
recorded as ΔAIC. Similarly, a GLM with a Poisson error distribution and a log link and a subsequent model comparison were used
to clarify which factors determined the number of attack attempts
that Calosoma adults needed to catch Lemyra larvae.
Finally, linear regressions were used to investigate the relationship between body weight and hair/mandible length in Lemyra,
Lymantria, and Calosoma. All analyses were performed using R version 2.15.1 (R Development Core Team 2012) and the lme4 package (Bates et al. 2011).
Results
Three (6.4%) and 6 larvae (10.6%) of Libythea and Lymantria,
respectively, were not attacked by Calosoma adults during the 10-min
period of each initial trial. However, the same Calosoma individuals attacked the larvae of both species on different days. Therefore,
all larvae of the 5 lepidopteran species were eventually attacked by
Calosoma adults (Figure 2a), although some were able to escape from
the attack (Figure 2b).
Behavioral Ecology
All Calosoma adults (i.e., 100%) successfully attacked Wilemania,
Libythea, and Orthosia larvae (Figure 2b; Supplementary Movie 1).
The dorsal thoraxes or abdomens of larvae were the main locations
injured by Calosoma mandibles (Supplementary Table 3). Almost
all Calosoma individuals were able to successfully catch Wilemania,
Libythea, and Orthosia larvae in their first attack, although a few
individuals required 2 attacks to catch larvae (Figure 3). Although
3 Calosoma individuals failed to catch Lymantria larvae, the others
(93.6%) were able to successfully do so (Figure 2b; Supplementary
Movie 1). Although predation rate by Calosoma adults did not differ between Lymantria (93.6%) and the 3 smooth species (100.0%;
Figure 2b), more attacks were required to catch Lymantria larvae
compared with the 3 smooth larvae (Figure 3).
Despite the extremely high success rates for Calosoma attacks on
4 lepidopteran species (93.6–100.0%), only 46.8% of Calosoma individuals successfully caught Lemyra larvae (Figure 2b; Supplementary
Movie 2). The other Calosoma individuals (53.2%) failed to catch
Lemyra larvae, although they made several attempts (median 3, range
1–30; Supplementary Movie 2). Even when Calosoma individuals
were able to finally catch Lemyra larvae, they required more attacks
to be successful (median 6, range 1–26; Figure 3; Supplementary
Movie 2). Our observations indicated that the dense and long hairs
of Lemyra larvae provided protection from biting by Calosoma mandibles (Figure 1b; Supplementary Movie 2). Because Lemyra larvae
do not have hairs on the ventral bodies, they frequently coiled up
to defend their ventral sides from Calosoma mandibles in a manner
similar to that of hedgehogs (Figure 1b; Supplementary Movie 2).
Although the ventral side of a Lemyra larva was injured by the beetle’s mandibles (Supplementary Movie 2), the proportion of body
parts injured by Calosoma did not differ among caterpillar species
(Supplementary Table 3).
No effects of body fluid and chemicals produced by all caterpillar species on Calosoma behavior could be observed. To determine the level of protection from Calosoma attacks provided by
hairs, we cut the dorsal hairs of Lemyra larvae using a small razor
(Supplementary Movie 3). Cutting these hairs resulted in higher
attack success rates (i.e., 205% increase; Figure 2b; GLMM: intercept, coefficient estimate ± standard error [SE] = –0.1278 ± 0.2923,
z = –0.437, P = 0.662; haircut treatment, coefficient estimate ±
SE = 3.2413 ± 0.7795, z = 4.158, P < 0.001) and fewer attacks
required for Calosoma adults to catch Lemyra larvae (Figure 3;
Supplementary Movie 3; GLMM: intercept, coefficient estimate ±
SE = 1.8712 ± 0.1504, z = 12.44, P < 0.001; haircut treatment, coefficient estimate ± SE = –1.7936 ± 0.1672, z = –10.73, P < 0.001).
