ORIGINAL PAPER

Evidence suggests that modified setae of the crab spidersStephanopis spp. fasten debris from the background

Felipe M. Gawryszewski

Received: 15 April 2013 / Revised: 13 November 2013 / Accepted: 18 November 2013

� Springer-Verlag Berlin Heidelberg 2014

Abstract Crab spiders (Thomisidae) are known by their

ability to change their body colouration via change in

epithelial pigments. However, the crab spider genus Step-

hanopis appears to match the colouration of the bark they

are sitting on by having debris attached to its dorsal cuticle.

The functional morphology, colouration, and evolution of

this phenomenon were investigated in Stephanopis cf.

scabra and S. cambridgei. Analysis under the microscope

revealed that debris originated from the bark they were

sitting on. Using scanning electron microscopy, three dif-

ferent types of setae likely related in the retention of debris

were found in S. cf. scabra and one in S. cambridgei. These

setae are branched and possess barbs, unlike the more fi-

liform setae found in other crab spider species. In addition,

the presence of debris improved the brightness background

matching of spiders against the bark, but not hue and

chroma matching. Ancestral character state reconstruction

suggested that presence of debris evolved two to three

times within Thomisidae. The evolution of both masking

and colour change among crab spiders indicates that they

are under a strong selection to avoid detection.

Keywords Masking � Camouflage � Thomisidae �Background matching � Crypsis

Introduction

Colouration patterns that conceal individuals are wide-

spread in animals (Cott 1966). Their ubiquity reflects a

strong selection pressure for avoidance of predators and/or

reduced detection by prey. In addition, the variety, and

sometimes lack of agreement, in the nomenclature for the

different strategies employed by animals to avoid being

detected or recognised (Stevens and Merilaita 2009) is also

an indication of the many different mechanisms and path-

ways associated with the evolution of concealment.

In several species of animals, individuals can change

their body colouration either over a few days (morpho-

logical colour change) or in a matter of minutes/seconds

(physiological colour change) (Cott 1966). In even more

intriguing cases, some animals reduce detection or recog-

nition by adding structures from the environment to their

bodies, a strategy termed masking (Cott 1966). Crabs of the

family Majidae add several different components of their

habitat (substrate debris, algae, sessile invertebrates) to

specialised hooked seta on their exoskeleton (Wicksten

1978, 1993). The nymphs of several bug species (Hemip-

tera, Reduviidae) have specialised seta and trichomes to

attach environmental debris and prey corpses (Weirauch

2006). The coat of environmental debris in these bug

species reduces the probability of detection by their ant

prey (Brandt and Mahsberg 2002), whereas the layer of

prey corpses on bugs reduces their chance of being preyed

upon by salticid spiders and other predators (Brandt and

Mahsberg 2002; Jackson and Pollard 2007).

The presence of exogenous material (soil, sand, debris,

etc.) on the cuticle has been described in some spider

families: Sicariidae (Duncan et al. 2007), Homalonychidae

(Roth 1984; Duncan et al. 2007), Microstigmatidae (Raven

and Platnick 1981), Paratropididae, Zodariidae, and

Communicated by A. Schmidt-Rhaesa.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00435-013-0213-4) contains supplementarymaterial, which is available to authorized users.

F. M. Gawryszewski (&)

Department of Biological Sciences, Macquarie University,

North Ryde, NSW 2109, Australia

e-mail: f.gawry@gmail.com

123

Zoomorphology

DOI 10.1007/s00435-013-0213-4

Pisauridae (Platnick and Forster 1993) (Mascord 1970;

Platnick and Forster 1993). In the best studied case, the

genera Sicarius Walckenaer, 1847 and Homalonychus

Marx, 1891 possess setae with protruding ‘hairlettes’. These

hairlettes are long and very thin (10–40 nm) and seem to

trap sand particles via intermolecular adhesion (Duncan

et al. 2007). Camouflage is generally the proposed function

of the masking behaviour found in animals. However, most

studies lack any objective test of the function of this

behaviour and whether or not they improve the degree of

camouflage.

Thomisidae, commonly known as crab spiders, is a

family of sit-and-wait spiders that exhibit a remarkable

degree of background matching (Mascord 1970). Species

that ambush pollinators on flowers often match the colour

of the flowers (at least to human eyes) (Mascord 1970). In

addition, these spiders have the ability to, over a few days,

change their colour and match the colour of the flowers.

