Proc. Arthropod. Embryol. Soc. Jpn. 47, 11–19 (2012)Ⓒ 2012 Arthropodan Embryological Society of Japan
ISSN 1341–1527

AUTOTOMIC STRUCTURES IN CENTIPEDES11[ORIGINAL PAPER]

Comparative Study of Autotomic Structures in Centipedes(Arthropoda: Chilopoda)
Akihiro MATSUI1) and Kensuke YAHATA2)
College of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305–8572, Japan
Current address: Graduate School of Science, Osaka University, Toyonaka, Osaka 560–0043, Japan
2)
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305–8572, Japan
E-mail: yahata@biol.tsukuba.ac.jp (KY)1)

Abstract
In the present study, autotomic structures of some centipede species are described. In each examined specimen of
Scutigeromorpha, Lithobiomorpha and Scolopendromorpha, discontinuous organization of exoskeletal cuticles was foundwithin the trochanters of walking legs. The position of these exoskeletal rifts corresponded precisely with autotomizing
sites in each species. In the scutigeromorph, Thereuonema tuberculata (Wood, 1863), exoskeletal rifts were found around theentire circumference of the trochanters of walking legs near the trochanter-prefemural articulations. In the lithobiomorphs,
Bothropolys montanus Verhoeff, 1938, B. acutidens Takakuwa, 1941, B. richthofeni Verhoeff, 1938 and Monotarsobius holstii(Pocock, 1895), exoskeletal rifts were present around two thirds of the circumference of trochanters, appearing from thedorsal side through to the ventral and posterior sides at the proximal skeletal annulus of the trochanter, which was locatednear the coxa-trochanteral articulations. In examined specimens of the scolopendromorphs, Scolopocryptops rubiginosus (L.
Koch, 1878) and S. quadristriatus (Verhoeff, 1934), exoskeletal rifts were limited to the dorso-posterior half of the trochanterʼs
circumference. In the geophilomorph, Pleurogeophilus sp., however, no discontinuity of exoskeletal cuticle was observed inany walking leg articles.

Introduction
Autotomy is a behavior seen in animals in which anorganism severs a part of its body in order to escape apredator. Although many species are known to exhibit
autotomic behaviors, studies of specific incidences ofautotomy are limited in some representative groups, e. g.,chordates, echinoderms and decapod crustaceans(Fleming et al., 2007). It has been shown that autotomies
occur at specific sites, and in most cases, that particular
structures designed to facilitate disconnection of distal
body parts exist at these sites. Some of these autotomic
structures are classified as autotomizers or autotomic
rifts, which allow for the rapid detachment of distal parts
from the body, while others are classified as post-autotomic
provisions, which reduce the damage caused by autotomic
injury.
Autotomic reactions are known to occur in many
centipede species (chilopods), however, research into the
autotomic structures of centipedes has been limited.
The class Chilopoda is composed of five orders;the Craterostigmomorpha, the Scutigeromorpha, theLithobiomorpha, the Scolopendromorpha and theGeophilomorpha. In Japan, the latter four orders,

centipedes inhabiting. Among these centipede groups,
limb autotomies are seen most frequently inscutigeromorphs, relatively often in lithobiomorphs,
rarely in scolopendromorphs, and very scarcely ingeophilomorphs (Takakuwa, 1941). It has been observedthat scutigeromorphs autotomize distal parts of walking
legs from the body at trochanter-prefemural articulations,whilst lithobiomorphs and scolopendromorphs do so at
coxa-trochanteral articulations (Takakuwa, 1940, 1941).
Herbst (1891) found thick discoidal diaphragmaticconnective tissue within the trochanteral segment of eachwalking leg in the scutigeromorph species, Scutigera
coleoptrata. According to his research, only a pedal nerve
and two veins extended through the diaphragm, while
neither muscle nor tracheae were present. These
characteristics have led to scutigeromorphan diaphragms
being identified as post-autotomic structures preventing
the outflow of blood while aiding in damage recovery after
limb autotomy. Herbst (1891) also noted the absence ofdiaphragmatic tissues in the walking legs of lithobiomorph
centipedes. No previous studies have presented
information regarding any autotomic structures inscolopendromorph centipedes, nor have they documented

