Arthropod Systematics & Phylogeny
71 – 78
68 (1)

71© Museum für Tierkunde Dresden, eISSN 1864-8312, 28.02.2010

Structural Features of Eggs of the Basal Phasmatodean
Timema monikensis Vickery & Sandoval, 1998
(Insecta: Phasmatodea: Timematidae)
YOSHIE JINTSU 1, *, TOSHIKI UCHIFUNE 2 & RYUICHIRO MACHIDA 1
1

Sugadaira Montane Research Center, University of Tsukuba, Sugadaira Kogen, Ueda,
Nagano 386-2204, Japan[jintsu@sugadaira.tsukuba.ac.jp; machida@sugadaira.tsukuba.ac.jp]
2 Yokosuka City Museum, Fukadadai 95, Yokosuka, Kanagawa 238-0016, Japan[toshiki-uchifune@city.yokosuka.kanagawa.jp]* Corresponding author
Received 19.xi.2009, accepted 12.i.2010.
Published online at www.arthropod-systematics.de on 28.ii.2010.

>

Abstract

Structural features of the eggs of a basal phasmatodean, Timema monikensis Vickery & Sandoval, 1998 (Timematidae) were
examined. The eggs of this species are soft and deposited coated with soil and/or other extraneous particles. The chorion,
which is transparent and weakly sclerotized, is composed of an endochorion and an exochorion. The non-inclined operculum
is located at the anterior pole of the egg. The chorion in the marginal region of the operculum is thinned to form an opercularcollar together with the chorion of the egg body. An inverted triangular micropylar plate is on the ventral side of the egg attached to the opercular collar. The micropylar plate is without external differentiations but is specialized inside the chorion.
A single micropyle, with a simple funnel-shaped chorionic opening, occurs on either side of the micropylar plate. The posterior mound, located at the posterior pole, is a thickened chorion rich in fine vertical striations, and the serosal cuticle beneath is thickened and highly specialized. The eggs of Timematidae were characterized and compared with those of Euphasmatodea and Embioptera. A phylogenetic discussion is presented, strongly supporting the assemblage of Timematodea,
Euphasmatodea and Embioptera as monophyletic.

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Key words

Timema, Timematidae, Phasmatodea, Embioptera, egg structures.

1.

Introduction

The genus Timema Scudder, 1895 (Timematidae),endemic to western North America, is usually considered the basalmost clade of Phasmatodea, i.e., thesister group of the remaining Phasmatodea (cf. BRADLEY & GALIL 1977; WHITING et al. 2003; BRADLER etal. 2003; BRADLER 2009). ZOMPRO (2004, 2005), in
contrast, suggested a close affinity between Timema
and Embioptera, based on several features including
egg structure. However, information on Timema eggs
remains sketchy, although there have been studies by
CLARK SELLICK (1997, 1998) and SELLICK (1997) on theeggs of Timema cristinae Vickery, 1993, T. douglasi
Sandoval & Vickery, 1996, T. chumash Hebard, 1920

and T. podura Strohecker, 1936 and by ZOMPRO (2004,
2005) on the eggs of T. podura. JINTSU et al. (2007) andJINTSU & MACHIDA (2009) examined in detail the eggs
of an embiopteran, Aposthonia japonica (Okajima,
1926), and pointed out a close structural resemblance
between the eggs of Embioptera and those of Phasmatodea, suggesting a close affinity between these
two orders. However, their view should be tested bya detailed examination of the eggs of Timema, representing the proposed basalmost clade of Phasmatodea.
Recently, we obtained a number of eggs of Timema
monikensis Vickery & Sandoval, 1998, and examined
their structural features.

