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Proc. Arthropod. Embryol. Soc. Jpn. 50, 1–5 (2016)Ⓒ 2016 Arthropodan Embryological Society of Japan
ISSN 1341–1527[ORIGINAL PAPER]

A New Type of Egg Burster in Sialis and Its Possible Phylogenetic
Significance: A Comment on the Article by Ando et al. (1985) (Insecta:
Megaloptera, Sialidae)
Yukimasa KOBAYASHI1) and Nobuo SUZUKI2)
Sayamadai 2–21–18, Sayamashi, Saitama 350–1304, Japan
Department of Movement Sciences, Japan Women’s College of Physical Education, Kitakarasuyama 8–19–1, Setagaya-ku, Tokyo
157–8565, Japan
E-mail: dineutus@hotmail.co.jp (YK)1)
2)

Abstract
In their article on the embryology of the alderfly Sialis japonica, Ando et al. (1985) reported the formation of comb-likestructures along the midline of the ventral side of the embryonic head of this species, but they did not refer to the function of thesestructures. By the examination of documentations on Sialis, we reconfirm the structures as an egg burster (EB) that is used to tearthe eggshell at the time of haching. We also demonstrate that the EB corresponds to a new type of EB that can be discriminatedfrom other types of EBs. This type of EB is suggested to be widely distributed in another megalopteran family, Corydalidae, and also
in the closely related order Neuroptera. The presence of this type of EBs in Megaloptera and Neuroptera affords us further evidenceof the sister group relationship between these two orders.

Introduction
On the basis of their scanning electron microscope(SEM) observations, Ando et al. (1985) reported thedevelopment of a pair of comb-like structures along the
midline of the ventral side of the head in an embryonic alderfly,
Sialis japonica Weele [S. mitsuhashii Okamoto is a synonym
for this species (Hayashi and Suda, 1995)] (Fig. 1, eb). The
structures, which assume the shape of a narrow inverted V,
occur in series composed of approximately 10 spines forming
two ridges. Previous studies have clearly demonstrated thatthese structures correspond to the egg burster (EB)-variously referred to as the egg tooth, oviruptor, Eizähne andEisprenger- that is used to tear the eggshell at the time ofhatching (Emden, 1925; Evans, 1972; Canterbury and Neff,1980). Interestingly, however, Ando et al. did not refer to thisfunction of the EB, probably because they were unaware of
the previous studies in which the function of this structure
was described. We reconfirm that these comb-like structures
are indeed the EB, and demonstrate that the EB corresponds
to a new type of EB that can be discriminated from other typesof EBs. In addition, we propose that this type of EBs is widelydistributed in another megalopteran family, Corydalidae, andalso in the closely related order Neuroptera.
Egg bursters of Sialis
The Sialis EB was first reported by Emden (1925).

Subsequently, the EBs were described in 14 North American
Sialis species by Evans (1972) and Canterbury and Neff
(1980). These authors already recognized the following
important points regarding the EB: 1) the Sialis EB is asclerotized tooth-like structure that is formed in theembryonic cuticle, 2) the EB is used to tear the eggshell at
eclosion, and is shed together with the embryonic cuticle
from the body, remaining attached to the teared eggshell after
eclosion. Both Emden (1925) and Canterbury and Neff (1980)
ended their observations only referring to EBs that remained
attached to the eggshells after eclosion, and no direct
observations of embryonic EBs were made. Evans (1972)
observed EBs on the embryonic heads of four Sialis spp.
However, none of these studies referred to the origin of theEB, i.e., what parts of the head are involved in the formation of
the EB.
Egg burster formation in Sialis embryos
According to Ando et al. (1985), the EB (referred to bythem as comb-like structures) originates in the proximal partof the labial appendages (Fig. 2A-D). In 4-day-old embryos,
the labial appendages arise as a pair of robust projections justposterior to the maxillary appendages (Fig. 2A, lb, mx). These
labial appendages then elongate medially and their distal
halves differentiate into future labial palpi (Fig. 2B, lbp). In
8-day-old embryos, the labial appendages shift anteromedially2

Y. KOBAYASHI AND N. SUZUKI

and become positioned close to each other in such a way thatthey are concealed below the developing maxillary
appendages (Fig. 2C). In 9-day-old embryos, the EB appears
as a narrow inverted V-shaped structure along the midline of
the gnathal regions, just posterior to the labrum and between
the antennae (Fig. 2D, eb). At this stage, the posterior end or
the base of the EB is continuous with the anterior part of the
fused labial appendages, which extend posteriorly as labial
palpi (Fig. 2D, lbp). Based on these observations, Ando et al.

