Odonatologica38(4): 359-363 December I, 2009
SHORT COMMUNICATIONS
Fine structure of theegg chorion in two
anisopteran dragonfliesfrom central India
R.J. Andrew
Post-Graduate Department of Zoology, Hislop College, Nagpur 440 001, India
Received June 13, 2009 / Revised and Accepted July 18, 2009
The fine of the chorion in structure egg Brachydiplax sobrina (Ramb.) and Or-
thetrum s. sabina (Dru.), is described using the scanning electron microscope. The
unwetted of B. sobrina and eggs are bluish-green spindle-shapedwhile those of O. s.
sabina are oval and brown in colour. The chorion is divided into light egg distinctly
an outer exochorion and an inner tough endochorion. The exochorion expands into
a thick, sticky, jelly-like structure in water duringoviposition, whereas the endocho-
rion remains The endochorion is thin unchanged. and smooth in O. s. sabina but in ,
B. sobrina the undersurface of the endochorion is pitted and rough. The apical mi-
is of chamber cropylar apparatus composed a sperm storage (atrium) and a median
which ofsub-terminal orifices. The atrium in projecting stalk, possesses a pair B. so-
brina is dome shaped with a tiny stalk whereas in O. s. sabina the micropylar appa-
ratus is triangularwith a longer stalk and a pair of almost apically placed orifices.
variations in Significant occur the shape and size of the micropylar apparatus. The
functional interrelationship of the micro morphological modifications in the chori-
onic structures is discussed.
INTRODUCTION
electron studies Scanning microscopic of the insect egg chorion have yielded important informationof great taxonomicand functional significance (HINTON,
In 1981; MARGARITIS, 1985). Odonata, however, such studies are confined to
few only a species (MILLER, 1987; IVEY, et. al.,1988; BECNEL & DUNKLE,
1990; TRUEMAN, 1990; 1991;SAHLEN, 1995; MAY,1995), including five from
India(ANDREW & TEMBHARE, 1992,95,96; ANDREW, 2002; ANDREW et. ah, 2006). 360 R.J. Andrew
The work undertaken present was to study the fine structure of post-oviposition in the chorion changes egg of the anisopteran, Brachydiplax sobrina (Rambur)
and Orthetrum s. sabina(Dru.).
MATERIAL AND METHODS
female Egg-laying were collected dragonflies during the post-monsoon period (July-Sept, 2002- 2005). The unwetted fertilized eggs were collected from the sub-genital plate by holding the wings flat and mimicking the action or a needle the thorax which initiated dipping by passing through egg Wet obtained shedding. eggs were by the abdominal in water containers initiate placing tip to egg
(ANDREW & Some also obtained shedding TEMBHARE, 1995). eggs were by dissecting the ovi-
duct and vagina. The exochorion of a few removed fine eggs was using tipped forceps or 10% KOH
treatment(TRUEMAN, 1991). The eggs were processed for SEM by the methods described by AN- DREW & TEMBHARE (1992, 1995).
RESULTS
BRACHYDIPLAX SOBRINA - The unwetted (Figs 1-4). eggs collected from the of the female subgenital plate are small, bluish-green in colour and spindle
shaped with linear dimensions of 360 ± 20 x 230 ± 15 pm. In water, the exocho- rion swells into a and the spongy, sticky, jelly-like structure eggs now measured
370 ± 20 270 ± 15 The endochorion is x pm. tough and 4.5 pm thick. Its under- surface is and pitted rough. The micropylar apparatus is well differentiated from
the exochorion circular by a depression (Figs 1,3) which corresponds to the pedi-
cel of the endochorion (Fig. 4). The micropylar is apparatus composed of a very large chamber dome-shaped spermstorage (atrium) about55 pm in diameterand
a stalk which is and micropylar flat short pm), with a of in (10 pair large (7 pm orifices diameter) (Fig. 3). The endochorionic circular pedicel is well developed.
It is ridged with a central and is located below the hump just micropylar appara- tus (Fig. 4).
ORTHETRUM SABINA SABINA The unwetted fertilized (Figs 5-8). eggs
are oval, brown and pale lightly sticky with linear dimensions of 370 ± 25 x 260
± 15 In the exochorion swells into pm. water, a spongy, sticky structure and the dimension of the to 385 ± 20 310 egg changed x ± 15 pm. The endochorion is smooth and is 5 ± 2 thick pm (Figs 6, 7). It is devoid of articulations and bears a tiny apical pedicel. The micropylar apparatus is triangular, nipple shaped, with a diameter of 38 at the base. The stalk is 30 with pm micropylar pm long a pair of circular almost orifices which apical are 5.5 pm in diameter(Figs 5, 8).