Model comparisons were conducted to investigate which factors
determine predation success and the number of attacks by Calosoma
adults. Backward stepwise selection indicated that the model
including only Lemyra weight provided the best fit in the analysis for predation success (Table 2; mean ± SD weight: eaten larvae = 224.0 ± 93.1 mg, N = 22; uneaten larvae = 291.8 ± 129.8 mg,
N = 25). The model including Lemyra weight and Calosoma weight
and another model including Lemyra weight, Calosoma weight, and
Calosoma mandible length had ΔAIC values <2.0 (Table 2). In
addition, the model including Calosoma and Lemyra weights provided the best fit in the analysis for the number of attacks that
Calosoma needed to catch Lemyra (Table 3). The model including
Lemyra weight, Calosoma weight, and Calosoma sex had a ΔAIC
value <2.0 (Table 3). Therefore, large Lemyra larvae were less likely
to be caught and consumed by Calosoma than small larvae.
Mean dorsal hair length was positively correlated with
body weight in both Lemyra and Lymantria larvae (Figure 4).
Sugiura and Yamazaki • Caterpillar defence against predators
Caterpillar species
979
(a)
(b)
Wilemania nitobei
N = 47
100.0
100.0
Libythea celtis
N = 47
100.0
100.0
Orthosia limbata
N = 47
100.0
100.0
Lymantria dispar
100.0
93.6
Lemyra imparilis
N = 47
100.0
46.8
Lemyra imparilis
(hair cut)
N = 47
100.0
95.7
N = 47
0
20
40
60
80 100 0
Attack rates by
Calosoma maximowiczi (%)
20
40
60
***
NS
***
80 100
Predation success by
C. maximowiczi (%)
Figure 2
(a) Attack rates of Calosoma maximowiczi on 6 types of caterpillars (5 lepidopteran species and shorn Lemyra imparilis). (b) Rates of predation success by Calosoma
adults on 6 types of caterpillars (5 lepidopteran species and haircut Lemyra). Statistical significance is shown (GLMMs): *** indicates P < 0.001 and NS
denotes P > 0.05 (see Supplementary Table 4 for details).
Caterpillar species
Wilemania nitobei
N = 47
a
Libythea celtis
N = 47
a
Orthosia limbata
N = 47
a
b
Lymantria dispar
N = 44
c
Lemyra imparilis
N = 22
ab
Lemyra imparilis
(hair cut)
N = 45
0
5
10
15
20
25
Numbers of attacks by Calosoma maximowiczi
Figure 3
Boxplot of the number of attacks that Calosoma maximowiczi needed to undertake to catch caterpillars of 6 types (5 lepidopteran species and haircut Lemyra
imparilis). Thick vertical lines within the boxes show median values. Right and left boxes show the 75th and 25th percentiles, respectively. Whiskers show
values within the 1.5 interquartile range. Circles indicate outliers. Different letters indicate significant differences of the number of attacks that Calosoma
adults needed to undertake to catch larvae differed among caterpillar types (GLMM, see Supplementary Table 5 for details).
Mandible length also increased with body weight in Calosoma
adults (Figure 4). However, the slope of the linear regression for Lemyra was much steeper than that for Lymantria and
Calosoma (Figure 4); that is, the dorsal hairs of Lemyra were
longer than those of Lymantria and the mandibles of Calosoma
for any body size (Figure 4). Furthermore, the dorsal hairs of
the individuals of Lymantria that Calosoma caught easily were
not longer than the beetles’ mandibles (Figure 4). The hairs
of Lemyra larvae were observed to have microscopic barbs
from the base to the tips (Figure 5a,b), whereas the hairs
of Lymantria larvae were not observed to have such barbs
(Figure 5c).