(Packard 1905; Gabritschevsky 1927; Thery 2007). The

crab spider genus Stephanopis Cambridge 1869 commonly

inhabits tree bark (Mascord 1970). Similarly to their rela-

tives, these spiders appear to match the colour of the bark

they are sitting on (Rainbow 1893; Mascord 1970; pers.

obs.). However, Stephanopis seem to be employing a dif-

ferent, or additional, background matching strategy. Some

individuals have been observed with debris attached to

their bodies (Mascord 1970, pers. obs.). These species have

a high density of setae and tubercles (Cambridge 1869;

Koch 1874), which may be involved in the retention of

debris. The mechanism that these spiders use to fasten

debris to their body, the kind of debris attached and its

effect on background matching is not known. In this study,

(1) the structures related to the retention of debris in

Stephanopis were investigated, (2) the effect of debris on

the colouration match between spider and bark was mea-

sured (background matching), and (3) the evolution of

presence of debris in crab spiders was analysed.

Methods

Study species

Thomisid spiders are found in flowers, tree bark, leafs,

stems, and leaf litter. This study focused on the species

Stephanopis cf. scabra L. Koch 1874, Stephanopis cam-

bridgei Thorell, 1870 (Fig. 1; ESM Figure 1), Thomisus

spectabilis Doleschall, 1859, Diaea evanida (L. Koch,

1867), Tharpyna campestrata L. Koch 1874, Tharpyna cf.

munda L. Koch, 1875, and Sidymella rubrosignata (Koch

1874) (ESM Figure 1). Species were identified using a tax-

onomic key (Ono 1988), comparison to specimens deposited

at the Australian Museum and a guide (Mascord 1970).

Stephanopis cf. scabra, Stephanopis cambridgei, Thar-

pyna campestrata, and Tharpyna cf. munda are bark-living

species, Thomisus spectabilis and Diaea evanida are found

on flowers, and Sidymella rubrosignata are found in green

shrubs and trees, seldom on flowers (Mascord 1970). From

a recent study, in a total of 638 specimens collected, S. cf.

scabra (N = 10), S. cambridgei (N = 1), T. campestrata

(N = 7), and T. cf. munda (N = 10) were found exclu-

sively on barks. Thomisus spectabilis (N = 207) and Diaea

evanida (N = 182) were found exclusively on flowers, and

Sidymella rubrosignata (N = 16) was collected using a

sweep net from the vegetation. It has not been found on

barks (Gawryszewski 2011). Recent molecular and mor-

phological phylogenies indicate that the genus Stephanopis

is paraphyletic. S. cambridgei and S. cf. scabra are more

closely related to some species of the genus Sidymella than

to each other (Benjamin 2011; Gawryszewski 2011).

In this study, Stephanopis cf. scabra and Stephanopis

cambridgei (Fig. 1) were collected in Hervey Bay,

Queensland (latitude -25.290�, longitude 152.892�), and

near Sydney, New South Wales, Australia (latitude

-33.713�, longitude 149.971�). Individuals were searched

for and hand-picked from tree bark. A piece of the bark

near the spider was also collected for spectrophotometric

measurements. For each individual, the presence of debris

on the spider body was checked and compared to the bark

where the spider had been collected using a dissecting

microscope. Spider age (juvenile, sub-adult, or adult) and

sex were also determined. Three specimens not used for

scanning electron microscopy were deposited at the Aus-

tralian Museum (KS.121424, KS.121425 and KS.121426).

Description of structures related to the retention

of debris in Stephanopis spp.

In order to describe the structures related to the retention of

debris, scanning electron microscopy (SEM) was per-

formed on S. cf. scabra individuals with debris (N = 3;

two adult females and one sub-adult female) and without

debris (N = 2; adult males) and one individual of Step-

hanopis cambridgei with debris (adult female). These

specimens were compared to other species of crab spiders

(Thomisidae): Thomisus spectabilis (n = 1; adult female),

Diaea evanida (n = 1, adult female), Tharpyna campe-

strata (n = 1; adult female), Tharpyna cf. munda (n = 1;

adult female), and Sidymella rubrosignata (n = 1; adult

female). Specimens were preserved in 70 % ethanol.