72

JINTSU et al.: Egg structure of Timema

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Figs. 1–5. Eggs of Timema monikensis Vickery & Sandoval, 1998. 1: An egg with its coating. Scale bar = 500 μm. 2: An egg justhatching, lateral view, ventral to the left. Scale bar = 500 μm. 3: An egg with the coating removed, containing an early germ band,
ventral view. One can see the protocephalon (arrow) of the embryo in the posterior region of the egg: the embryo takes its positionat the posterior pole of the egg, a little biased to the ventral side. Scale bar = 500 μm. 4: Eggs with the coating removed, containing
an embryo that has acquired a definitive form. 4A: Ventral view. Scale bar = 500 μm. 4B: Lateral view, ventral to the left. Scalebar = 500 μm. 4C: Anterior view, showing the operculum, ventral to the top. Scale bar = 100 μm. 4D: Posterior view, showing theposterior mound, ventral to the top. Scale bar = 100 μm. 5: Micropylar plate and micropyles. Scale bars = 100 μm. 5A: Micropylar plate. Lines X and Y represent levels of sections shown in Fig. 8. 5B: Micropylar plate with paired micropyles, light fieldmicroscopy with transmission illumination. Arrows point to the micropyles. 5C: Enlargement of a micropyle (left one in 5B), differential interference contrast microscopy: two images with different levels in focus are merged into one frame. Scale bar = 50 μm.
Ab = abdomen; EB = egg body; GB = germ band; Mp = micropyle; MpC = micropylar canal; MpP = micropylar plate; Op = operculum; OpC = opercular collar; PM = posterior mound; Th = thorax.

73

Arthropod Systematics & Phylogeny 68 (1)

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Fig. 6. SEM micrographs of eggs of Timema monikensis Vickery & Sandoval, 1998. A: Ventral view. The micropylar plate is notrevealed, implying that it is devoid of external structural differentiation. Scale bar = 500 μm. B: Anterior view, showing the operculum, ventral to the top. Scale bar = 100 μm. C: Micropyle (arrow). This specimen was completely cleaned by treatment with
antiformine for 30 min and 5% KOH for 30 min prior to the processing for SEM, so as to reveal the micropyles; the micropyles
are not detected in the specimen shown in A, which was cleaned only in 30% antiformine for 5 min. Scale bar = 10 μm. D, E: Posterior mound, ventral (D) and posterior (E) views. Scale bars = 50 μm. EB = egg body; Op = operculum; OpC = opercular collar;
PM = posterior mound.

2.

Material and methods

Second or third instar juveniles of Timema monikensis(basically a parthenogenetic species) were collected in
March and April 2009 on Ceanothus and Cercocarpus
leaves in Los Angeles County, California. They werereared on Ceanothus cuttings until they were adults.
Adults were kept individually in petri-dishes with soilbrought back from the T. monikensis habitat, and fed
on Ceanothus cuttings and a mixture of wheat gluten,
sugar and vitamins, enabling females to produce anddeposit their eggs. The eggs were incubated at 20˚C
without humidity control and entered diapause in several months. Eggs in diapause were used in this study.
After being dipped in water for a while, the eggs
were transferred to a 10% solution of dishwashingdetergent (“Joy” by Procter & Gamble) for 1 h, to remove material coating the eggs. Then, the eggs were
immersed in 30% antiformine (4% hypochlorite solution) for 5 min and cleaned in water by means offorceps and a small brush. To observe the micropyles
under SEM, the eggs were further treated in antiform-

ine for 30 min and then in 5% KOH for 30 min, tocompletely remove the coating. The cleaned eggs
were fixed either in Karnovsky’s fixative according to
MACHIDA et al. (1994a,b), or in Bouin’s fixative.
General features of the eggs were observed undera stereomicroscope, Leica MZ12. To observe micropyles by light microscopy, torn egg membranes were
mounted in a lactic acid solution, Heinz liquid, andexamined with a Nikon OPTIPHOTO for light field
images or a Leica DM6000B for differential interference contrast images.
For SEM, the fixed eggs were dehydrated in a graded ethanol series, dried with a critical point dryer, andcoated with gold, and the specimens were observed
under an SEM TOPCON SM-300.
Some of the fixed eggs were processed into 2-μmthick methacrylate sections according to MACHIDA etal. (1994a,b). Sections were stained with 10% Mayer’s acid haemalum at 60˚C, 0.5% eosin G for 1 h anda 0.5% fast green FCF 80% ethyl alcohol solution for
5 min.
Diapause was easily broken by placing the eggs in
wet conditions. Forty to 50 days later, the juveniles
hatched.