Behavioral Ecology
980
Table 2
Comparisons of models explaining the predation success by Calosoma maximowiczi in Lemyra imparilis larvae
Response variable
Model (fixed variable)a
AICb
ΔAICc
Predation success
L weight
C weight + L weight
C mandible length + L weight
C sex + L weight
C sex + C weight + L weight
C weight + C mandible length + L weight
C mandible length
C weight
C sex + C mandible length + L weight
C sex
C sex + C weight + C mandible length + L weight
C sex + C weight
C weight + C mandible length
C sex +C mandible length
C sex + C weight + C mandible length
64.85
66.26
66.65
66.80
68.00
68.25
68.59
68.59
68.65
68.93
69.94
70.42
70.46
70.56
72.16
0
1.41
1.80
1.95
3.15
3.40
3.74
3.74
3.80
4.08
5.09
5.57
5.61
5.71
7.31
Calosoma maximowiczi adult; L, Lemyra imparilis larva.
bold value indicates the lowest AIC (GLM, coefficient estimate ± SE; intercept, 1.25572 ± 0.773041; caterpillar weight, –0.005391 ± 0.002812).
cThe ΔAIC column shows the difference in AIC between the model with the lowest AIC value and each other model.
aC,
bThe
Table 3
Comparisons of models explaining the number of attacks that Calosoma maximowiczi needed to catch Lemyra imparilis larvae
Response variable
Model (fixed variable)a
AICb
ΔAICc
Numbers of attacks
C weight + L weight
C sex + C weight + L weight
C weight + C mandible length + L weight
L weight
C sex + L weight
C mandible length + L weight
C sex + C weight + C mandible length + L weight
C sex + C mandible length + L weight
C weight
C sex
C mandible length
C sex + C weight
C sex +C mandible length
C weight + C mandible length
C sex + C weight + C mandible length
234.22
236.08
236.22
236.76
237.32
237.84
238.04
239.25
252.88
252.96
253
254.31
254.83
254.88
256.2
0
1.86
2.00
2.54
3.10
3.62
3.82
5.03
18.66
18.74
18.78
20.09
20.61
20.66
21.98
Calosoma maximowiczi adult; L, Lemyra imparilis larva.
bold value indicates the lowest AIC (GLM, coefficient estimate ± SE; intercept, 0.7384981 ± 0.3813254; C. maximowiczi weight, 0.0008605 ± 0.0003958;
L. imparilis weight, 0.0039055 ± 0.0008618).
cThe ΔAIC column shows the difference in AIC between the model with the lowest AIC value and each other model.
aC,
bThe
Discussion
Although larval hairs and microscopic barbs are generally believed
to play an important role as physical defences against predator
attacks, few studies have investigated their effectiveness (Wagner
2009; Greeney et al. 2012). Here, we used adults of Calosoma beetle
as predators to investigate the importance of hairs in Lemyra and
Lymantria larvae. All Calosoma individuals attacked any lepidopteran
larvae (Figure 2a), indicating that they do not reject a particular
prey species before the attack. Our experiments also showed that
long hairs can protect Lemyra larvae from Calosoma mandibles
(Figures 2b and 3), suggesting that the hairs with microscopic barbs
may have a defensive function. Although model comparisons suggested that various factors could influence predation success for
Calosoma adults, the models including Lemyra weight provided the
best fit in the analysis of predation success (Table 2) and of the
number of attacks that Calosoma adults needed to catch Lemyra
larvae (Table 3). Therefore, large Lemyra larvae were more likely to
escape from predator attacks than small larvae (Tables 2 and 3).
However, the predator was able to catch any size of larvae after
they were shorn (Figure 2b). Large Lemyra larvae, which are more
conspicuous to their predators, have much longer hairs than small
larvae (Figure 4). Therefore, Lemyra dorsal hair length, rather than
body size, was important for successful defence against predator attacks. Although Calosoma adults eventually consumed most
Lymantria larvae, they had to attempt more attacks to catch the hairy
larvae of Lymantria than the smooth larvae of Wilemania, Libythea,
and Orthosia (Figure 3). Thus, the shorter dorsal hairs of Lymantria
larvae may also function as a physical barrier against predators.
