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

13

Panoistic Ovarioles of the Dobsonfly,
Protohermes grandis(Megaloptera, Corydalidae)

Morio MATSUZAKI
Asako SASAKIand
Makoto KOMURO

Synopsis
The morphology of ovarioles of the megalopteran Protohermes grandis is described. Each of thepaired ovaries of this insect consists of 145-150 panoistic ovarioles having no nurse cells. A fully developed ovariole contains a linear array of about 14 developing oocytes of different stages; fIrst four anterior
oocytes show the late previtellogenic, next 4-5 the vitellogenic, and about fIve posterior the fully grownones. The process of the ovariole development roughly resembles to those of the most insects with thepanoistic ovaries. However, this is entirely different from that of the sister-groups, the megalopteranSialidae and the Raphidioptera which have the telotrophic ovarioles.

Introduction
The order Megaloptera is classified into two families, Sialidae and Corydalidae respectively. On the oogenesis of Sialis belonging to the family Sialidae, two electron microscopic
studies were published by Matsuzaki and Ando (1977) and BOning (1979). According tothem, the ovariole type of Sialidae is the telotrophic one which is fundamentally similar to
those of the Hemipteran and polyphagous Coleopteran insects. Recently it has also been reported that insects belonging to the order Raphidioptera closely related to the Megaloptera
have the same telotrophic ovarioles (BOning, 1980; Matsuzaki and Kano, 1980). In theanother family Corydalidae included to the Megaloptera, however, the female reproductive
system has not been investigated up to the present, therefore it is not known on the type ofovariole present in the insects of this family.

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Matsuzaki, M. et al.

The present study was designed to investigate the ovarian structure, development of theoocytes and the related tissue (follicular cells) during the early oogenesis by means of electron microscopy.

Materials and Methods
In this study, female adults and larvae of Protohermesgrandis Thumberg were used. The
ovary was fixed in either 1% solution of osmium tetroxide buffered with O.lM sodium cacodylate at pH 7.4 or 4% glutaraldehyde buffered with same buffer (pH 7.4) for 3-4 hours at
about 4°C. In the latter case, the materials rinsed in cacodylate buffer were then postfixedin 1% OS04 in the same buffer for 3 hours. Then they were stained in saturated uranyl acetate for 60 min. Following dehydration, the ovaries were embedded in Epon 812. Ultrathin
sections were stained with lead citrate for 60 sec. and observed with JEM-T7 electron microscope.

Observations
Morphology of the Ovarioles
Each ovary of the dobsonfly, Protohermes grandis consists of 145-150 panoistic ovarioles,
segmentally arranged on the lateral oviducts. In this respect Protohermes shows rather aclose affinity to the Thysanura belonged to the Apterygota, and hemirnetabolan panoistic
ovaries, such as the Odonata, Orthoptera, Grylloblattodea and Thysanoptera etc.
Fully developed ovarioles of females just before the oviposition are about 10mm from thetip of the germarium to the base of the vitellarium, and three regions can be distinguished;
the terminal filament, germarium and vitellarium. The terminal filament measured about 13
pm in diameter is composed of a row of flattened mesodermal cells with elongated nuclei
generally lying their long axis perpendicular to the long axis of terminal filament (Fig. 1).
No transverse sep.tum may be observed in the region between the terminal filament and thegermarium. These ovariole structures are similar to those of the sister-groups, such as megalopteran Sialis (Matsuzaki and Ando, 1977; BUning, 1979) and neuropteran Chrysopa(Matsuzaki, 1978). The terminal filament posterioriy continues into the germarium maximum width of which is enlarged up to about 40 pm.
The germarium is composed of the oogonia, young oocytes and prefollicular cells, and theoogonia occupy the anterior region of the germarium. In the larval stage oogonial mitosis isoccasionally observed in the anterior portion of the germarium. The oogonia clustered in
the anteriorgermarium are sU.bspherical or ovoid in shape, measured 13-15 pm in diameter,and contain large spherical or subspherical nuclei (10-12 pm in diameter). The chromatinmaterial in the nuclei of the oogonia is in a dispersed state. Three or four small masses of
the nucleolar material (I .0-1.5 pm in diameter) are dispersed to closely opposite to theinner layer of the nuclear membrane. The oogonial cytoplasm contains mitochondria,
rough-surfaced endoplasmic reticulum, a few Golgi bodies and numerous free ribosomes.
The mitochondria are randomly distributed in the cytoplasm, and they are largely spherical(about 0.3 pm in diameter) or oval (0.6 X 03 pm) in shape and have less developed cristae,Panoistic ovariole of dobsonfly