Samples were dehydrated through successive steps of

15 min in solutions of 80, 90, and 100 % (twice) ethanol,

followed by 1:1 ethanol:hexamethyldisilazane (HMDS)

and 100 % HMDS (three times). Specimens were left to air

dry for 48 h and gold coated (Emitech K550 Gold Sputter)

for SEM imaging (JEOL JSM 6480 LA).

Zoomorphology

123

Seta density for each specimen was calculated from two

top view SEM images of the prosoma, at approximately

1409 magnification. Density of seta was calculated by

counting number of seta and dividing it by the estimated

image area, using software ImageJ 1.47v (US National

Institutes of Health, Bethesda, USA). Only seta related in

the retention of environmental debris in Stephanopis spp.

and their counterparts in other crab spider species were

analysed. Other specialised setae such as trichobothria,

proprioceptor setae near articulations and chemosensory

setae (Barth 2002) were not investigated.

Analyses of the effectiveness of debris as camouflage

To examine the effectiveness of debris as camouflage, the

spectrophotometric properties of Stephanopis spp. with and

without debris were compared to the properties of the tree

bark where spiders were collected from. The reflectance

spectrum of the dorsal spider opisthosoma and bark were

measured using a USB2000 spectrophotometer (Ocean

Optics Inc., Dunedin, USA) connected to a light source

(PX-2 light source, Ocean Optics Inc., Dunedin, USA) and

an optical fibre probe. The probe consists of six illumi-

nating fibres around one reading fibre (400 ± 8 lm

diameter fibres; Ocean Optics Inc., Dunedin, USA). The

probe was positioned 45� above the samples using a probe

holder (RPH-1 Probe Holder, Ocean Optics Inc., Dunedin,

USA). The centre of the probe was positioned at approxi-

mately 3 mm above the samples, which gives a measure-

ment area of approximately 6 mm2 (area calculated

assuming an optic fibre acceptance angle of 25.4�; Ocean

Optics Inc., Dunedin, USA; http://www.oceanoptics.com/

Products/fiberspecs.asp). The reference spectrum was taken

using the WS-1 Diffuse Reflectance Standard (Ocean

Optics Inc., Dunedin, USA;[98 % reflectance from 250 to

1,500 nm) or the WS-1-SL Diffuse Reflectance Standard

(Ocean Optics Inc., Dunedin, USA; [96 % reflectance

from 250 to 2,000 nm). The dark spectrum was taken from

the black background (black cotton cloth) against which

the measurements were done. Five measurements from

each spider/bark were averaged. A slight reflectance, from

650 to 700 nm, of the black standard used for the mea-

surements created a drop in reflectance data in this range.

This was fixed by substituting the original raw black

standard reference spectrum after 650 nm by an adequate

dark reference spectrum collected later, from a black

plastic block. Next, reflectance spectrum was recalculated

using the raw reflectance spectrum, original white refer-

ence spectrum, and the corrected black reference spectrum

(for formula, see Ocean Optics Inc 2005).

There are several models that try to estimate how a

reflectance spectrum is perceived by different animals (e.g.

Chittka 1992; Vorobyev and Osorio 1998). However, the

precise prey and predators that potentially are the selective

forces on Stephanopis colouration are not known. For this

reason, a more conservative approach of extracting colour

parameters (brightness, hue, and chroma) directly from

reflectance spectra was used. The relative total brightness

of spiders and barks was calculated by integrating their

reflectance spectra (i.e. calculating the area below the

spectrum) and dividing it by maximum total brightness

(brightness of a 100 % reflective spectrum; Montgomerie

2006). Hue was defined as the wavelength at maximum

reflectance (Montgomerie 2006). Chroma was calculated

by subtracting minimum reflectance from the maximum

reflectance and then dividing the result by the average

reflectance across all wavelengths (Andersson et al. 2002;

Montgomerie 2006).