Hairy and spinous caterpillars are found in at least 5 superfamilies of Lepidoptera—Zygaenoidea, Lasiocampoidea, Bombycoidea,
Papilionoidea, and Noctuoidea—suggesting that larval hairs and
spines have repeatedly evolved in the order (Stehr 1987; Sugi 1987;
Wagner 2005; Greeney et al. 2012). We investigated here 2 hairy
Sugiura and Yamazaki • Caterpillar defence against predators
Length of dorsal hairs / mandibles (mm)
8
981
Lemyra imparilis
N = 30
6
Lymantria dispar japonica
N = 30
4
2
Calosoma maximowiczi
0
0
200
400
600
800
N = 47
1000
Body weight (mg)
Figure 4
The relationship between body weight and mean length of dorsal hairs in Lemyra imparilis and Lymantria dispar japonica (or mandible length in Calosoma
maximowiczi). Each blue rhombus, red circle, and black triangle indicates each individual of Lemyra, Lymantria, and Calosoma, respectively. Lines represent bestfit linear regressions; Lemyra (blue line), y = 0.0062145 × x + 3.1532047, r2 = 0.8453, F1,28 = 153.00, P < 0.0001; Lymantria (red line), y = 0.0019482 × x +
1.6782740, r2 = 0.5749, F1,28 = 37.87, P < 0.0001; Calosoma (black line), y = 0.0005984 × x + 2.0765043, r2 = 0.239, F1,45 = 14.14, P = 0.0005.
Figure 5
Dorsal hairs (i.e., secondary setae) of Lemyra imparilis and Lymantria dispar japonica larvae. (a) Dorsal hairs of Lemyra. (b) Microscopic barbs on dorsal hairs of
Lemyra. (c) Dorsal hairs of Lymantria. Scales: (a) 1.0 mm, (b) 0.1 mm, (c) 1.0 mm.
species belonging to the subfamilies Arctiinae and Lymantriinae
(Noctuoidea: Erebidae), in which all species are known to have
larval hairs (Sugi 1987; Wagner 2009). Larval hairs have probably evolved in the ancestor of each of these 2 subfamilies or
in their common ancestor (cf., Zahiri et al. 2012). Additionally,
many species of the subfamily Arctiinae are known to have larval hairs with microscopic barbs (Stehr 1987; Sugi 1987; Wagner
2009), suggesting that the barbs evolved in an ancestor taxon of
the subfamily Arctiinae. Given that the larval hairs of Arctiinae
and Lymantriinae are likely to be homologous and that the general morphology of the hairs is very similar among all Arctiinae,
the results reported here should not be regarded as a particularity of the 2 species we studied. Hairs of lepidopteran larvae may
also have other functions (Bowers 1993; Greeney et al. 2012). For
example, long hairs can provide sensory inputs triggering behaviors
that reduce predation risk. The hairy larvae of Orgyia leucostigma
(Smith) (Erebidae: Lymantriinae) drop from their host plants in
response to high hair bending velocities and walk away in response
to low hair bending velocities (Castellanos et al. 2011). Lymantria,
which belongs to the same subfamily (Lymantriinae), has also been
observed to drop from plants in response to Calosoma attacks in the
field (Sugiura S, personal observation). Lymantria larvae have several long hairs (mean 7.2 mm) on their lateral surface, which may
function as triggers to escape from Calosoma attacks. When Lemyra
hairs were touched by Calosoma mandibles, Lemyra larvae were frequently observed coiling up (Figure 1b). This behavior suggests that
Lemyra larvae might also drop from their host plants if touched by
their predators, although our experiments were conducted on soil.
Therefore, hairs of Lemyra and Lymantria larvae may not only function as a physical barrier but also as touch detectors when under
Behavioral Ecology
982
predator attack. Caterpillar hairs may also aid in thermoregulation
(Bowers 1993). In Lymantria, larval hairs have been reported to aid
in thermoregulation, enhancing heat gain when basking (Knapp
and Casey 1986) or reducing heat loss (Casey and Hegel 1981).
Because L. imparilis is known to overwinter as middle instar larvae
(Hondo 1981), the thermoregulatory function of hair would aid
growth in the spring.