Fig. 1 Latero-ventral (A) and ventral (B) views of a 10-dayold Sialis japonica embryo, showing the egg burster(eb) (Ando et al.ʼs comb-like structures). Modified
from Ando et al. (1985) with permission from The
Arthropodan Embryological Society of Japan.
ab9 and 10: ninth and tenth abdominal segments, at:antenna, hc: head capsule, lr: labrum, md: mandible,
mx: maxilla, th1.l: prothoracic leg. Scale = 100 µm.

(1985) concluded, “This structure (EB) is originated from theanterior part of the labial rudiments.” (p. 198). They also
regarded the structure as a ʻdentate ligulaʼ (p. 200).
The labial origin of the Sialis EB asserted by Ando et al.
(1985) appears to be natural as the general morphogenetic
movement of the cephalognathal region containing theproximal part of the fused labial appendages is strongly shifted
anteriorly, resulting in this region sometimes coming into
contact with the labrum (e.g., trichopteran embryogenesis,
Kobayashi and Ando, 1990). It is likely that Ando et al. (1985)
did not directly observe the primordial EB before theformation of the V-shaped structure, as this primordial EB
would have been concealed by the developing antennae (Fig.
2C, circle with white stippled line) in 8-day-old embryos. We
presume that the most proximal endites of labial appendages(Fig. 2B, C, arrows), or glossae, are fused to each other anddevelop into the EB. However, in a discussion concerning theorigin of the EB with Dr. Rolf. G. Beutel, he addressed that
this part (Fig. 2C, circle with white stippled line) is the future
hypopharynx and not the labium, which means that the Sialis
EB is hypopharyngeal in origin (Rolf G. Beutel, pers. comm.).
In thysanopteran embryogenesis, the hypopharynx develops
from the epidermis of the mandibular and maxillary segments,
and possibly also the intercalary segment (Heming, 1980). In
the embryonic whirligig beetle Dineutus mellyi, the future
hypopharynx, which is probably derived from the sterna of themandibular and maxillary segments, is located just anterior to
the fused labial appendages (see Fig. 5 in Komatsu andKobayashi, 2012). Thus, while we are still inclined to regard
the EB as being labial in origin, the possibility that somehypopharyngeal elements participate in the formation of the
EB should not be excluded completely. For the determinationof the exact origin of the Sialis EB, histological observations
on the serial sections of the parts concerned need to be
conducted. However, in the interim, we propose that the EB
of Sialis should be tentatively called the Sialis-type EB,
without referring to its embryonic origin.

Fig. 2 Successive stages of developing Sialis japonica embryos (ventral views), showing the morphogenetic movement of the
cephalognathal region. Modified from Ando et al. (1985) with permission from The Arthropodan Embryological Society of Japan. A.
4-day-old embryo. B. 6-day-old embryo. C. 8-day-old embryo. The left maxillary palp and left labial palp were removed. D. 9-day-old
embryo. The regions corresponding to the maxillary appendages and labial appendages are highlighted in blue and red, respectively.
at: antenna, eb: egg burster, ga: galea, int: intercalary segment, lb: labial rudiment, lbp: labial palp, lc: lacinia, lr: labrum, md:
mandible, mx: maxillary rudiment, mxp: maxillary palp, stom: stomodaeum, th1: prothoracic segment, th1.l: prothoracic leg. whitearrow: glossa, white stippled line: see text. Scales = 50 µm.