To test whether or not spider colouration was correlated

to the bark colouration, a linear regression or a spearman

Fig. 1 Photographs under the dissecting microscope of a Stephanopis cf. scabra, alive, and b Stephanopis cambridgei, ethanol preserved

Zoomorphology

123

correlation test between the spider (dependent variables)

and bark was performed. The effect of the presence of bark

debris on spider colouration was tested by comparing spi-

der colour of individuals with debris to individuals without

debris, using a t test or a Mann–Whitney test. Bark colour

parameters of spiders with debris were compared to bark

colour parameters of spiders without debris using a t test or

a Mann–Whitney test. Finally, the effectiveness of debris

in terms of background matching was tested by comparing

the absolute difference between spider and bark brightness,

hue, and chroma, for individuals with and without debris,

using a t test or a Mann–Whitney test. Use of separated

tests instead of a full ANOVA model was opted because of

the co-variance between the presence of debris, the bark,

and spider colours. The dependent variables were ln-

transformed, when necessary, to fulfil statistical assump-

tions. Statistical tests were validated visually by looking for

deviation from normality of residuals, heterogeneity, and

violation of independence (Zuur et al. 2009). Nonpara-

metric tests were used when parametric assumptions could

not be fulfilled.

Ancestral character state reconstruction

Phylogenetic trees available from the literature were used

to reconstruct ancestral character states of presence/

absence of exogenous material on the cuticle of Thomisi-

dae species. One morphology-based phylogeny (Benjamin

2011) and two molecular phylogenies were used (Benjamin

et al. 2008; Gawryszewski 2011). From Benjamin et al.

(2008), the majority rule consensus tree available was used.

Presence of debris on the cuticle was coded as follow: (0)

presence debris has never been reported, (1) presence of

debris has been reported, and (unknown) for species, within

genus Stephanopis and Borboropactus, from which there is

no data on the presence of exogenous material. This was

done because data on the presence of debris from species

within these genera are limited. Data on Borboropactus, for

instance, state that species within this genus possess debris

attached to their bodies, but do not give any specific spe-

cies name (Platnick and Forster 1993). Trees were pruned

to remove identical taxa. Ancestral states for each phylo-

genetic tree were estimated using the likelihood recon-

struction method (Markov k-state 1 parameter model—

Mk1) in Mesquite 2.75 (Maddison and Maddison 2011).

Results

General patterns

Debris was found on the dorsal side of pedipalps, legs,

prosoma, and opisthosoma of Stephanopis species. There

was no debris attached to the ventral side of spiders

(n = 14). Debris was only attached to the body areas where

specialised setae were present (Fig. 2). This was evident

when comparing the prosoma of one specimen without

environmental debris (Fig. 2a) to a specimen with debris

(Fig. 2b; ESM Figure 2). Debris was firmly attached to

those setae. Setae were damaged when debris was removed

with a brush or forceps. Under the dissecting microscope,

spider debris was similar in colour and shape to the debris

found on the surface of the piece of bark from where each

spider was collected. This suggests that the debris found on

spiders was acquired from the tree bark where spiders were

collected from. Debris was found attached to specialised

setae in both sexes, and sub-adult and adult spiders.

Description of the seta morphology of Stephanopis spp.

and other crab spider species

Three types of seta related to the retention of debris were

identified in Stephanopis cf. scabra (Figs. 2, 3; ESM Fig-

ures 2 and 3) and one type in Stephanopis cambridgei

(Fig. 4; ESM Figure 4). The seta sockets were tightly

attached to the seta shaft. As a consequence, their ability

for displacement was probably limited (Fig. 3; ESM

Figures 3 and 4). These setae were remarkably different

Fig. 2 SEM images of Stephanopis cf. scabra prosoma with a small and b large amounts of debris; c detail showing areas with high density of

type I seta. White arrows point to type II setae, and black arrows point to type III setae

Zoomorphology

123

from the setae on the ventral side of spiders (ESM Fig-

ure 5) and to the setae of other crab spider species (ESM

Figures 5 and 6). The cuticle areas without setae presented

scaly patches of muscle attachment while cuticle areas with

setae presented a smoother surface (Figs. 2, 4).

Stephanopis cf. scabra type I setae were the most

common (x ± SD; 343 ± 63 seta/mm2; four images, two

individuals) and distinguished setae associated with the

retention of environmental debris. They were distributed on

the dorsal side of pedipalps, legs, prosoma, and opistho-

soma. On the prosoma, they were densely concentrated on

defined patches (Fig. 2; ESM Figure 3b). Specially in a

patch spanning from the medial to the anterior prosoma

(ESM Figure 3b). They were approximately 80 lm long

Fig. 3 On top, illustration of

Stephanopis cf. scabra and type

I, II, and III setae (dorsal view).