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A. MATSUI AND K. YAHATA

the presence of autotomizers or autotomic rifts in any
chilopodan orders.
From these background, we have been conducting ahistological study of the walking legs of some centipede
species, in order to reveal the morphological or histologicalmechanisms of autotomy (i. e., autotomic structures andidentified autotomic rifts). In this paper, detailed
characteristics of centipede autotomic structures aredescribed and discussed with respect to evolutionaltransitions among chilopodan orders.
Materials and Methods
Centipede species used in the present study aresummarized in Table 1. Specimens were collected fromchannels, underneath the bark of felled trees and leaf litterin Tsukuba and Sakuragawa Cities (Ibaraki Prefecture,
Japan), from August to December, 2010.
To compare limb structure between legs that had
been autotomized and those that had not, several
specimens were made to autotomize some of their walking
legs through artificial manipulation. In order to observe
the skeletal elements of walking legs, several specimensthat underwent autotomy and several that did not were
fixed with 90% ethanol and then treated with 1% potassiumhydroxide at 60 C for several days. Other specimens were
fixed in Bouinʼs solution, dehydrated in a graded ethanoln-butanol series and cleared with 100% methyl benzoate
before being embedded in paraffin. Serial sections ranging
between 5 and 7 µm in thickness were stained with
Mayerʼs hematoxylin and eosin. Exoskeletal articulationsof walking legs were distinguished based on their folding
structures and stainability with eosin, as the sclerotized
cuticle of each article was evidently more eosinophilicthan the flexible inter-articular membranous cuticles.
Names of muscles used in the present paper were basedon Mantonʼs nomenclature (Manton, 1965).
Technical problems encountered during histologicalexamination restricted our analysis to the observation of
the anterior walking legs, thereby preventing the
elucidation of histological information regarding theposterior legs.

Table 1.

Results
Thereuonema tuberculata (Scutigeromorpha)
In the skeletal materials of each walking leg in thescutigeromorph, Thereuonema tuberculata (Wood, 1863), amelanized brownish streak was observed encircling theentire circumference of the trochanter at the section near
the trochanter-prefemural articulation (Fig. 1). In each
autotomized walking leg, this brownish streak formed a
rim around the sectional end of the remaining part of theleg (Fig. 2), suggesting that the distal part of the leg had
been disconnected at the melanized streak.
Exoskeletal cuticles of the trochanters had
thicknesses ranging about 20–30 µm, thicker than the
cuticles associated with other articles, which were only 15µm in thickness (Figs. 3a, 4). In the proximal part of eachtrochanter, the skeletal cuticle formed the proximal
skeletal annulus of the trochanter (PSAT), reached
thicknesses of 40–120 µm, and was especially thick on theventral side (Figs. 3a, 4). In the distal part of the trochanter,the skeletal cuticle reached thicknesses of 30–40 µm (Figs.
3–6).
In the trochanter of each walking leg, the epidermal
layer extended continuously from the proximal part to the
distal part of the leg, while the extension of the exoskeletal
cuticular layer was interrupted at the thickened distal
section immediately proximal to the trochanter-prefemural
articulation constriction (represented by the arrowheads
in Figs. 3–6). These exoskeletal rifts encircled the entire
circumference of each trochanter. That the outer edges ofthese rifts were melanized was apparent due to theirbrownish coloration (Figs. 3–6), which was the same asthat of the melanized streaks observed in the skeletal
elements (Figs. 1–2).
Within the trochanter of each walking leg, thickdiscoidal diaphragmatic connective tissue was found at a
position immediately proximal to the exoskeletal rift andimmediately distal to the PSAT, to which the proximal
trochanteral muscles were adhered (Figs. 3a, 4–6). The
diaphragm adhered tightly to the epidermal epithelium,
and the epidermal epithelial cells adhering to thediaphragm were concentrated and extended (Figs. 3a, 4, 6).