DISCUSSION
The libellulidsB. sobrinaand O. s. sabinaexhibit an exophytic modeof oviposi- tion where the eggs during oviposition extrude from the genital opening and ac- cumulateunderthe ninthabdominal sternum on the sub-genital plate and are then Egg chorion in Brachydiplax sobrina and Orthetrum sabina 361
washed-off by flicking the abdomi- nal tip in the water, as reported fora large number of dragonflies (COR-
BET, 1999). In libellulids that ex- hibit an exophytic modeof oviposi- tion, the eggs are generally oval/sub- spherical to facilitate quick sinking to avoid being consumed by fishes and other aquatic animals. Further-
these settle in more, eggs can easily small recesses in the substrate of the water body (CORBET, 1962). The exochorion of the libellulids B. sob- rina and O. s. sabina exhibits typical libellulidfine morphological chang-
in es water by swelling into a sticky spongy jelly-like mass (TRUEMAN,
1991; SAHLEN, 1995; ANDREW
&TEMBHARE, 1996; ANDREW,
2002). The expanded thick exocho- rion protects the growing embryo and firmly anchors the egg. It further Figs 1-4. SEM micrographsof Brachydiplaxsobrina
(Rambur): (I) cluster of unwetted spindle-shaped provides extended areas for tiny par-
the apical — ticles stick it eggs showing micropylar apparatus; to to and form a pro- (2) fractured section of the egg showing the pitted tective camouflage. The swelling in undersurface of the endochorion; - (3) side view B. sobrina is less comparative then of the micropylar apparatus showingthe micropylar that observed in O. sabina. s. This orifice on a short micropylar stalk; note the dome- is probably due to differences in the shaped sperm storage chamber; - (4) egg without exochorion showingthe ridged, conspicuous,circular water body used for oviposition. O. pedicel with aprominent central hump. s. sabina oviposits in shallow still wa- ter or slow moving roadside water puddles whereas B. sobrina oviposits in much
deeper water bodies (MITRA 2006). SAHLEN (1995) observed that ions in the water and of also affect the of overwintering eggs degree expansion of the exo- chorion.
The endochorion of found in exophytic eggs is generally unsculptured as O. s.
but the sabina, undersurface in B. sobrina is pitted and rough. A sculptured out- er surface of the endochorion has been reported in Ladona deplanta (IVEY et. al., 1988) and Bradinopyga geminata (ANDREW & TEMBHARE, 1996). The pedicel of the endochorion is well developed in both species studied but in B. sobrina it is and with projecting heavily ridged a central hump, as found in R. v. variegata (ANDREW & TEMBHARE, 1994). An endochorionic pedicle is ob- served in of most the libellulideggs examined under the SEM (IVEY et. al., 1988; 362 R.J. Andrew
TRUEMAN, 1991; ANDREW &
TEMBHARE, 1996) although
SAHLEN(1995) did not observe the
while pedicel studying libellulideggs
under the SEM.
SEM studies of the micropylar ap-
paratus have disclosed an evolution-
in since ary pattern Anisoptera, a
progressive reductionin the number
of micropylar orifices is noticed from
14 in Pelaturidaeto 2 in Libellulidae.
This reduction requires less sperm
material for fertilization and accel-
erates the rate of oviposition (IVEY,
et. The al., 1988). micropylar appa-
ratus is also used as an important
taxonomictool in the classification
ofclosely related species of odonates
(MAY, 1995).
In the present study, the dragon-
flies exhibit a typical ‘libellulid’mi- 5-8. Figs SEM micrographs of Orthetrum s. sabina cropylar apparatus (TRUEMAN, wetted (Drury): (5) egg showing expanded sticky ex- 1991), but the large ochorion — dome-shaped and anterior micropylar apparatus; (6) unwetted with the exochorion off show- chamber and of B. egg peeled humped pedicel
the smooth - fractured section ing endochorion; (7) sobrina is similarto thatof R. v. var- showing the thin, tough endochorion; — (8) micro- iegata. The circular depression of with pylar apparatus a triangular stalk and almost exochorion around the micropylar apical micropylar orifice. is well B. apparatus developed in so- hrina and similar found in the of a depression is eggs R . v. variegata and Tra- mea virginia (ANDREW & TEMSHARE, 1994; ANDREW, 2002). The flatand short micropylar stalk of B. sohrina is also unusual since most of the lihellulids
T. and (excluding virginia) O. s. sabina have cylindrical stalks (BECNEL & DUN-
KLE, 1990; SAHLEN, 1995; MAY, 1995; ANDREW & TEMBHARE, 1996;
ANDREW, 2002). The micropylar stalk is modified in accordance with the shape and size of the fertilizationpore of the vagina for rapid and efficient fertilization, as found in the libellulid. T. Virginia (ANDREW &TEMBHARE, 1994). almost In all odonates, the eggcolour varies from yellowish brownto dark brown to black. K. Inoue observed bluish (pers. comm.) green eggs of Macrodiplax cora at Guam Island in (USA) the Pacific Ocean and Hydrobasileus croceus at Ishi- gaki Island in Ryukyu Archipelago, Okinawa prefecture, Japan. Thus B. sohrina is the third odonate in the species world to exhibit bluish greeneggs and the first from the oriental region. Eggchorion in Brachydiplax sobrina and Orthetrum sabina 363
ACKNOWLEDGEMENT
Partial financial support from the University Grants Commission, Pune, India under the Minor
Research Project scheme is hereby acknowledged.
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