15

Fig. 1. Low electron micrograph of the boundary between terminal filament (TF) andgermarium (GE). QG oogonia, QC young oocyte, PFN prefollicular cell nucleus.
X 2,300
Fig. 2. A stack of annulate lamellae (AL) associated directly with the rough-surfaced endoplasmic reticulum (ER) observed in close association with the oogonial nucleus(N) in the anterior portion of germarium.
X 15,000
Fig. 3. Electron micrograph showing adjacent two oogonia in the anterior part of germariurn. A single strand of annulate lamellae (AL) is seen in contact with the outer or
inner nuclear membrane. N oogonial nucleus, CM oogonial cell membrane. Arrow heads indicate another annulate lamellae.
X 10,000

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Matsuzaki, M. et al.

but highly elongated forms can not be observed. The rough-surfaced endoplasmic reticulumof various length and forms are small in number most of which distribute around the nucleus
in the ooplasm. Annulate lamellae are often observed in the oogonia. In many cases they
are positioned close to the nuclear membrane, and are a single to several strands in occurrence (Figs. 2 and 3). As observed in Fig. 2, the interlamellar spacing is qUite uniform, andfrequently both ends of each strand of the annulate lamellae are directly associated with therough-surfaced endoplasmic reticulum in continuity. Young oocytes in the posterior regionof the germarium slightly increase in volume, and its nucleus likewise enlarges to about 17 X
13 J.Lm and contains usually two masses of nucleoli. Prefollicular cells surrounding the 00gonia arid early young oocytes are poorly developed similar to those in other panoistic ovarioles.
The vitellarium of the ovariole is situated between the germarium and pedicel. In the caseof the late larval ovarioles, the oocytes grow very slowly and they are retained in the earlyprevitellogenic stage. In the most anterior portion of the vitellarium the first young oocyte
arranged in a single file is somewhat oblong (ca. 55 X 70 J.Lm), and its long axis is perpendicular to that of the ovariole. The 14th oocyte located in the most posterior portion of thevitellarium becomes more elongated and measured approximately 200 X 86 J.Lm in size. In
the ovarioles of females just before the oviposition, on the other hand, the number of 00cytes in the vitellarium is also about 14; anterior about four oocytes are about 140 X 70 J.Lm
to 300 X 100 J.Lm in size and are in a late previtellogenic stages, next 4-5 oocytes of about
320 X 120 J.Lm to 360 X 150 J.Lm are in the early to middle vitellogenic, and the last average
five oocytes are approximately 1,300 X 550 J.Lffi in size and are extremely the same withmature oocytes. This fact may suggest that a female produces at least approximately 1,500
eggs at the same time. The mature eggs are cylindrical, with rounded end, and a conspicuous
micropylar apparatus is observed at the one side of their anterior end.

Morphological Features of Developing Oocytes and Follicular Cells during Oogenesis
As described above, the development of oocytes is comparatively slow
Germinal vesicle
during the larval period. Accompanying by the oocyte growth, the germinal vesicle becomesalso enlarged and round in shape, and somewhat irregular in the outline. The first oocyte
located in the most anterior portion of the vitellarium measures about 32 X 32 J.Lm in size
and three times of the maximum diameter of the germarial oogonia, the size of the 14th oocyte is about six times of that of the first oocyte in the fully developed vitellarium of adultfemales. At the late stage of the vitellogenesis the germinal vesicle becomes somewhat ellipSOidal in shape and reaches its maximum size (ca. 120 X 86 J.Lm). That is, this is about
ten times in the major axis of the oogonial nucleus. During this process, the nucleolar masses appear in numerous small fragments which unevenly disperse throughout the nucleoplasm, and these nucleoli disappear in the late Vitellogenesis. This condition is similar to
those of the panoistic ovarioles of hemimetabolan insects and primitive polytrophic ovariolesof neuropteran Chrysopa (Gruzova et al., 1972; Matsuzaki, 1978) and Stenoponia of theSiphonaptera (King and Teasley, 1980).
Mitochondria
During the early previtellogenesis at the late larval period, the mitochondria are the most numerous and conspicuous organelle found in the ooplasm. In the earlyPanoistic ovariole of dobsonfly

17

Fig. 4. Perinuclear cytoplasm of the larval previtellogenic oocyte. Numerous mitochondria(M) and smooth-surfaced endoplasmic reticulum (ER) are observed. N nucleus.
X 10,000
Fig. 5. Annulate lamellae (AL) in the late larval previtellogenic oocyte. M mitochondria.
X 9,000
Fig. 6. Previtellogenic young oocyte (DC) and flattened follicular cells (FC). MV microvilli between oocyte and follicular cells. Arrow head indicates microvilli between
two oocytes. M mitochondria.
X 9,000

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Matsuzaki, M. et al.