Species of centipedes examined in the present study with corresponding collection data.

Species
Order Scutigeromorpha
Thereuonema tuberculata (Wood, 1863)

Collection data
Tsukuba City, August 2010;
Sakuragawa City, October–December 2010

Order Lithobiomorpha
Bothropolys montanus Verhoeff, 1938
B. acutidens Takakuwa, 1941
B. richthofeni Verhoeff, 1938
Monotarsobius holstii (Pocock, 1895)

Sakuragawa City, October 2010
Sakuragawa City, October 2010
Sakuragawa City, October 2010
Tsukuba City, December 2010

Order Scolopendromorpha
Scolopocryptops rubiginosus (L. Koch, 1878)
S. quadristriatus (Verhoeff, 1934)

Tsukuba City, December 2010
Sakuragawa City, October 2010

Order Geophilomorpha
Pleurogeophilus sp. (species unidetified)Tsukuba City, December 2010

AUTOTOMIC STRUCTURES IN CENTIPEDESThe pedal nerve cord was observed running through thediaphragm (Fig. 6).
The proximal trochanteral muscles, which consistedof the retractor trochanteris and depressor trochanteris,adhered to the proximal part of the PSAT (partly shown in
Figs. 3a, 4, 6), while the distal muscles, the flexores
unquiculi prefemoris, depressor femoris and depressor
femoris anterior, adhered to the distal end of thetrochanteral or prefemural sclerites (partly shown in Figs.
3a, 4, 6; two superficial muscles, the flexores unquiculi
prefemoris and depressor femoris anterior, were omitted
from present figures). The flexores tibiae to trochanter was
the only distal muscle adhered to the apodeme from thetrochanter portion of the anterior coxa-trochanter pivot(Figs. 4, 5). The apodeme was located at a position
immediately distal to the exoskeletal rift and the diaphragm(Figs. 4, 5).

Fig. 1

Fig. 2

Fig. 313

The edges of sectional ends of autotomized walking
legs were encircled by the melanized proximal borders of
exoskeletal rifts exteriorly and terminated at thediaphragms interiorly (as indicated by the arrowheads in
Fig. 7). The diaphragms of autotomized legs shrank along
the distal-proximal axis, but not along the transverse axis
(Fig. 7). The pedal nerve cord was torn at the site of limb
autotomization (Fig. 7).
Bothropolys species and Monotarsobius holstii(Lithobiomorpha)
Autotomic structures observed in the lithobiomorph
species examined [Bothropolys montanus Verhoeff, 1938,
B. acutidens Takakuwa, 1941, B. richthofeni Verhoeff, 1938,
and Monotarsobius holstii (Pocock, 1895)] were similar toeach other, with no substantial differences observed
between them.

Lateral view of the exoskeletal preparation of the proximal part of the 13th walking leg in Thereuonema tuberculata. Dorsal and anterior sides areoriented towards the top and left sides of the image, respectively. Arrowheads indicate a ring consisting of a melanized streak within thetrochanter, immediately proximal to the trochanter-prefemural articulations. cx: coxa, pf: prefemur, tr: trochanter. Scale = 500 µm.
Antero-lateral view of the exoskeletal preparation of the proximal part of the autotomized 13th walking leg in Thereuonema tuberculata. Dorsal
and anterior sides are oriented towards the top and left sides of the image, respectively. Sectional end is encircled by a ring consisting of amelanized streak within the trochanter (indicated by the arrowhead). cx: coxa, tr: trochanter. Scale = 500 µm.
a. Sagittal-section of the 3rd walking leg in Thereuonema tuberculata. Dorsal and proximal sides are oriented towards the top and left sides of theimage, respectively. The boundary between the trochanter and prefemur was distinguished based on other sections. Arrowheads indicate theexoskeletal rift located at the position immediately proximal to the trochanter-prefemural articulation constriction. Thick discoidal
diaphragmatic connective tissue (dp) can be observed immediately proximal to the exoskeletal rift and immediately distal to the proximal
skeletal annulus of the trochanter (PSAT). b and c. Extended parts of dorsal and ventral exoskeletal rift shown in a. Outer edge of the rift ismelanized. ap.cx: apodeme of coxa, pf: prefemur, pn: pedal nerve, tr: trochanter; dep.fe.: depressor femoris, dep.tr.: depressor trochanteris, fl.ti.tr.:
flexores tibiae to trochanter, ret.tr,: a group of retractor trochanteris. Scales = 100 µm in a, and 50 µm in b and c.