A NEW TYPE OF EGG BURSTER IN SIALIS

Egg bursters of Corydalidae
In another megalopteran family, Corydalidae, or
dobsonflies, the presence of an EB has been reported in four
genera (Corydalus, Chauliodes, Neohermes, and Orohermes) by
Smith (1922) and Evans (1972). Each of the corydalid EBs,
unlike those of Sialis, assumes as a single blade with minuteirregularly shaped teeth. The EB is located on the ventral sideof the labrum, but they are not connected.
Egg bursters of Neuroptera
Among the 17 families in Neuroptera, EBs have been
identified in eight; the Sisyridae, Osmylidae, Coniopterygidae,
Mantispidae, Hemerobiidae, Chrysopidae, Nemopteridae, andNymphidae (Hagen, 1852; Smith, 1922; Withycombe, 1923,
1925; Emden, 1925; Kuroko, 1961; New, 1983; Popov, 2002;
Candan et al., 2005; Konopová and Zrzavý, 2005). Based on theFig. 3 Ten-day-old embryo (A) of Osmylus chrysops Linné[= Osmylus fulvicephalus (Scopoli)] and its egg
burster (eb) (B, ventral view; C, left side view) (After
Hagen, 1852).
at: antenna, hc: head capsule, md: mandible, th1.l:
prothoracic leg.

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absence of an EB in embryos of Ascalaphus ramburi, it isconsidered that EB formation probably does not occur in the
Ascalaphidae (Kamiya and Ando, 1985). Similarly, theMyrmeleontidae is said to lack an EB (Hinton, 1981). Thus,
although the state of the EB has not been examined in sevenother neuropteran families to date, EBs are presumed to be
widely distributed in Neuroptera. The neuropteran EB, as in
the Corydalidae, is located on the ventral side of the embryonic
head, posterior to the labrum and between the antennae where
it assumes as a single blade or ridged structure with mostly
minute teeth; however, the shape of the blade is highly variable
among genera (Figs. 3, 4A-H); e.g., the blade is long and thin in
Osmylus (Osmylidae) and Sisyra (Sisyridae), and thick androbust in Semidalis (Coniopterygidae) and Hemerobius(Hemerobiidae). The EB of Nemoptera bipennis isconspicuously hill-shaped without a blade (Fig. 4H). As in the
Megaloptera, neuropteran EBs are also shed with the eggshellat eclosion.
Various types of egg bursters in Dicondylia
Extending our view to all hexapods, the presence of an
EB has been reported in 22 of the 29 dicondylian insect orders.
Emden (1946) classified hexapod EBs into three types
according to their location and relation to the embryoniccuticle. The first type is an acral or frontal EB, located on thefrontal region of the head. This type of EB is confined to theembryonic stage and shed with the embryonic cuticle
immediately after the rupture of the chorion at eclosion. The
second type is a persistent frontal EB, that is also located on
the embryonic head; however, this type is retained during thefirst larval stage. The EBs of the third type are scattered on
the dorsa of the thoracic and abdominal segments, and areretained during the first larval stage. The third type has only
been recorded in several polyphagan coleopteran families(Chrysomelidae, Coccinellidae, etc.).
Since Emdenʼs classification, EBs have been discovered

Fig. 4 Egg bursters of neuropteran species (A, C, D, E and G, right-side views; B and F, left-side views; H, ventral view) (A - G, after
Withycombe, 1923; H, after Withycombe, 1925). A. Sisyra fuseata (Sisyridae). B. Semidalis aleurodiformis (Coniopterygidae). C.
Sympherobius pygmaeus (Hemerobiidae). D. Hemerobius lutescens (Hemerobiidae). E. Hemerobius paganus (Hemerobiidae). F.
Nothochrysa capitata (Chrysopidae). G. Chrysopa flavifrons (Chrysopidae). H. Nemoptera bipennis (Nemopteridae).