Grey areas denote patches

where type I setae were found in

high densities. Type III setae

were especially concentrated on

the distal opisthosoma. SEM

images showing a type I, b &

c type II, d type III seta on the

distal opisthosoma, and e area

with high concentration of bark

debris around a type III seta

Zoomorphology

123

and 8 lm in diameter. Their sockets projected setae par-

allel and in close contact with the cuticle. The lateral and

ventral sides presented long branches (approximately

6.5 lm long) projecting from the main seta shaft (Fig. 3;

ESM Figure 3). The debris seemed firmly attached to type I

setae (Fig. 3a).

Stephanopis cf. scabra type II setae were located on the

pedipalps, legs, prosoma, and opisthosoma. They were

found alongside the type I seta. On the prosoma, they were

distributed in rows, radiating from the median prosoma and

on six prosomal humps at intermediate position, three at

each side of the carapace. Type II seta sockets were located

on a protrusion of the cuticle, which positioned these setae

higher than type I setae. They were less common than type I

setae (36 ± 14 seta/mm2; four images, two individuals). In

addition, they were curved, forming an arc above the cuti-

cle. The dorsal side of type II seta presented small barbs—in

relation to seta size—whereas the ventral side presented a

high density of longer barbs (Fig. 3). These setae were

approximately 100 lm long and 15 lm in diameter.

Fig. 4 Top left, illustration of Stephanopis cambridgei debris seta. SEM images showing S. cambridgei debris setae of a variables sizes and

length/width ratios (arrows); b dorsal side of the seta; c ventral side of the seta; and d area of the prosoma of S. cambridgei without debris

Zoomorphology

123

Stephanopis cf. scabra type III seta was the least

common type (9 ± 1 seta/mm2; four images, two indi-

viduals). They were found in several areas of legs, ped-

ipalps, prosoma, and opisthosoma, but were specially

concentrated on the distal part of the opisthosoma

(Fig. 3). The sockets projected the setae in an upward

position (90� perpendicular from the cuticle), above other

types of setae. In addition, on the prosoma and opistho-

soma, type III setae were present on top of a protrusion of

the cuticle (Fig. 3). Frequently in these protrusions, sev-

eral (7 or 8) type III setae were found together (Fig. 3).

Their branches were symmetrically distributed and pro-

jected upward following the seta length. Their size varied

from 45 to 100 lm long and 10–15 lm in diameter. They

were rigid and appeared black under the dissecting

microscope.

Stephanopis cambridgei setae were cuneiform with

asymmetrically distributed barbs (similarly to S. cf. scabra

type II setae). On the dorsal side they were striated, with

small protuberances on the distal area. The number of

barbs increased in the direction of the seta ventral side, to

the point that the venter was composed of a dense number

of barbs (Fig. 4). They were densely distributed

(218 ± 33 seta/mm2; two images, one individual) and

varied in length (35–135 lm), width (15–65 lm) and in

length/width ratio (2–5) (Fig. 4; ESM Figure 4).

In comparison to Stephanopis spp., setae from other

species of crab spiders were filiform (ESM Figure 5 and 6).

They lacked branches and did not present barbs similar to

Stephanopis spp. seta (ESM Figure 5). The exceptions

were Tharpyna campestrata and T. cf. munda legs that

contained setae with distinctive long barbs at alternate

sides of the shaft (ESM Figure 5). Both species of Thar-

pyna also inhabit tree bark. Furthermore, the density of

setae was lower than in Stephanopis spp.: Thomisus

spectabilis (27 ± 12 seta/mm2; two images, one individ-

ual), Diaea evanida (51 ± 9 seta/mm2; two images, one

individual), Sidymella rubrosignata (63 ± 6 seta/mm2;

two images, one individual), Tharpyna campestrata

(72 ± 6 seta/mm2; two images, one individual), and T. cf.

munda (136 ± 15 seta/mm2; two images, one individual).