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A. MATSUI AND K. YAHATA

Skeletal materials in the walking legs of eachlithobiomorph species examined consisted of a trochanter
composed of a small ring (Fig. 8). Furthermore, no obvious
melanized streak was observed within the examinedtrochanters. In each autotomized walking leg, the sectionalend of the limb was not visible as it had been drawn into thecoxa (Fig. 9).
Exoskeletal cuticles of the trochanters were of thesame thickness as those found in the other articles (Figs.
10a, 11). In the trochanters of Bothropolys acutidens, themajority of exoskeletal cuticles were approximately 10 µm
in thickness. The proximal part of each trochanter formed

Fig. 4

Fig. 5

Fig. 6

Fig. 7the PSAT, while the skeletal cuticle ranged 20–25 µm inthickness and was especially well developed on the ventral
side (Figs. 10, 11). Increased thickness of the distal part ofthe skeletal cuticle was not observed.
In the trochanter of each walking leg of thelithobiomorphs examined, an exoskeletal rift similar to theone observed in the scutigeromorph, Thereuonema
tuberculata, was observed (indicated by the arrowheads in
Figs. 10, 11). The position of the exoskeletal rifts in thelithobiomorphs examined differed from the position
observed in T. tuberculata, as they were located near the
top of the PSAT, immediately distal to the coxa–

Sagittal-section of the 3rd walking leg in Thereuonema tuberculata. Dorsal and proximal sides are oriented towards the top and left sides of theimage, respectively. Arrowheads indicate the exoskeletal rift. ap.cx: apodeme of coxa, ap.tr: apodeme of trochanter, dp: diaphragm, pf: prefemur,
pn: pedal nerve, PSAT: proximal skeletal annulus of the trochanter, tr: trochanter; dep.fe.: depressor femoris, dep.tr.: depressor trochanteris, fl.ti.tr.:
flexores tibiae to trochanter, ret.tr,: a group of retractor trochanteris. Scale = 100 µm.
Horizontal-section of the 1st walking leg in Thereuonema tuberculata. Anterior and proximal sides are oriented towards the top and left sides of
the image, respectively. Arrowhead indicates the exoskeletal rift, which is located at the position immediately proximal to the apodeme of thetrochanter (ap.tr) and immediately distal to the diaphragm (dp). pf: prefemur, tr: trochanter; fl.ti.tr.: flexores tibiae to trochanter. Scale = 50 µm.
Sagittal-section of the 3rd walking leg in Thereuonema tuberculata. Dorsal and proximal sides are oriented towards the top and left sides of theimage, respectively. Arrowheads indicate the exoskeletal rift. Pedal nerve (pn) crosses through the diaphragm (dp). pf: prefemur, tr: trochanter;dep.fe.: depressor femoris, ret.tr,: a group of retractor trochanteris. Scale = 100 µm.
Horizontal-section of an autotomized 2nd walking leg in Thereuonema tuberculata. Anterior and proximal sides are oriented towards the top andleft sides of the image, respectively. Autotomic sectional end is closed by the diaphragm (dp) and encircled by the remaining half of theexoskeletal rift (as indicated by the arrowheads). Pedal nerve (pn) is severed. dep.tr.: depressor trochanteris, ret.tr,: a group of retractor
trochanteris. Scale = 100 µm.