74

JINTSU et al.: Egg structure of Timema

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Figs. 7, 8. Sections of eggs of Timema monikensis Vickery & Sandoval, 1998. 7A: Parasagittal section of an egg, ventral to the left.
Scale bar = 500 μm. 7B: Enlargement of the egg membranes of the egg body, differential interference contrast microscopy. Scale bar= 10 μm. 7C: Enlargement of the area around the opercular collar, differential interference contrast microscopy. An arrow shows
the line of detachment that borders the part of the opercular collar derived from the operculum and that derived from the egg body.
Scale bar = 20 μm. 7D: Enlargement of the posterior mound, differential interference contrast microscopy. An asterisk shows theserration in the outer surface of the serosal cuticle. Scale bar = 20 μm. 8: Transverse sections of the micropylar plate through theapproximate levels indicated by lines X (A) and Y (B) in Fig. 5A, differential interference contrast microscopy. Scale bars = 20 μm.
8A: Transverse section of the anterior half of the micropylar plate. The endochorion is thickened. The arrow and arrowhead show
the micropyle and micropylar canal, respectively. 8B: Transverse section of the posterior half of the micropylar plate. The exochorion is thickened and porous in structure. Ab = abdomen; Br = brain; Ch = chorion; EnCh = endochorion; ExCh = exochorion;
Hg = hindgut; Mg = midgut; Op = operculum; OpC = opercular collar; PM = posterior mound; SeC = serosal cuticle; Th = thorax.

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Arthropod Systematics & Phylogeny 68 (1)3.

Results

The eggs of Timema monikensis are coated with soil
and/or other particles ingested by females (Fig. 1).
Figs. 3, 4, 6 show the eggs with this coating removed.
The eggs are ellipsoidal, about 1 mm wide and about
2 mm long, and are soft, and light brown because theegg’s inside is seen through the thin, transparent chorion (Figs. 3, 4A,B).
The operculum (Op) is located at the anterior pole
(Figs. 3, 4A–C, 6B). It is a low dome-shaped disc (Figs.
4C, 6B, 7A) and not inclined, and its marginal region
forms the opercular collar (OpC; Figs. 3, 4A–C, 6A,B,
7A,C) together with the chorion of the egg body. The
operculum is devoid of a knob or capitulum.
By light microscopy, a whitish, inverted-triangulararea is found attached to the opercular collar on theventral side of the egg: the micropylar plate (MpP;
Figs. 3, 4A,B, 5A,B). The micropylar plate does notshow any structural differentiation externally, and is
not recognizable under the SEM (Fig. 6A). Lateral toeach margin of the micropylar plate is a single micropyle (Mp; arrows in Fig. 5B). The two micropyles arefunnel-shaped openings through the chorion, without
any specializations such as a micropylar hood or tube
(Figs. 6C, 8A). From the micropyle, the micropylar
canal (MpC) runs inwards medioposteriorly, acrossthe chorion (Fig. 5B,C).
In this study, we examined about 80 eggs of T.
monikensis, all of which were in diapause. Approximately 90% were at a stage where the embryos had
acquired a definitive form (Fig. 4A,B). In the remaining eggs (10%), the embryos were at the early germ
band stage (GB; Fig. 3). Although diapause occurs ata single, species-specific stage in most insect eggs (cf.
DENLINGER 2002), it is known that one or two diapauses may occur in Euphasmatodea, depending on species
or environmental condition (BEDFORD 1978). Since theventer of embryos in the final stage of development
faces that side of the egg where the micropylar plate
is located (Fig. 4A,B), this plate of T. monikensis canbe designated as being on the ventral side of the egg.
Early germ bands were located around the posteriorpole of the egg, slightly toward the ventral side (Fig.
3). Thus, it is likely that the embryos of this species
do not rotate longitudinally about the anteroposterior
axis of the egg, unlike those of other phasmatodeans(FOURNIER 1967; BEDFORD 1970, 1978).
A knob-like chorionic projection (PM) approximately 100 μm in diameter occurs at the posterior poleof the egg (Figs. 3, 4A,B,D, 6A,D,E, 7A,D), whichis comparable to the structure CLARK SELLICK (1997,1998) named the “posterior mound” in eggs of other
Timema spp.

