Recent Advances in Insect Embryology in Japan
Edited by H. Ando and K. Miya.
ISEBU Co. Ltd., Tsukuba 1985

131

Blastodermic Cuticles of the Jumping Bristletail,
Pedetontus unimaculatus(Microcoryphia, Machilidae)

Ryuichiro MACHIDA
andHiroshi ANDO

Synopsis
The formation and structure of blastodermic cuticles of the machilid, Pedetontus unimaculatus are described and illustrated in detail.
The formation of blastodermic cuticle commences in the stage of germ rudiment formation, and completes generally by about the time of the elongation of germ disc. The blastodermic cuticle is composedof three layers. The outermost layer (2-4l'm-thick) is dark brown in color. Its surface assumes a polygonal pattern of which polygons are ca. 1OI'm in diameter, and at the center of each polygon a pointed
process (ca. 2-3I'm-high) stands. The outermost layer also has various-shaped and sized nodal projections
on its surface. The middle layer (4-8I'm-thick) is hyaline. The innermost layer (5-10I'm-thick) is also
hyaline and bears radial striation.
The basic plan of blastodermic cuticle in Pedetontus unimaculatus is in good agreement with those inmachilids hitherto studied. Among the features of machilid blastodermic cuticles the formation in theearlier stages of development is the most noticeable within the ectognathous insects.

Introduction
In not a few insects, the secondary egg membranes, blastodermic or serosal cuticles, areformed during embryogenesis. They play an important role of the protection of egg. The
blastodermic cuticles are also known in machilid eggs. Larink (1969, 1972, 1979) reported
the formation and structure of blastodermic cuticles of some machilids (four genera, fivespecies). He (1979, 1983) pointed out that the blastodermic cuticles of machilids havecommon features in the formation and basic structures: i) formation in the early stage ofembyogenesis, ii) three-layered construction composed of the outermost pigmented and132

Machida, R. and H. Ando

two hyaline ones, and iii) radial-striated structure in the innermost layer. He also suggested
that the surface structures of machilid blastodermic cuticles are clearly specific in species.
We have been studying the embryonic development of a machilidan Pedetontus unimaculatus (Machida, 1981; Machida and Ando, 1981). In a series of our work, here we
describe and illustrate the formation and structure of the blastodermic cuticle of the insect
in detail.

Materials and Methods
Imaginal insects were collected from Shimoda, Shizuoka Pref., Japan. The methods formaintaining them, collecting eggs and fixing the materials were the same as described previously (Machida, 1981). Sections were prepared with the normal paraffin and the JBA
embedding methods. For scanning electron microscopy, the gelatinous layer and chorion
were removed with rolling the egg on a double-coated tape ("Scotch") struck to the slide
glass. Some of eggs were ultrasonicated in a detergent solution. After fixation and thepenetration of egg membranes with needle, eggs were dehydrated and dried according to thecritical point method. Specimens mounted with nail enamel on stubs were coated with goldand examined with a JEOL JSM-T200.

Results
The general character of the egg of Pedetontus unimaculatus was described previously(Machida, 1981). The egg is covered with a outermost gelatinous layer (CL) and a chorion(CH) ca. 1-2,um and 3,um in thickness respectively (Fig. 1). In the egg just after oviposion,
the chorion is elastic, but soon it hardens and turns into light brown from hyaline. Eggs
adhere to each other in a clump or to the substratum with the gelatinous layer. In Pedetontus unimaculatus, the blastodermic cuticle forms in early stages of the embryonic development as in the other machilids. The stages in the description follow the previous work
(Machida, 1981).
The blastoderm forms and the development enters into the germ disc formation. The
blastodermic cuticle begins to be secreted during the stage of germ rudiment formation(early in Stage O. First, externally to the blastoderm a light brown cuticle appears (Fig. 2).
It reaches '2-4 pm in thickness, and darkens (Fig. 3). This cuticular layer is named blastodermic cuticle 1 (BLC 1). A hyaline rough-structured layer (coating layer, CL, in Figs. 2, 4,
11, 12) is observed at the surface of BLC 1. The coating layer of Pedetontus unimaculatus
corresponds to the Oberjl'achemembran reported by Larink (1969, 1972, 1979) for theother machilids. Wide variations are observed in the thickness of coating layer (CL) regionally in each egg or individual eggs (c! Figs. 2,4). In Fig. 12, two sublayers of the coating
layer are distinguished. Nothing is known on the relation of the coating layer and vitelline
membrane. The former may be only a desiccated fluid material existed betweerr the chorion
and blastoderm. The surface structure of BLC 1 is illustrated in Figs. 7-12. It has a polygonal pattern of which polygons are ca. 10,um in diameter, and a pointed process stands,
2-3JLm or lower in height, at the center of each. BLC 1 also has various-shaped and sized
nodal projections on its surface. The chorion and the egg contents are now separated fromeach other with the processes and nodal projections. In a few days after oviposition, the egg
color changes from orange into dark brown with the formation of BLC 1.