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Y. KOBAYASHI AND N. SUZUKI

in many orders, and EBs of the first type have been recorded
in 19 orders including Megaloptera and Neuroptera. Emden(1925, 1946) already regarded the megalopteran andneuropteran EBs as being associated with the head.
Subsequent interpretations of EB location in orders excluding
the Megaloptera and Neuroptera have revealed that 17 otherorders have EBs located on the vertex of the head capsule,frons or clypeus (e.g., Kobayashi and Ando, 1990; Mashimo etal., 2014). On the other hand, as mentioned previously, since
the megalopteran and neuropteran EBs have been observedon the ventral side of a head capsule, they should bediscriminated from the first type.
A new type of egg burster in Megaloptera and
Neuroptera: A potential synapomorphy between these
two orders
Relatively little is known about the embryonic origin of
the EB in the Corydalidae and Neuroptera. In their article
examining the formation of the embryonic cuticle in several
insect orders, Konopová and Zrzavý (2005) investigated theformation of the EB in the neuropteran Chrysopa perla. In thecaption of their “Figs. 7 and 8”, they used the phrase “Note thelabral egg-tooth” (p. 350) and the term ʻlabral egg-toothʼ was
also used in the text of the article (p. 349). It therefore seems
clear that Konopová and Zrzavý (2005) consider the EB as
belonging to the labrum. However, judging from its location
between the antennae it is strongly suggested that the
Chrysopa EB could also be classified as a Sialis-type EB withno connection to the labrum. It is therefore presumed that theEBs of Megaloptera and Neuroptera are derived from thesame embryonic part, but the exact part is not yet been
identified; the EBs of these two orders could therefore be
regarded as the Sialis-type EB.
It is also possible that the Sialis-type EB could be a new
type of EBs that is only known in Megaloptera and Neuroptera,
and that its presence could be regarded as a potential
synapomorphy between these two orders. Although the
Megaloptera, Neuroptera, and Raphidioptera are known toconstitute the superorder Neuropterida (Mickoleit, 1973), thephylogenetic relationships among these orders have beenhighly debated, with some authors questioning whether the
Megaloptera are monophyletic (see Kristensen, 1995; Aspöck
et al., 2012). Traditionally, the Megaloptera and Raphidioptera
have been considered sister taxa (Hennig, 1953), primarily
due to similarities in skeletomuscular characteristics of thebasal abdominal segments (Achtelig, 1975) and the structureof the female postabdomen (Hünefeld et al., 2012). On thebasis of comprehensive morphological and molecularevidence, Beutel et al. (2010) also proposed the clade
Megaloptera + Raphidioptera, although in their analysis the
Megaloptera are paraphyletic [i.e., Sialidae + (Corydalidae +Raphidioptera)]. Although some molecular phylogenetic
analyses have provided evidence for the clade Megaloptera +Raphidioptera (Whiting, 2002; Wiegmann et al., 2009), thehypothesis for the sister-group relationship of Megaloptera +Neuroptera that was first proposed by Boudreaux (1979) has
been strengthened by an increasing body of morphological(Aspöck et al., 2001; Aspöck, 2002; Krenn, 2007; Aspöck andAspöck, 2008; Zhao et al., 2014) and molecular phylogenetic
evidence (Haring and Aspöck, 2004; Kjer et al., 2006; Cameron
et al., 2009; Peters et al., 2014). Consequently, the clade
Megaloptera + Neuroptera has gained increasing acceptance,
as has the monophyly of the Megaloptera. Indeed, thepresence of the Sialis-type EB in both the Megaloptera and
Neuroptera would provide further evidence for the sistergroup relationship between these two orders (i.e.,
synapomorphy). However, in order to resolve this argument,
the state (presence or absence) of the EB in Raphidioptera
should be investigated. From an examination of the figures ofthe fully grown Inocellia japonica embryo (Tsutsumi, 2008),
who first described embryogenesis of Raphidioptera, an EB isindeed absent in this species (his Fig. 26Aʼ). The absence of an
EB in Inocellia therefore adds evidence favorable for the clade
Megaloptera + Neuroptera to the phylogenetic discussion of
Neuropterida.
Acknowledgments: We extend our sincere gratitude to
the late Prof. Emeritus Hiroshi Ando, the late Dr. Kozo
Miyakawa, and Mr. Satoru Shimizu, for providing us withnumerous SEM micrographs of Sialis. We are grateful to Prof.
Rolf G. Beutel and for his comments on the origin of the EB of
Sialis. We also thank Prof. Ryuichiro Machida and Mr.
Akimasa Kamiya for their help im many aspects and criticalcomments on the manuscript.
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