The relationships between a predator’s weapons and the armor
of a prey species have often been discussed in terms of a coevolutionary arms race (reviewed by Brodie and Brodie 1999; Abrams
2000). However, little supporting evidence has been found in many
predatory–prey interactions (Brodie and Brodie 1999), and the relationship between Calosoma mandibles and Lemyra hairs could not be
considered as part of such an arms race. Because Calosoma uses the
smooth larvae of various lepidopteran species as prey (Figure 2;
Kamata and Igarashi 1995), Lemyra dorsal hairs may not generate a selective pressure on the mandibles of Calosoma adults to
become longer than caterpillar hairs. Lemyra larvae also suffer from
the attack of various other natural enemies (Hondo 2003), suggesting that predation by Calosoma adults may not be the sole selective
pressure on the evolution of hairs in this species. Therefore, an
assemblage of natural enemies may put selective pressure on Lemyra
larvae to develop longer dorsal hairs. For example, vertebrates such
as birds may prey on Lemyra larvae, and birds (except for cuckoos)
do not prefer hairy caterpillars over smooth caterpillars (Heinrich
and Collins 1983; Barber et al. 2008), probably because the handling cost (e.g., time taken to remove hairs) would be higher than
that for smooth caterpillars. Lindstedt et al. (2008) used a hairy caterpillar species, Parasemia plantaginis (Linnaeus) (Erebidae: Arctiinae),
to show that Parasemia hairs may be an effective defence mechanism
against wary bird predators.
Parasitoids such as parasitic wasps and flies are also known to
prey on Lemyra larvae (Hondo 1981, 2003). Parasitoid oviposition
can be prevented by the dense and long hairs of caterpillars (Gross
1993), but more diverse parasitoid species have been reported to
attack more hairy caterpillars than smooth caterpillars (Stireman
and Singer 2003). Hairy caterpillars, which are not very susceptible
to predators, may provide “refugia” (enemy-free space) for parasitoids (Gentry and Dyer 2002; Stireman and Singer 2003). In fact,
a relatively large number of parasitoid species has been known to
attack Lemyra larvae (Kobayashi and Taketani 1993; Hondo 1981,
2003). Although we used a single predator species in this study, previous studies with hairy larvae of other moth species have used various types of predators such as ants, wasps, and bugs to clarify the
effectiveness of prey defences against predator attacks (Olmstead
and Denno 1993; Dyer 1995, 1997; Eisner and Eisner 2000;
Murphy et al. 2009). The variation in defence effectiveness may
result from many interacting factors, including the attack strategy,
size, learning ability, natural history of the predators/parasitoids,
etc. Therefore, predation of Lemyra larvae should be investigated in
predators other than Calosoma to elucidate the selective agents leading to the evolution of larval hairs.
Our study is the first to clarify the adaptive significance of caterpillar hair length, although past research has shown that the
presence of hairs is known to affect the feeding behavior of invertebrate predators (Dyer 1995, 1997). Like hair length, caterpillar
spine length may be also important for physical defences against
predator attack. However, the length of stinging spines (multicellular processes with poison glands) may be less important than that
of harmless ones, probably because their poisonousness alone is an
effective deterrence to predators. Hairs and spines of other animals
such as urchins, sticklebacks, and planktonic rotifers are well known
as physical defences against predators (Edmunds 1974). Similarly,
spines, trichomes, and hairs in vascular plants have also been
proposed to be physical defences against herbivores (Levin 1973;
Eisner 2003). Hair and spine lengths of such animals and plants
may be important defensive characters (Halbach 1971; Edmunds
1974), and the experimental cutting of hairs or spines, such as in
this study, can be the first step in testing this hypothesis.
Supplementary Material
Supplementary material can be found at http://www.beheco.
oxfordjournals.org/.
Funding
This research was partly supported by a Grant-in-Aid for Scientific
Research (26450065).
We thank G. Machado and 3 anonymous reviewers for their valuable comments on the manuscript. We thank S. Fujie and M. Ito for providing several individuals of Calosoma maximowiczi. We also thank K. Takakura for
advice on our analyses.
Handling editor: Glauco Machado
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