Blastodermic cuticles of machilids

3

6

Figs. 1-6. Photographs showing the successional stages of blastodermic
cuticle formation in Pedetontus unimaculatus. Scales: 20 J1.m.
1. Egg with newly formed blastoderm.
Egg is surrounded with a gelatinous
layer and a chorion. 2. Egg in germ rudiment formation. Blastodermic cuticle 1(BLC 1) is under formation. A hyaline layer (CL) is observed over BLC 1. 3.
BLC 1 thickens and becomes darker. It bears many processes on its surface. 4.
An additional hyaline layer, blastodermic cuticle 2 (BLC 2) is under formation.
A hyaline layer (CL) is faintly seen over BLC 1.
5. Third layer, blastodermic
cuticle 3 (BLC 3), is added under BLC 2. BLC 3 is also hyaline and bears radialstriations. BLC 2 is heavily stained with chromotrope 2R in this section. The
blastodermic cuticle completes generally by about the time of elongation of germdisc. The egg of this section is one in the earliest stage of germ band (Stage 2).
6. Near hatching, the blastodermic cuticles 2 and 3 are digested. This is a photograph of exuvia. Only the BLC 1 remains intact.
BL blastoderm, BLC I, 2, 3 blastodermic cuticle I, 2, 3, CH chorion, CL
layer coating blastodermic cuticle, GL gelatinous layer.

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Machida, R. and H. Ando

In no time, a hyaline cuticular layer, blastodermic cuticle 2 (BLC 2), is added innerly to
the BLC 1 (Fig. 4). It is 4-81lm in thickness, and tends to be heavily stained with dyes suchas eosin and chromotrope 2R. The surface facing the BLC 1 assumes a polygonal patterncorresponding to that of BLC 1. Soon, an additional cuticular layer, blastodermic cuticle 3,
is formed just under the BLC 2 (Fig. 5). It is 5-101lm in thickness, and it bears radial striation, as in Trigoniophthalmus alternatus and Lepismachilis sp. (Larink, 1979). The striatedstructures in Pedetontus unimaculatus may correspond to the micro canals as Larink (1979)
observed for Trigoniophthalmus altematus and Lepismachilis sp.
In this way the blastodermic cuticles are formed, and the formation completes generally
by about the time of elongation of germ disc, at most delayed by the earliest germ bandstage (Stage 2). The definitive general construction of the egg membrane system in Pedetontus unimaculatus is as illustrated in Fig. 13. No changes is observed in the blastodermic
cuticles during the following embryonic stages until near the hatching. The thickness of cuticular layers varies from 12-20J.Lm or more in individual eggs. These differences in thickness
depend on those of the blastodermic cuticles 2 and 3. In each egg, the blastodermic cuticles
have almost a uniform thickness, only under the germ project innerly the cuticular layers
into a protuberant process ca. 25J.Lffi in height, as in Petrobius brevistylis (Larink, 1969). It
is related to fold-like structure of the embryonic membrane formed under the ~erm disc.
After the completion of blastodermic cuticles, the chorion becomes fragile, and is latercracked often. Now the cuticular layers perform a principal part of egg protection instead of
the chorion. We often found that the mycocelia invade the space between the chorion andblastodermic cuticle, or that the space is filled with water when the egg is incubated in a
wetty condition. The cuticular layers of Pedetontus unimaculatus have a high ability for theprotection of egg, as the case in the other machilids (Wygodzinsky, 1941; Larink, 1969,1972).
Just before hatching (Stage 14), blastodermic cuticles 2 and 3 disappears (Fig. 6), andthe larva hatches. Probably some kind of enzyme may be responsible for the disappearanceof blastodermic cuticles. But we cannot provide any supportive evidences for it at thepresent. The digestion of egg membranes is not followed well in the apterygote insects. In a
thysanuran Thermobia domestica, an evidence is presented for the serosal cuticle is digested
by a substance secreted from pleuropodia (Woodland. 1957).