Effectiveness of debris as camouflage and ancestral

character state reconstruction

A total of 13 individuals of Stephanopis cf. scabra and one

of Stephanopis cambridgei were collected. Half of these

individuals had debris attached to their bodies. The

reflectance spectra of spiders and bark showed that indi-

viduals with debris had a lower overall reflectance (i.e. they

tended to reflect less light across all wavelengths) than

individuals without debris (Fig. 5; ESM Figure 7). Bark

reflectance also followed this same pattern, i.e. bark where

spiders with debris were collected from had a lower overall

reflectance than bark of spiders without debris. In addition,

average spectrum of spiders with debris seemed more

similar to their bark spectrum than the spectrum of spiders

without debris to their bark spectrum (Fig. 5; ESM

Figure 7).

There was a positive significant relationship between

spider and bark brightness (adj. R2 = 0.42, F1,12 = 10.58,

p = 0.007; y = 4 ? 52*x; Fig. 6), but not for hue (Spear-

man; N = 14, q = 0.36, S = 291.95, p = 0.21) and

chroma (adj. R2 = 0.11, F1,12 = 2.57, p = 0.13). The

overall brightness and chroma of spiders with debris was

lower than spiders without debris, but above the statistical

threshold of 0.05 (brightness: t12 = 1.92, p = 0.078;

chroma: t12 = 1.80, p = 0.096; Fig. 7). Hue of spiders was

similar regardless of the presence of debris (Mann–Whit-

ney, N = 14, W = 17.5, p = 0.40; Fig. 7). The overall

brightness of barks where spiders were collected from was

lower for spiders with debris than without (t12 = 2.56,

p = 0.025; Fig. 7), but not hue (Mann–Whitney, N = 14,

W = 32, p = 0.36). Chroma of barks of spiders with debris

was lower than bark of spiders without debris, but not

statistically significant (t12 = 1.90, p = 0.082; Fig. 7). The

absolute difference between spider and bark brightness was

significantly lower for spiders with debris than for spiders

300 400 500 600 7000

510

1520

2530

35Wavelength (nm)

Ref

lect

ance

(%

)

No (N = 7)

300 400 500 600 700

05

1015

2025

3035

Wavelength (nm)

Ref

lect

ance

(%

)

Yes (N = 7)Fig. 5 Mean reflectance spectra

of Stephanopis spp. (S. cf

scabra, n = 13 and S. cf.

cambridgei, n = 1) with and

without debris. Black and white

circles represent spiders and

barks, respectively. Black and

grey error bars represent

standard deviations of spiders

and barks, respectively

Zoomorphology

123

without debris (t12 = 2.23, p = 0.045; Fig. 7), but not hue

(Mann–Whitney, N = 14, W = 29.5, p = 0.54; Fig. 7) and

chroma differences (t12 = 0.54, p = 0.60; Fig. 7).

Ancestral character state reconstruction over all three

phylogenetic hypotheses indicated independent evolution

of exogenous material on the cuticle at least twice within

Thomisidae. Presence of debris probably has evolved three

times within this family if presence of debris is confirmed

in some of the taxa classified as ‘unknown’ (ESM

Figure 8).

Discussion

Crab spiders are well known for their ability to change

colour in order to match the background (Packard 1905;

Gabritschevsky 1927; Thery 2007; Thery and Casas 2009).

In Stephanopis, however, a completely different strategy

has evolved in order to achieve colour change, namely the

retention of debris. Both Stephanopis cf. scabra and S.

cambridgei had bark debris attached to their bodies. Debris

was fastened to specialised setae—hereafter debris setae—

on the dorsal side of spiders. Areas without setae, most

showing scaly structures typical of muscle attachments

(Foelix 1996), and areas with non-specialised setae (i.e.

venter) did not carry debris. Stephanopis was found

exclusively on trees and debris found on their bodies came

from the bark surface. Debris setae were remarkably dif-

ferent from the setae of other crab spider species that do not

have debris on their bodies and to ventral setae of Step-

hanopis spp.

Functional morphology of crab spider setae

There is some degree of convergence of seta morphology

among different animal taxa, especially the ‘hairlettes’ of

the non-closely related Sicarius and Homalonychus spiders

(Duncan et al. 2007). Nonetheless, the setae found in ants

(Holldobler and Wilson 1986), crabs (Wicksten 1978,

1993), Sicarius and Homalonychus (Duncan et al. 2007),

and Stephanopis spp. spiders are notably different, despite

having a similar function. This variation in setae mor-

phology could be the result of selection for retaining more

efficiently different types of particles.