AUTOTOMIC STRUCTURES IN CENTIPEDES

Fig. 8Ventral view of the exoskeletal preparation of the proximal part of the 2nd walking leg in Bothropolys richthofeni. Anterior and proximal sides areoriented towards the top and left sides of the image, respectively. cx: coxa, pf: prefemur, st: sternite, tr: trochanter. Scale = 500 µm.
Fig. 9 Ventral view of the exoskeletal preparation of the proximal part of the autotomized 2nd walking leg in Bothropolys richthofeni. Anterior andproximal sides are oriented towards the top and left sides of the image, respectively. Autotomic sectional end is drawn into the coxa and thetrochanter is not outwardly visible. cx: coxa, st: sternite. Scale = 500 µm.
Fig. 10 a. Sagittal-section of the 2nd walking leg in Bothropolys acutidens. Dorsal and proximal sides are oriented towards the top and left sides of theimage, respectively. Arrowheads indicate the exoskeletal rift within the proximal skeletal annulus of the trochanter (PSAT), at a position
immediately distal to the coxa-trochanteral articulation. b and c. Extended parts of the dorsal and ventral exoskeletal rift shown in a. cx: coxa, pf:
prefemur, pn: pedal nerve, tc: trachea, tr: trochanter; dep.fe.: depressor femoris, dep.fe.a: depressor femoris anterior, dep.tr.: depressor trochanteris,fl.ti.: flexores tibiae, lev.tr,: a group of levator trochanteris. Scales = 100 µm in a, and 50 µm in b and c.
Fig. 11 Horizontal-section of the 3rd walking leg in Monotarsobius holstii. Anterior and proximal sides are oriented towards the top and left sides of theimage, respectively. Arrowhead indicates the exoskeletal rift, which can be observed on the posterior side but not on the anterior side. ap.tr:
apodeme of trochanter, pf: prefemur, tc: trachea, tr: trochanter; dep.fe.: depressor femoris. Scale = 100 µm.
Fig. 12 Sagittal-section of an autotomized 2nd walking leg in Bothropolys acutidens. Dorsal and proximal sides are oriented towards the top and left sidesof the image, respectively. Autotomic sectional end is encircled by the remaining half of the exoskeletal rift (as indicated by the arrowheads).
Pedal nerve (pn) and tracheae (tc) are severed due to autotomization. cx: coxa, tr: trochanter; dep.tr.: depressor trochanteris, lev.tr,: a group oflevator trochanteris. Scale = 50 µm.