Discussion
The formation and structure of the machilidan blastodermic cuticle were reported forPetrobius brevistylis, Petrobius maritimus, Tr;goniophthalmus alternatus, Dilta hybernica
and Lepismachilis sp. (Larink, 1969, 1972, 1979). The blastodermic cuticles in Pedetontus
unimaculatus are in good agreement with those in the above four genera and five species
machilids in basic plans: i) formation in the very early stage of embryogenesis, ii) threelayered construction, iii) outermost pigmented layer (BLC 1) with a complex surface
structure, iv) hyaline middle and innermost layers (BLC 2 and 3), and v) radial-striatedstructures in the innermost layer (BLC 3). It is supportive for Larink's (1979, 1983) viewon the features of the machilid blastodermic cuticles. He also pOinted out that the surface
structures of the outermost layer are clearly specific in species, and we have no objection
also to his pointing-out. The surface structure of the outermost layer in Pedetontus uni-

Blastodermic cuticles of machilids

Photographs ·showing surface structures of the blastodermic
Figs. 7-12.
cuticle (BLC I) of Pedetontus ·unimaculatus. The specimens in thesephotographs are in the middle stages of embryogenesis (Stages 8-10).
See the text.
7. Dechorionated egg. Scale: 5001Jm.
8. General surface structure of BLC 1.
Scale: 101Jm. 9. Nodal projections of BLC I. The BLC 1 is thinly coated with a
10. Ibid. Scale:10lJm.
11. BLC 1 andlayer, the coating layer. Scale: 501Jm.
coating layer (CL). Scale: 101Jm. 12. Ibid. Scale: 101Jm.

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Machida, R. and H. Andomaeulatus is relatively simple among the machilid hitherto studied.
The blastodermic cuticle in the Microcoryphia and the serosal cuticle formed in most of
pterygote insects and the Thysanura (Heymons, 1897; Sharov, 1953; Woodland, 1957) arehomologized with each other. It deserves specieal emphasis, within the ectognathous insects,
that in machilids the cuticles are formed in the very early stages of development: i. e., generally from the germ rudiment formation- to germ disc-stages. It may be related to the factthat the chorion in machilids is not so protective, or that the structure to be corresponding
to the amnioserosal fold formed in the Thysanura-Pterygota is not well developed or, instrict sense, lacking in Pedetontus unimaeulatus, probably the machilids (unpublished). The
conditions in machilids resemble those in collembolans (et Jura, 1972). Within the Antennata possessing only the serosa or blastoderm as the embryonic membrane, the blastodermic
cuticles are formed in the Collembola (Jura, 1972), Diplura (Uzel, 1898; Tiegs, 1942),
Symphyla (Tiegs, 1940), Pauropoda (Tiegs, 1947) and Diplopoda (Dohle, 1964). The
formation occurs generally in the early stages of development in these aminals, although it
might be not easy to compare the stages in different animals. Only among these the formation is relatively delayed in the Diplura.

GL

CL

CH

Fig. 13., Diagram illustrating the structure of egg membrane in Pede tontus unimaculatus.
See the text. BL blastoderm, BLC 1, 2, 3 blastodermic cuticle 1, 2,3, CH
chorion, CL layer coating blastodermic cuticle, GL gelatinous layer.

Acknowledgements
We thank to Profs. K. Miya and M. Kurihara of Iwate University for their valuable suggestions on the preparation of JBA sections. The work was carried out while the seniorauthor, R. Machida, was in receipt of a grant from the Japan Society for the Promotion of
Science which is gratefully acknowledged.

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137

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Authors' address:

Dr. R. Machida and Prof. H. Ando,
Sugadaira Motane Research Center,
University of Tsukuba, Sanada,
Nagano 386-22, Japan