Stephanopis spp. did not show any structure similar to

the hairlettes found in Sicarius and Homalonychus (Duncan

et al. 2007). There was also no evidence of an association

of Stephanopis setae with glandular pores such as in

Reduviidae (Weirauch 2006). However, simply applying

debris on top of a spider does not cause debris to be

attached to the cuticle (pers. obs.). This, together with the

fact that debris is firmly attached to the spider body, sug-

gests that some sort of adhesion may occur (e.g. use of

silk). Future behavioural experiments and specific histo-

logical techniques in order to search for glandular pores

could clarify this question. Furthermore, setae in spiders

are usually innervated (Barth 2002). In Stephanopis spp.,

especially long setae such as type III in Stephanopis cf.

scabra, could be important mechanosensors. Analysis of

the nerve cells associated with these setae could help

revealing other functions of debris seta.

In Stephanopis, seta barbs and branches (type I and II in

S. cf. scabra and seta from S. cambridgei) likely contribute

to the retention of particles by mechanically fastening

debris. Branches increase the total contact area with par-

ticles, and barbs may pierce particles. High density of seta

probably also contribute to the mechanical retention of

debris (Duncan et al. 2007). Moreover, variation in type

and size of debris setae in Stephanopis may more effi-

ciently hold different sizes of debris, similarly to what have

been proposed for the seta in Basicerotini and Ste-

gomyrmecini ants (Holldobler and Wilson 1986). Type III

seta may not contribute significantly to the retention of

debris because it does not possess long branches or barbs.

In addition, some quantitative differences between male

and female setae may not have been identified in this study

0 5 10 15 20

05

1015

20

Bark brightness (%)

Spi

der

brig

htne

ss (

%)

670 675 680 685 690 695 700

670

675

680

685

690

695

700

Bark hue (nm)

Spi

der

hue

(nm

)

10 15 20 25 30 35

1015

2025

Bark chroma

Spi

der

chro

ma

Fig. 6 Relationship between spider and bark. a Brightness, b hue, and c chroma. Dark circles represent Stephanopis cf. scabra spiders with

debris and white circles spiders without debris. Black square represents S. cambridgei

Zoomorphology

123

because specimens without debris used for SEM were all

males.

In comparison to Stephanopis cf. scabra and S. cam-

bridgei, most seta of other Thomisidae species sampled

lacked branches and barbs. Even in bark-living spider

species (Tharpyna cf. munda and Tharpyna campestrata)

or an closely related genus (Sidymella rubrosignata), dor-

sal setae from prosoma and opisthosoma were distinct from

seta found in Stephanopis spp. In addition, SEM images of

several species of Thomisids (Cebrenninus rugosus,

Xysticus fraternus, Aphantochilus rogersi, Geraesta mkw-

awa, Mecaphesa asperata, and Phrynarachne sp.) from

another study (Benjamin 2011) also did not show the

presence of seta with branches. Interestingly, in this same

study, Borboropactus nyerere (Thomisidae)—genus

known by having debris on their bodies (Platnick and

Forster 1993)—does show branches projecting laterally

from setae (see Figure 26D in Benjamin 2011). Images of

Onocolus sp. and Stephanopis sp. also suggest presence of

branches (see Figures 60D and 69D in Benjamin 2011).

no yes

46

810

1214

16

Spi

der

brig

htne

ss (

%)

no yes

46

810

1214

1618

Bar

k br

ight

ness

(%

)

no yes

24

68

Brig

htne

ss a

bs. d

iffer

ence

(%

)

no yes

675

680

685

690

695

700

Spi

der

hue

(nm

)

no yes

675

680

685

690

695

700

Bar

k hu

e (n

m)

no yes

05

1015

2025

Hue

abs

. diff

eren

ce (

nm)

no yes

1012

1416

1820

2224

Spi

der

chro

ma

no yes

1015

2025

3035

Bark debris present

Bar

k ch

rom

a

no yes

05

1015

20

Chr

oma

abs.

diff

eren

ce

Fig. 7 Box-plots of Stephanopis spiders with (n = 7) and without

(n = 7) bark debris attached to their bodies; top brightness of spiders

and barks and absolute difference between spider and bark brightness;

middle hue of spiders and barks and absolute difference between

spider and bark hue; bottom chroma of spiders and barks and absolute

difference between spider and bark chroma

Zoomorphology

123

Additionally, images of Stephanopis cambridgei and Bor-

boropactus sp. confirm the presence of debris (see Fig-

ure 5D and Figure 6C and Benjamin 2011), and the type of

seta found in S. cambridgei (see Figure 67 Benjamin

2011). Description of Onocolus species—genus closely

related to Stephanopis—did not state presence of debris on

their bodies (Lise 1981).