oocytes situated in the anterior position of the vitellarium, the mitochondria fairly increasein number and somewhat grow in size (about 1.5 X 0.3 tnn) than those in the germarial 00gonia. Subsequently, as the oocytes grow the mitochondria conspicuously increase in number especially in the cytoplasm around the germinal vesicle (Fig. 4). These mitochondria aremostly round or short-rod shaped with irregular cristae, and rod-shaped mitochondria are
1.5-3.3 /lm in length. However, sometimes the very long rod-shaped (maximum length aproximately 6.0 /lm), Y-shaped and dumbbell-shaped mitochondria are observed, and it isthought that they are to be interpreted as the terminal stage of mitochondrial division asreported by Liu and Davies (1973) and others. Moreover, numerous smooth-surfaced
endoplasmic reticulum which may be originated from the nuclear membrane are foundwithin the ooplasm embedded numerous mitochondria around the germinal vesicle (Fig. 4).
In the late previtellogenic oocytes of the adult ovarioles the mitochondria are dispersed into
the cortical and subcortical ooplasm. While their number decreases in the ooplasm close to
the germinal vesicle. These mitochondria are largely subspherical or short rod-shaped withmany transverse cristae and are slightly smaller in size (0.6 X 0.5-1.0 X 0.6 /lm) than thosefound in the former stage.
Annulate lamellae
In the germarial oogonia, as mentioned above, the annulate lamellae
occur in contact with the nuclear membrane, and are also connected to the rough-surfaced
endoplasmic reticulum (Fig. 2). In some cases the annulate lamellae are found as a single
strand within the nucleus (Fig. 3) as observed in the oocyte nucleus of Ophion (King andRichard, 1968). Anagasta (Cruickshank, 1972), Aedes (Fiil, 1974) and preblastodermal
nuclei of Oncopeltus eggs (Gassner and Sears, 1977). In Protohermes, in the young vitellarial oocytes the annulate lamellae are scattered in stacks in the endoplasm and subcorticalooplasm (Fig. 5). From the early to the late vitellogenic stage, the stacks of annulate lamellae developed within the cytoplasmic electron dense masses are observed here and there in
the cortical and subcortical ooplasm. Similar results were also obtained in the Libellula
oocytes (Kessel and Beams, 1969), in the nurse cells of trichopteran Parastenopsyche(Matsuzaki, 1972), and homopteran Bothrogonia (Matsuzaki, 1975).
Follicular epithelium
In Protohermes ovarioles, during larval period the epithelium enclosing the young previtellogenic oocytes is incompletely formed by scattered squamous
follicular cells, because the oocytes and their follicular cells during this period grow slowly insize and number. Accordingly, the cytoplasm of these cells is poor in the organelles, i. e.,
only some mitochondria and rough-surfaced endoplasmic reticulum may be seen (Fig. 6).
In the boundary between the oocyte. and follicular epithelium small microvilli develop,
among which an electron opaque substance may be seen. Subsequently, as the oocyte
growth, the follicular cells gradually increase in number by means of mitotic division andtheir size markedly increase at the same time (Fig. 7). Consequently, at the early stage ofthe vitellogenesis the oocyte becomes to be enclosed in the cuboidal follicular epithelium
which is approximately 8 /lm in height (Fig. 8). The cytoplasm of follicular cells contains
numerous ribosomes, and other organelles such as the mitochondria, rough-surfaced endoplasmic reticulum etc., also increased in number. The microvilli are more n'umerous ascompared with those of the previtellogenic stages, and the micropinocytotic vesicles or pinosomes become to be found in the cortical ooplasm (Fig. 8). From the middle to the latePanoistic ovariole of dobsonfly

19

Fig. 7. Late previtellogenic oocyte (OC) and somewhat flattened follicular epithelium(FE). M mitochondria, MY microvilli.
X 6,000
Fig. 8. Early vitellogenic oocyte and cuboidal follicular epithelium. AL annulate lamellae,
FN follicular cell nucleus, M mitochondria, MY microvilli, Y proteid Yolk.
X 6,000
Fig. 9. Late vitellogenic oocyte and surrounding the columnar follicular epithelium. AL
annulate lamellae, ER rough-surfaced endoplasmic reticulum, FN follicular cell
nucleus, G Golgi body, M mitochondria, MY microvilli.
X 6,000
Fig. 10. Conspicuously elongated follicular cells (FC) on the anterior most portion of oocyte (OC). Whorls of the endoplasmic reticulum (ER) is developed in the cytoplasm near the follicular cell nucleus. MY microvilli.
X 6,000

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Matsuzaki, M. et al.

vitellogenic stage the follicular epithelium attains their maximum size (approximately 9 tIm
in height) and is columnar in shape (Fig. 9). As shown in Fig. 9, the mitochondria with theelectron dense matrix and rough-surfaced endoplasmic reticulum increase remarkably innumber. The Golgi bodies are also found in some places of the follicular cells. The part of
the follicular epithelium investing the anterior pole of the oocyte is significantly thick (1820 tIm in height) (Fig. 10). The cells in this part have the synthetic function of the micropylar apparatus and the egg membranes i. e. vitelline membrane and chorion. Therefore thedevelopment of the cell organelles especially the whorls of rough-surfaced endoplasmic reticulum is noticeable.

