http://www.collembola.org/publicat/egg.htm - Last updated on 2009.05.19 by Frans Janssens

Frans Janssens, Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium

Model of the formation of the polar caps of the egg of Collembola

Fig.0. Egg cluster of Proisotoma minuta.
Hatching juvenile at arrow.
Janssens, F. © 2007.12.10

Fig.00. Egg cluster of Isotomurus cf. maculatus.
Zungri, D. © 2008.05.09

In Isotoma viridis, a mature female lays one cluster of 27-54 eggs (Milne, 1960).

Fig.1. Eggs at oviposition.
Lynch, T. © 2001

"La forme de l'oeuf, sa couleur et le tissu de l'enveloppe varient non seulement d'un genre à l'autre, mais encore d'espèce à espèce; les oeufs à enveloppe solide sont en général très peu transparents, lisses, d'une couleur brune plus ou moins foncée et plus souvent oblongs que sphériques: ils appartiennent surtout au genre Podure.
Ceux à enveloppe molle offrent plus de variété dans la forme et la texture de la membrane extérieure; ils sont tantôt oblongs ou ovoïdes, tantôt sphériques ou en sphéroïde aplati de deux côtés; leur couleur est généralement pâle, ou plutôt blanche, mais légèrement lavée de blue, de jaune, de rose ou de violet, selon les espèces. Leur transparence permet de suivre jusqu'à un certain point le développement de l'embryon. La membrane extérieure est lisse dans la plupart des espèces, pointillée ou réticulée dans quelques unes; dans ce dernier cas, les oeufs sont parfois velus; garnis de poils longs et serrés; d'autres sont plutôt épineux que velus; les épines longues, flexibles et un peu frisées comme de la laine, affectant toutes les formes et toutes les directions, sont larges à leur base et aiguës à leur extrémité; elles naissent chacune d'une espèce de bulbe formé de deux renflements placés l'un au dessus de l'autre, dont le premier ou l'inférieur est hémisphérique, et le second ou le supérieur, en disque arrondi; c'est au centre de ce dernier qu'est attachée l'épine.
Dans les oeufs réticulés et velus, c'est du point de jonction des lignes qui forment les mailles du réseau, que sortent les poils; ils sont droits ou perpendiculaires au centre de l'oeuf et n'ont jamais de bulbe pour base." (Nicolet, 1842:16-17).

Fig.1b. Eggs of Pseudosinella rolfsi
Bernard, E.C. © 2006

The eggs of Isotma viridis are smooth surfaced and globular and measure 0.21 mm in diameter before development (Milne, 1960). Oviposition always occurs on the surface and on laying the eggs have an orange-red pigmentation (Milne, 1960).
At oviposition (fig.1, 1b), many eggs of Collembola are spherical and smooth (Lubbock, 1873:82). The eggs have a smooth external shell (Kühnelt, 1961:162), called the 'chorion' (Wigglesworth, 1965:1). E.g., the eggs of Isotoma Walkerii (now Desoria albella) are spherical, glistening white, with the chorion very transparent. (Packard, 1871? cited from Lubbock 1873:84). In Hypogastruridae, the eggs are spherical, whitish, opaque and shiny; the chorion is smooth, in general; the egg is moist and sticky (Thibaud, 1970:127). In Folsomia candida, the fresh egg is milky white and has a diameter of 0.110-0.126 mm; the surface of the egg shell is finely granulated (Gao, Bu, Luan & Yin, 2006:519). In Hypogastruridae, at oviposition, the egg diameter is about 140-260 micron (Thibaud, 1970:128). In subterraneous species, the number of eggs per batch decreases, while the size of the eggs increases (Thibaud, 1970:130).
In Hypogastruridae, the incubation period (time between oviposition and eclosion) is about 24-45 days at 9-12°C (Thibaud, 1970:130). In Folsomia candida, the incubation period is about 7 to 10 days (Gao, Bu, Luan & Yin, 2006:519). In Megalothorax, the incubation period is about 16 days at 20°C (Blancquaert & Mertens, 1979:126). In Hypogastruridae, the eggs can develop and hatch under water (Thibaud, 1970:181).

Fig.2. Eggs of Collembola:
Protaphorura armata (left)
Deuteraphorura inermis (middle)
Orchesella cincta (right)
After Handschin in Kühnelt, 1961:162

Nicolet, 1842:19 is the first to describe the forming of polar caps (fig.6): "Dès que les yeux commençent à paraître, l'oeuf se comprime de manière à prendre la forme d'un sphéroïde aplati; l'enveloppe extérieure se fend dans son plus grand diamètre et forme deux hémisphères qui s'éloignent insensiblement l'un de l'autre; leur sommet s'affaisse, et leur donne bientôt l'apparence de bonnets grecs, appliqués de chaque côté de la nouvelle membrane, qui devient alors, jusqu'à l'éclosion, membrane extérieure de l'oeuf."