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A. MATSUI AND K. YAHATA

trochanteral articulations (Figs. 10a, 11). The presence of
these exoskeletal rifts was restricted to two thirds of thecircumference of the trochanters, ranging from the dorsal
to the ventral, and through to the posterior side.
The proximal trochanteral muscles, which consistedof a group of levator trochanteris and a group of rotator
trochanteris, as well as the retractor trochanteris anddepressor trochanteris, adhered to the proximal part of the
PSAT, just proximal to the exoskeletal rift (partly shown in
Fig. 10a), while two distal muscles, the flexores tibiae and apart of the depressor femoris, adhered to the distal part of
the PSAT, immediately distal to the exoskeletal rift (e. g.,
Fig. 10a). Three other distal muscles, the depressor femoris
anterior and posterior, and the coxa-femur muscle,
however, ran across the trochanter and adhered to thecoxal sclerite (partly shown in Fig. 10a). No diaphragmatic
structures were found within the trochanters of walkinglegs.
In the lithobiomorph species examined, the sectionalends of autotomized walking legs were extremely narrow
and were encircled by exoskeletal rifts (Fig. 12). Theseobservations suggested that the distal parts of walking
legs were disconnected from the proximal parts at theexoskeletal rifts within the trochanter during
autotomization. Three cross-boundary muscles, the pedal
nerve cord, and some tracheae that extended across the
autotomic border were severed during autotomization(Fig. 12).
Scolopocryptops rubiginosus and S. quadristriatus(Scolopendromorpha)
The two scolopendromorph species examined,
Scolopocryptops rubiginosus (L. Koch, 1878) and S.
quadristriatus (Verhoeff, 1934), exhibited no substantial
differences in both general limb structures and structures
related to autotomization.
Skeletal elements in the walking legs of eachscolopendromorph species examined consisted of a shorttrochanter (Fig. 13), which was particularly short on thedorsal side. No obvious melanized streak was observed
within trochanters. In each autotomized walking leg, thesectional end of the limb was not visible as it had drawn
into the coxa (Fig. 14).
In Scolopocryptops rubiginosus, exoskeletal cuticles oftrochanters were thicker than those observed in otherarticles, with thicknesses ranging 15–20 µm. Exoskeletal
cuticles of trochanters were up to 40 µm thick in the dorsal
region and up to 80 µm thick in the ventral region (Fig.
15a). The entire trochanter comprised the PSAT (Fig. 15a).
In the trochanter of each walking leg in the
Scolopocryptops species examined, an exoskeletal riftsimilar to those observed in the former two groups wasfound (indicated by the arrowheads in Figs. 15, 16).
Exoskeletal rifts were limited to the dorso-posterior halfof the circumference of walking leg trochanters. Positionof the exoskeletal rifts seen in Scolopocryptops species
differed from those observed in the scutigeromorph,
Thereuonema tuberculata, and the lithobiomorphs,
Bothropolys spp. and Monotarsobius holstii, as they werelocated somewhat proximally to the top of the PSAT, at a
position located roughly in the middle of the trochanter(Figs. 15, 16). Outer edges of these rifts were melanized,
as indicated by their brownish coloration (Fig. 15b).
In autotomized limbs sectional ends were encircled
by exoskeletal rifts on the dorso-posterior side (indicatedby the arrowhead in Fig. 17), while on the ventro-anteriorside, they terminated at the trochanter-prefemur
articulations, at a position immediately distal to the PSAT
(Fig. 17). These observations in Scolopocryptops spp.
suggested that the autotomizing plane of their walking
legs slant across the trochanter.
Proximal trochanteral muscles, consisting of a groupof levator trochanteris and the depressor trochanteris,adhered to the proximal part of the PSAT, proximal to theexoskeletal rift in the dorso-posterior region (partly shownin Fig. 15a). On the other hand, the distally located ventral
part of the depressor femoris, adhered to the distal part of
the PSAT, immediately proximal to the trochanterprefemural articulation (partly shown in Fig. 15a). Fiveother distal muscles, the depressor femoris anterior, flexores
unguiculi to trochanter, protractor femoris longus, retractor
femoris, and coxa-femural muscle ran across the trochanterand adhered to the proximal end of either the trochanteralsclerite or the coxal sclerite (partly shown in Fig. 16),crossing the autotomic boundary. In autotomized limbs,
these cross-boundary muscles were severed, theirremnants having been observed in the examined
specimens (e. g., dep.fe. in Fig. 17). In addition, a pedal
nerve cord and some tracheae crossed the autotomic
boundary in each walking leg (Figs. 15a, 17). No
diaphragmatic structures were observed within thetrochanters of walking legs.
Pleurogeophilus sp. (Geophilomorpha)
Walking legs of the unidentified Pleurogeophilus
species examined were individually torn at several interarticular parts due to artificial manipulation (e. g., femurtibia, tibia-tarsus; Fig. 18). The sclerotized exoskeletal
cuticular layer of each limb article was smooth andcontinuous throughout the entire limb, from coxa to tarsus(Fig. 19), and after histological examination, no autotomic
structure, such a diaphragm or exoskeletal rift, could be
identified in specimensʼ walking legs.
Discussion
In the trochanters of walking legs in thescutigeromorph, Scutigera coleopterata, the presence ofdiaphragmatic connective tissue, which serves as a postautotomic structure, has been observed (Herbst, 1891). In
the present study, similar diaphragms were found in anotherAUTOTOMIC STRUCTURES IN CENTIPEDES