Discussion
The above mentioned ovarian development in Protohermes gFandis belonging to Corydalidae of the Megaloptera roughly agrees with that known in many other insects having panoistic ovaries, they are the ThysanUFa (Bitsch, 1980a, b; Bitsch and Bitsch, 1982; Cone and
Scalzi, 1967) included in the Apterygota,. several primitive orders belonging to the Hemimetabola, such as Odonata (Ando, 1962; Beams and Kessel, 1969: Halkka and Halkka, 1968;
Kessel and Beams, 1969; Kessel and GaniQn, 1979; Matsuzaki, 1971; Seshachar and Bagga,1963), Orthoptera (Allen and Cave, 1968; Bassemir, 1977; Cave and Allen.1969; FavardSereno, 1966; Favard-Sereno and Durand, 1963a, b; Gupta, 1968;. Kunz, 1969; Matsuzaki,1971), Blattaria (Anderson, 1964; Gresson and Threadgo!d, I962; Ksiazkiewicz-Ilijewa,
1977; Ogi, 1973; Scheurer,1969; Tanaka,1973; Wiiest,1979; Zinsmeister and Davenport, 1970), Mantodea (Ogi and Iwaikawa, 1980), Grylloblattodea (Matsuzaki et aI, 1979),
Isoptera (Truckenbrodt, 1970), and Thysanoptera (Haga and Matsuzaki, 1980), althoughsome variation of the style of ovarian development may be observed in some of these insects.
The ovaries of these insects have no nurse cells, but their oocyte nuclei or germinal vesicles
develop well instead. For instance, the germinal vesicle of Protohermes ovarioles grows
about ten times in the maximum diameter larger than that of the germarial oogonia: Inthese insects RNA synthesis during the oogenesis would reasonably be carried out only by
the oocyte nucleus. Same results are also obtained in some primitive polytrophic ovaries ofChrysopa (Gruzova et aI, 1972; Matsuzaki, 1978) and Stenoponia (King and Teasley, 1980).
Namely, in Chrysopa the germinal vesicle of the middle vitellogenic stage 5 shows a maximum major axis, which is about ten times longer than that of stage 1 (Matsuzaki, 1978). On
the other hand, King and Teasley (1980) have described the relative growth rates of thenuclei of oocytes and those of nurse cells of Stenoponia and Drosophila during the oogenesis. According to them, in Drosophila the volume of each oocyte nucleus grows only
1.5 times, whereas that of each nurse cell nucleus grows between 4 and 5 doublings. In
Stenoponia this tendency was reversed. The volume of the oocyte nucleus increased 5 times,
whereas each nurse cell nucleus only doubled its volume.
In general the follicular cells are fully developed by the onset of the vitellogenesis. For
instance, in Galloisiana ovaries the columnar follicular cells attain their maximum thickness(ca. 35 tIm) during the vitellogenesis (Matsuzaki et aI., 1979). On the other hand, in Protohermes ovaries in the present study, the follicular cells show the comparatively slow
growth and are only about 9 tIm in their maximum thickness. The precursors of the proteid

Panoistic ovariole of dobsonfly

21

yolk substances are supplied from the follicular cells to the oocyte. After this processceased, the follicular cells secrete the material for the vitelline membrane and chorion, andthen degenerate.
Several differences in the fine structure of Protohermes grandis oogenesis were noticed
among the other panoistic ovaries described in the present paper, although those ovaries arevery similar from a functional point of view. Another interesting group having a panoistic
ovaries is the mecopteran Boreus and Siphonaptera which are often placed in advanced
groups because they are considered close to the Diptera. As described above, King andTeasley (I980) found that Stenoponia has the primitive polytrophic ovaries as well as thoseof the Neuroptera. It is very interesting to elucidate the evolutional correlation between thepanoistic and polytrophic ovaries in the holometabolous insects.

Acknowledgements
We wish to express our sincere gratitude to Dr. H. Ando, Professor of University ofTsukuba for his cordial guidance and helpful criticism of the manuscript.

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Authors' address:

Prof. M. Matsuzaki, and Miss A. Sasaki,
Biological Laboratory, Fukushima University, Fukushima 960-12, Japan
Mr. M. Komuro, Kagitori 1-10-13, Sendaishi, Miyagi 982,Japan