Figs. 7, 8 show sections of eggs of T. monikensis.
The chorion is composed of an exochorion (ExCh)and endochorion (EnCh), and in areas other than the
operculum, opercular collar, micropylar plate and posterior mound, is 8–10 μm thick, of which about 70%
is exochorion. We could not find a vitelline membrane
and it may be too fragile to observe under a light microscope. In eggs containing fully developed embryos,a thick serosal cuticle (SeC) is found beneath the chorion that is almost equal to the latter in thickness (Fig.
7B). The chorion of the operculum is about twice as
thick as that of the egg body (Fig. 7C), but is thinned
at its margin to form the opercular collar together withchorion of the egg body (Fig. 7C). That part of theopercular collar derived from the operculum and thatderived from the egg body are clearly delimited by adetachment line (Fig. 7C): the operculum opens as theopercular collar ruptures along this detachment line,
and the juvenile hatches (Fig. 2). The chorion in thearea of the micropylar plate is thickened about 20 μm
and is specialized inside (Fig. 8): in the anterior half
the endochorion is thickened (Fig. 8A), while in theposterior half the exochorion is thickened and porousin structure (Fig. 8B). In the posterior mound, the chorion is heavily thickened (about 20–30 μm), with theexochorion and endochorion fusing; the chorion of theposterior mound has numerous vertical striations inside (Fig. 7D). Beneath the posterior mound, the serosal cuticle is strongly thickened and specialized with aserrated outer surface (Fig. 7D).

4.

Discussion

Our observations of the structural features of T. monikensis eggs are consistent with previous findings madeon eggs of T. cristinae, T. douglasi, T. chumash andT. podura (SANDOVAL & VICKERY 1996; CLARK SELLICK 1997, 1998; SELLICK 1997; ZOMPRO 2004, 2005).
Based on that information and including the new findings presented here (indicated by italics), the eggs ofTimema can be characterized as: 1) ellipsoidal, about
2 mm long, soft and with a coating of soil and/or otherextraneous particles when deposited; 2) having a transparent chorion which is weakly sclerotized and composed of an endochorion and exochorion; 3) having a
low dome-shaped operculum, which is not inclined,
is devoid of a capitulum, and located at the anteriorpole of the egg; the chorion of the marginal region of
the operculum is thinned to form an opercular collar
together with the chorion of the egg body; there is aline of detachment between the two in the opercular

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JINTSU et al.: Egg structure of Timema

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Fig. 9. Eggs of the embiopteran Aposthonia japonica (Okajima, 1926). A: An egg coated with extrinsic substances such as silk,plant tissues and excrements, ventral view. Scale bar = 500 μm. B: An egg with the coating removed, ventral view. The operculum surrounded by the opercular collar is at the anterior pole of the egg. The micropylar plate is attached to the opercular collar
on the ventral side of the egg. The micropylar tube cannot be distinguished in this photo. Scale bar = 500 μm. C: Enlargement of
the micropylar plate. The micropylar tube cannot be distinguished in this photo. Scale bar = 50 μm. D: SEM of the micropylar
plate, accompanied by the micropylar tube. The micropylar plate cannot be distinguished in this photo. Scale bar = 50 μm.
E: Posterior view, showing the polar mound. Scale bar = 100 μm. MpP = micropylar plate; MpT = micropylar tube; Op = operculum; OpC = opercular collar; PM = polar mound.

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Fig. 10. Egg of the phasmatodean Neohirasea japonica (de Haan, 1842). Scale bars = 500 μm. A: Ventral view. The operculum
is at the anterior pole of the egg, and the micropylar plate is in the center of the ventral side of the egg. B: Anterior view, showing the operculum, ventral to the top. C: Posterior view, showing the chorionic projection at the posterior pole of the egg, which
is often named the “posterior area”, “pseudoplate” etc., ventral to the top. ChP = chorionic projection at posterior pole of egg;
MpP = micropylar plate; Op = operculum.