Fig.3a. Transparent eggs showing ocular patches Chamberlain, P.M. © 2002

Fig.3aa. Willowsia buski from the USA Egg of 0.3 mm showing ocular patches Zungri, D. © 2009.04.25

Directly as the eggs of Isotoma viridis are laid they begin to enlarge due to uptake of water from the atmosphere. In an atmosphere of 100% relative humidity eggs reach maximum size in a few hours. Further enlargement does not take place until the growth of the embryo begins after about 20% of the developmental period (Hale, 1965c). The eggs become coloured due to the formation of pigment in the body of the embryo, however eye spots appear before the body colouration. Development can be characterized as epimetabolic (Weber, 1968), which means a gradual differentiation occurs during development.

The egg swells soon after deposition (Davidson, 1932:867; Schaller, 1970:51; Thibaud, 1970:130-132), due to absorption of water (Thibaud, 1970:130). The eggs do not remain spherical at all; they increase in size rapidly after oviposition, to remain stable halfway the incubation period (Thibaud, 1970:130-131). Due to the formation of a dorsal organ, the form of the egg changes by the rising of the dorsum of the growing embryo (Schaller, 1970:51). As a result, usually on the second to fourth day of embryonic development, the chorion bursts (Schaller, 1970:51). In many species, the chorion does not split randomly, but symmetrically at the equator of the egg (Davidson, 1932:867; Schaller, 1970:51; Thibaud, 1970:132). Due to the swelling of the egg, the two chorion caps thus formed, gradually separate to form two symmetric caps at the poles of the egg, the 'polar caps' (fig.2) (Nicolet, 1842:19; Davidson, 1932:867; Schaller, 1970:51). The separating chorion caps reveal the embryo surrounded by a transparant membraneous inner shell (called the 'vitelline membrane' (Wigglesworth, 1965:1)) (Davidson, 1932:867; Thibaud, 1970:132). In Hypogastruridae, between the 12th to 15th day, the limbs can be recognised through the transparant vitelline membrane; around the 16th day, in pigmented species, the two ocular patches can be seen (Thibaud, 1970:132) (fig.3a and 3b).

Fig.4. Egg of Orchesella cincta
After Bretfeld in Schaller, 1970:51

In some cases, the eggs which are at first apparently smooth, after a few days will be found to be covered with long hairs (Lubbock, 1873:83). The after the chorion split freed vitelline membrane surface carries in many species (such as in Orchesellinae and Tomoceridae) numerous processes, (fig.2 right, fig.4) in other species (such as in Onychiuridae) flattened nipples (fig.2 middle) (Kühnelt, 1961:162; Schaller, 1970:51). These structures enable the egg to adhere firmly to the substrate on which it has been deposited (Kühnelt, 1961:162) (protection against flotation due to in the soil penetrating rain water) (Schaller, 1970:51).

The dorsal organ consists of filamentous processes arising from a pit in the anterior dorsal region and spreading over the entire surface of the embryo beneath the chorion (Wigglesworth, 1965:5). It is suggested that these may be concerned in the absorption of water (Wigglesworth, 1965:5).

Fig.4a. Folsomia candida Egg in anaphase Chorion is split in two polar caps After Gao et al. 2006 Fig.3.5

A schematic animation of the formation of the polar caps of the egg of Folsomia candida WILLEM, 1902 based on observations of a specimen culture kept from 1994 to 1996 (Janssens, 1994:6-10; Janssens, 1996:120-125): fig.5 shows three orthogonal views at the same time: upper left: frontal view, upper right: lateral view, and lower left: dorsal view. Due to the swelling of the egg, the chorion bursts at the equator of the egg. The two halves of the chorion are pushed towards the poles of the egg, while the originally spherical egg evolves into a laterally flattened ellipsoid shape. The chorion halves fold in the shape of a craterlike depression at the poles, forming in such a way two polar caps. The polar caps are eventually 'ejected' from the egg surface (not shown).

Fig.5. Polar caps formation on the egg of Folsomia candida WILLEM, 1902 Animation by Janssens, F. © 1999

Fig.6. "Rupture de l'enveloppe externe de l'oef." Desoria cinerea = Vertagopus cinereus After Nicolet, H. 1842 Pl.1 fig.8,9

Fig.3b. Embryo of Sminthurides aquaticus in transparant egg ready to hatch.
Robertson, A. © 2009.05.17

In Sminthurides aquaticus, at the end of the embryonic development, the eggshell is completely transparant revealing the fullgrown embryo ready to hatch.

Fig.3c. Newly hatched Folsomia candida.
After Tully, T. 2008 ©

In Folsomia candida, at the end of the embryonic development, the diameter of the egg reaches 0.180-0.185 mm. (Gao, Bu, Luan & Yin, 2006:519). Freshly deposited eggs are orange-brown. The egg swells during embryonic development, the chorion bursts and forms two polar caps. The newly hatched instar is about 0.35 mm in length.

Fig.3d. Newly hatched Isotomurus cf. palustris.
Janion, C. © 2007

Newly hatched young of Isotoma viridis measure 0.57-0.63 mm in length and 0.15 mm in head width. A red pigment, possibly carotenoid is present in the body fluid of this species, giving the young a pale red colouration (Milne, 1960). The surface pigmentation only becoming apparent after the first ecdysis (Milne 1960).

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