Effectiveness of debris as camouflage and evolution

of debris retention

The positive relationship between spider and bark bright-

ness and the small differences between spider and bark

brightness, chroma, and hue, suggest that Stephanopis

individuals match the colour of the bark they sit on. In

addition, the presence of debris seemed to improve back-

ground matching, because the absolute difference in

brightness between spider and bark was lower for spiders

with debris compared to spiders without debris (Fig. 7).

Interestingly, individuals with bark debris were found sit-

ting on bark with darker colours (lower brightness),

whereas individuals without debris were found on bark

with lighter colours (higher brightness). Chroma data also

followed this same pattern, although not statistically sig-

nificant. Coupled with the finding that the natural colour-

ation of the spiders is already somewhat similar to the

colouration of lighter barks (Figs. 5, 7), this suggests that

spiders only acquire bark debris when the bark colouration

is different than their own body colouration.

Very little is known about Stephanopis spp. prey and

predators. The only piece of information states that

S. scabra feeds on spiders of the family Salticidae and

Hersiliidae, but does not provide detailed data (Mascord

1970). Salticids (jumping spiders) have well-developed

vision (Foelix 1996; Barth 2002), and therefore, back-

ground matching might increase the chances of Stephano-

pis catching this type of prey. Despite of that, a similar

body colouration to the background does not necessarily

provide a selective advantage (e.g. Brechbuhl et al. 2009).

Moreover, animals can avoid detection and recognition not

only through visual concealment but also by other forms of

camouflage, such as olfactory camouflage (Dettner and

Liepert 1994; Ruxton 2009). The layer of dust present in

the assassin bug Paredocla sp., for instance, is efficient in

reducing detection by ants that the bugs feed on (Brandt

and Mahsberg 2002). Future experiments using potential

Stephanopis prey and predators could provide additional

information on the function of debris in these species.

Topologies of phylogenetic trees used in this study vary.

Borboropactus, for instance, appears as a close clade to

Stephanopis in Benjamin (2008) and Benjamin (2011), but

not in Gawryszewski (2011). Nonetheless, ancestral char-

acter reconstruction analyses indicated that, surprisingly,

presence of exogenous material on the cuticle has evolved

two to three times within crab spiders. Retention of debris

evolved independently in Stephanopis cf. scabra and S.

cambridgei. These species are separated by Sidymella

species, which are not known for masking behaviour. In

this study, for instance, Sidymella rubrosignata did not

show debris and only presented filiform setae. In addition,

another non-closely related genus, Borboropactus, is

known to carry debris on its body (Platnick and Forster

1993). The evolution of both morphological colour change

and presence of debris in Thomisidae suggest that indi-

viduals in this family are under strong selection for

reducing the probability of being detected by predators

and/or prey.

In conclusion, data suggest that Stephanopis cf. scabra

and S. cambridgei have specialised setae that mechanically

fasten bark debris. This phenomenon evolved indepen-

dently within Thomisidae two to three times. The fact that

(1) debris was found only in areas where debris setae were

present and (2) seta with branches and barbs were found

almost exclusively in species known to carry debris is

compelling evidence that the modified setae found in

Stephanopis are associated with the retention of particles.

Future studies on debris setae and on the natural history of

Stephanopis spp. are likely to yield interesting insights into

the evolution of background matching more broadly.

Acknowledgments Author would like to thank Marie E. Herber-

stein for guidance and comments on this manuscript; Matthew Bul-

bert, James O’Hanlon, Marie E. Herberstein, Jasmin Ruch, and Olga

Kazakova for helping with the collection of crab spiders; Debra Birch,

Nicole Vella, and Scott Fabricant for support and guidance during the

SEM imaging; Anne Wignall, Dinesh Rao, Isabel Waga, and anon-

ymous reviewers for comments on this manuscript; and Macquarie

University and the Joyce W. Vickery Scientific Research Fund of The

Linnean Society of New South Wales for financial support.

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