collar, and the opercular collar ruptures along thisline on hatching; 4) having an inverted triangular micropylar plate on the ventral side attached to the opercular collar; the plate is not externally differentiatedbut is specialized within the chorion; 5) possessing apair of micropyles, each a simple funnel-shaped chorionic opening, on either side of the micropylar plate;a micropylar canal runs medioposteriorly from eachmicropyle and penetrates the chorion; and 6) having

a knob-shaped structure or posterior mound at theposterior pole; the posterior mound consists of thickened chorion rich in fine vertical striations, where theexochorion and endochorion fuse; the serosal cuticle
beneath the posterior mound is thickened and highly
specialized.
Orientation of the egg is defined by the orientationof the embryo in the final stage of development. According to this definition, the micropylar plate is onArthropod Systematics & Phylogeny 68 (1)the ventral side of the eggs of Timema, while in thoseof other Phasmatodea (Euphasmatodea) it is on thedorsal side (e.g., CLARK SELLICK 1998; ZOMPRO 2004).
As JINTSU et al. (2007) pointed out, the dorsal positionin Euphasmatodea results from longitudinal rotationof the embryo along the anteroposterior axis of the egg
during embryogenesis (e.g., FOURNIER 1967; BEDFORD
1970). In T. monikensis eggs the embryo does not rotate during embryogenesis. Consequently, the micropylar plates are most likely on homologous sides ofthe eggs in both Timema and euphasmatodean eggs,
and the different positions only indicate a shortcoming in the definition; the actual difference between
Timema and euphasmatodean eggs concerns the presence or absence of embryonic rotation.
ZOMPRO (2004) suggested a close relationship between Timematidae and Embioptera based on several characteristics including egg features such as 1) a
prominent opercular collar, 2) a micropylar plate thatexternally is not specialized and 3) a micropyle close
to the opercular collar; as a result, he considered Phasmatodea to be polyphyletic. Although ZOMPRO’s idea isinteresting, it requires further consideration. Euphasmatodeans have tough eggs mimicking plant seeds
with a heavily sclerotized chorion and elaborate micropylar plate (cf. CLARK SELLICK 1997, 1998; ZOMPRO
2004). These features of Euphasmatodea are apparently apomorphic. However, more plesiomorphic isprobably the condition seen in Timematidae and Embioptera, whose eggs have a weakly sclerotized chorion that must be protected with a coating of extrinsic materials at time of deposition (for Timematidae,
Fig. 1; CLARK SELLICK 1997, 1998; ZOMPRO 2004; for
Embioptera, Fig. 9A; KERSHAW 1914; ROSS 2000). At
least two of the features proposed by ZOMPRO (2004)
as evidence for a close affinity between Timematidae
and Embioptera (1 and 2 above) may be correlatedwith the likely plesiomorphic condition the eggs ofthese two groups share: that is, because the chorion
is only weakly sclerotized, the micropylar plate is not
so elaborate and the opercular collar is prominent. (Asfor character 3, see below: the number of micropyle(s)is revealed to differ between these two groups!)
Our study revealed the posterior mound in T. monikensis to closely resemble the polar mound of embiopteran eggs in having: 1) a thickened chorion where theexochorion and endochorion fuse, 2) numerous striations inside, and 3) a thickened and highly specialized
serosal cuticle beneath it. These features clearly playa significant role in eggs of Timematidae and Embioptera, perhaps, as a hydropyle or aeropyle. The congruence may reflect homology between these mounds,suggestive of a closer affinity between Timematidae
and Embioptera. However, eggs of Euphasmatodea
often have specialized chorionic structures at the posterior pole, known as the “polar area” or “pseudoplate”