Fig. 13 Ventral view of the exoskeletal preparation of the proximal part of the 6th walking leg in Scolopocryptops rubiginosus. Anterior and proximal sides
are oriented towards the top and left sides of the image, respectively. cx: coxa, f: femur, pf: prefemur, st: sternite, tr: trochanter. Scale = 500 µm.
Fig. 14 Ventral view of the exoskeletal preparation of the proximal part of the autotomized 6th walking leg in Scolopocryptops rubiginosus. Anterior andproximal sides are oriented towards the top and left sides of the image, respectively. Autotomic sectional end is drawn into the coxa and thetrochanter is not outwardly visible. cx: coxa, st: sternite. Scale = 500 µm.
Fig. 15 a. Sagittal-section of the 4th walking leg in Scolopocryptops rubiginosus. Dorsal and proximal sides are oriented towards the top and left sides of
the image, respectively. Arrowhead indicates the exoskeletal rift within the dorsal section of the proximal skeletal annulus of the trochanter(PSAT). b. Extended part of the dorsal exoskeletal rift shown in a. Outer edge of the rift is melanized. cx: coxa, pf: prefemur, pn: pedal nerve, tc:
trachea, tr: trochanter; dep.fe.: depressor femoris, dep.tr.: depressor trochanteris, lev.tr,: a group of levator trochanteris. Scales = 100 µm in a and 50µm in b.
Fig. 16 Horizontal-section of the 11th walking leg in Scolopocryptops quadristriatus. Anterior and proximal sides are oriented towards the top and left
sides of the image, respectively. Arrowhead indicates the exoskeletal rift, which can be observed on the posterior side of trochanter. pf:
prefemur, tr: trochanter; dep.fe.: depressor femoris, dep.fe.a: depressor femoris anterior. Scale = 50 µm.
Fig. 17 Sagittal-section of an autotomized 4th walking leg in Scolopocryptops rubiginosus. Dorsal and proximal sides are oriented towards the top and left
sides of the image, respectively. Autotomic sectional end is encircled by the remaining half of the exoskeletal rift (indicated by the arrowhead)dorsally and by the proximal skeletal annulus of the trochanter (PSAT) ventrally. Pedal nerve (pn) is severed. Torn depressor femoris (dep.fe.),remains at the distal side of the ventral PSAT. cx: coxa; dep.tr.: depressor trochanteris, lev.tr,: a group of levator trochanteris. Scale = 100 µm.

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A. MATSUI AND K. YAHATA

scutigeromorph species, Thereuonema tuberculata,
however, they were not observed in the other centipede
species examined. As such diaphragms seems to be aunique characteristic of scutigeromorphan centipedes.
Scutigeromorphs also have a kind of specialized plasma
cell which plays a role in hemolymph clotting (Hilken et al.,2003); these cells may also function in the autotomizationof scutigeromorph limbs.
In the present study, discontinuous organization of
exoskeletal cuticles within trochanters were identified in
the centipede species examined except for thegeophilomorph species. These exoskeletal rifts wereshared among centipedes which exhibited autotomization
behaviors,
scutigeromorphs,
lithobiomorphs
andscolopendromorphs, but absent in geophilomorphs, which
are not known to autotomize limbs. The position of these
exoskeletal rifts corresponded with the autotomizing sitesof each species. With these results in mind, it was
concluded that exoskeletal rifts are the prerequisite
structure for limb autotomization in centipede species.
Previous authors have noted that autotomization of
distal parts in the walking legs of scutigeromorphs occurs
at the trochanter-prefemural articulations, while, inlithobiomorphs and scolopendromorphs, it occurs at thecoxa-trochanteral articulations (e. g., Takakuwa, 1940,
1941). The present study, however, revealed that
autotomies occur at exoskeletal rifts within the trochanterof all examined scutigeromorph, lithobiomorph andscolopendromorph species, without exception. These
results clearly suggest that past authors misidentified thepositions of autotomizing sites.
Exoskeletal rifts in scutigeromorph species arelocated near the trochanter-prefemural articulations andmost parts of the trochanters remain visible after