77(e.g., ZOMPRO 2004), which are likely homologous to
the posterior mound or polar mound in timematid andembiopteran eggs. Indeed, there are phasmatodeansin which such polar structures are seemingly lacking(e.g., CLARK SELLICK 1997, 1998; ZOMPRO 2004), buteven if the chorion is not specialized externally, it may
have special features inside. More study is needed, andthere is no clear support for a clade Timematidae +Embioptera based on egg structure.
Attention needs to be paid to the fact that T. monikensis eggs have a pair of micropyles associated withthe micropylar plate, whereas the eggs of Euphasmatodea (e.g., CLARK SELLICK 1997, 1998; ZOMPRO 2004)
and Embioptera (JINTSU et al. 2007; JINTSU & MACHIDA
2009) have only one. Taking into account that possession of a single micropyle on the ventral side of theegg is unique to euphasmatodeans and embiopteransamong Polyneoptera (cf. HINTON 1981; JINTSU et al.
2007), this feature could be regarded as a potential“synapomorphy” of these two groups. However, theexclusive phylogenetic correlation of Embioptera andEuphasmatodea may be incorrect, according to recent
morphological (BRADLER 2009) and molecular (WHITING et al. 2003; TERRY & WHITING 2005) data where
monophyly of Timematidae + Euphasmatodea is wellsupported.
Whatever the interrelationships among Timematodea, Euphasmatodea and Embioptera, it should be emphasized that these three groups share several features:
1) a detachable operculum, 2) a micropylar plate on
the “ventral side” of the egg (as in the phasmatodeans discussed above), 3) a small number of micropyles (1 or 2) associated with the micropylar plate, and
4) a specialized chorionic structure at the posteriorpole (as in the case of the phasmatodeans discussed
above) (for Embioptera and Phasmatodea, see Figs. 9
and 10, respectively; cf. HINTON 1981; CLARK SELLICK
1997, 1998; ZOMPRO 2004; JINTSU et al. 2007; JINTSU& MACHIDA 2009). Only eggs of these three groups
bear such features (cf. HINTON 1981; CLARK SELLICK
1998; JINTSU et al. 2007; JINTSU & MACHIDA 2009), andthe monophyly of a clade comprising these three taxa
would appear well-supported based on egg structure.
To clarify the interrelationships within this clade, determination of the states of the relevant egg characters in the stem lineage is necessary. A comprehensive
structural analysis of eggs throughout the Polyneoptera is desired, including detailed comparisons within
Embioptera and of the chorionic thickening at the posterior pole of the egg in various Phasmatodea, together
with a careful survey of the number and positioning of
the micropyles in the eggs of other polyneopterans.
A clade comprising Timematidae, Euphasmatodea,
and Embioptera has found support in several recent
phylogenetic studies (e.g., WHITING et al. 2003; TERRY& WHITING 2005; KJER et al. 2006; BRADLER 2009);

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JINTSU et al.: Egg structure of Timema

however, there is also much evidence in conflict withthis hypothesis (summary in KLASS 2009). Furthermore, a monophyletic Phasmatodea including Timema
appears to be well-supported by both morphological(BRADLER 2009) and molecular data (TERRY & WHITING
2005, who also included morphological data); yet, inthe combined analyses of KJER et al. (2006) the relationships between Timema, Euphasmatodea, and Embioptera are ambiguous.

5.

Acknowledgments

We are greatly indebted to Dr. Tanja Schwander of Simon Fraser University for kindly providing us with materials. We are
grateful to Dr. K.-D. Klass of Museum für Tierkunde Dresden
and two anonymous reviewers for valuable comments and to
Dr. S.D. Schoville of UC Berkley for help in planning thisstudy. We also thank Mr. S. Masuda of Leica Japan and Mr. I.
Hamano of Hamano Microscope for their technical help. This
study was supported by the Japan Society for the Promotion of
Science (JSPS) Research Fellowship for Young Scientists to YJ(21-350) and a Grant-in-Aid from the JSPS (Scientific Research
C: 21570089) to RM. This is contribution No. 217 from theSugadaira Montane Research Center, University of Tsukuba.

6.

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ZOBODAT - www.zobodat.at
Zoologisch-Botanische Datenbank/Zoological-Botanical Database
Digitale Literatur/Digital Literature
Zeitschrift/Journal: Arthropod Systematics and Phylogeny
Jahr/Year: 2010
Band/Volume: 68
Autor(en)/Author(s): Machida Ryuichiro, Jintsu Yoshie, Uchifune Toshiki
Artikel/Article: Structural Features of Eggs of the Basal Phasmatodean Timema
monikensis Vickery & Sandoval, 1998 (Insecta: Phasmatodea: Timematidae) 71-78