autotomization. Exoskeletal rifts in lithobiomorphs, on theother hand, are located near the coxa-trochanteralarticulations. In the lithobiomorph limb, coxal sclerites
overhang the proximal parts of trochanters at the locationof exoskeletal rifts. Trochanters in scolopendromorphs arevery small, and their dorsal sides are hidden by pleurites.
Due to these characteristics, trochanters in lithobiomorphand scolopendromorph autotomized limbs remained only
as small pieces, difficult to detect by outward observation.
Results suggested that frequency of autotomization
events was correlated with the relative sizes of exoskeletal
rifts. Based on our observations, scutigeromorphs, which
autotomized limbs more frequently than the othercentipede groups examined, exhibited exoskeletal rifts
along the entire circumference of the trochanter;scolopendromorphs, which rarely underwent limb
autotomization, exhibited exoskeletal rifts along only halfof the trochanteral circumference; and geophilomorphs,
which are not known to autotomize limbs at all, exhibitedno exoskeletal rifts on the trochanter.
Results also suggested that frequency of autotomies
correlated with characteristics of the internal structures of
the walking legs, such as the organization of both muscles
and tracheae. Autotomy in lithobiomorph andscolopendromorph centipedes, which rarely undergo limb
autotomization, requires tearing of several muscles andtracheae at the autotomic boundary, while inscutigeromorphs, which autotomize more frequently, it
does not require any muscle tearing whatsoever.
Scutigeromorphs are the only centipede group known touse hemocyanin in order to transport oxygen (Mangum etal., 1985), a possible explanation for the lack of tracheae
within their walking legs.
Muscular systems of centipede limbs have beenFig. 18 Ventral view of the exoskeletal preparation of the proximal part of 2nd–4th walking legs in Pleurogeophilus sp. Anterior and proximal sides areoriented towards the top and left sides of the image, respectively. The 2nd and walking leg was torn at tibia-tarsal articulation, while the 3rd at
femur-tibial one. cx: coxa, f: femur, pf: prefemur, st: sternite, ta: tarsus, ti: tibia, tr: trochanter. Scale = 500 µm.
Fig. 19 Horizontal-section of the 12th walking leg in Pleurogeophilus sp. Anterior and proximal sides are oriented towards the top and left sides of theimage, respectively. Within the walking leg, neither an exoskeletal rift nor a diaphragm can be observed. Tarsus is not shown in this section. cx:
coxa, f: femur, pf: prefemur, ti: tibia, tr: trochanter. Scale = 100 µm.

AUTOTOMIC STRUCTURES IN CENTIPEDES

documented in detail by Manton (1965) with consideration
paid to mechanisms of locomotory function. Muscularsystems of scutigeromorph limbs seems to be specialized,
and are distinctive among centipedes (see Table 4 in
Manton, 1965). Scutigeromorphs are the only centipedesthat have no muscle running across the trochanter. Resultsof the present study suggested that the distinctiveness ofthe muscular system in scutigeromoph limbs may
correlate with the autotomic structures. It is possible thatevolutionary transition of the muscular system in limbs
could have allowed scutigeromorphs to shift the site of
autotomization on the trochanter distally, allowing theimpregnable diaphragms to serve as an autotomic damage
reducer.
The class Chilopoda is divided into two subclasses,
the Notostigmophora and the Pleurostigmophora. The
former consists of a single order, Scutigeromorpha. Amongthe pleurostigmophoran orders, the Lithobiomorpha is themost basal clade while the Scolopendromorpha and theGeophilomorpha form a terminal clade. This system ofclassification has been supported by several studies basedon morphological data (e. g., Dohle, 1985; Shear andBonamo, 1988) and molecular data (e. g., Giribet et al.,1999). According to this system, exoskeletal rifts seen invarious centipede species should share a common
evolutionary origin, while the diaphragm reported only in
Scutigeromorpha among myriapod groups should beconsidered an autoapomorphic feature. In addition, it ispossible that the Geophilomorpha lost their exoskeletal
rifts secondarily, a possible cause being their body shape,
which is specialized for interstitial life. Geophilomorphs
are characterized by a long body with many diminutive
legs which is suitable for interstitial life, makes escapefrom predators unlikely. These results suggested asfollows; while scutigeromorphs adapted their specialized

19

characteristics for frequent autotomies in order to avoid
predators inhabiting ground, geophilomorphs secondarily
lost their autotomic structures due to their interstitiallifestyles.
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