Presentation Biology Living habits Infestation Control methods
Author: François DROUET.
Photographs: François DROUET.
(unless indicated).
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The egg (maturation, incubation)
The description of the egg of Silba adipata McAlpine, accompanied by observations of unhatched eggs and empty eggs envelopes (chorions), is the subject of a specific chapter. In this chapter, I give information and I provide observations about the maturation and the incubation of the egg. According to the following plan: maturation, the egg just after laying, revealing the yolk under a stereomicroscope, number of eggs carried by a mature female, incubation.
MATURATION
Maturation of the egg is the phase during which it forms within the female reproductive system, more precisely in an ovariole. Ovarioles are the fundamental units of ovaries. Each ovariole is an epithelial tube in which oocytes form at one end and complete development as they reach the other. The terminal filament and the germarium, which contains germ cells, are at the distal end. The ovariole joins a lateral oviduct through which the mature egg will pass to reach the common oviduct, then the vaginal duct, leading to the ovipositor.
According to the observations of F. SILVESTRI, in the summer period and for individuals fed with water and honey, the maturation of the Black Fig Fly 'egg takes place in about ten days. Reference: SILVESTRI F., 1917, Sulla Lonchaea aristella Beck. (Diptera : Lonchaeidae) dannosa alle infiorescenze e fruttescenze del caprifico e del fico, Bollettino del Laboratorio di Zoologia Agraria in Portici, vol.12, pp. 123 -146.
THE EGG JUST AFTER LAYING
Silba adipata McAlpine: egg under an ostiolar scale of an immature fig - 'Bellone' variety.
(the scale was cut with a scalpel and turned over).
Silba adipata McAlpine: egg under an ostiolar scale of an immature fig - 'Bellone' variety.
Details on the contents of the egg just after laying (text and figure below) are provided by the insect development biology course of Sihem AGUIB. Reference: AGUIB Sihem, 2020, cours Biologie du développement des insectes (L3 entomologie), Frères Mentouri University, Constantine (Algeria).
The newly laid egg is filled with yolk, a nourishing substance used for the development of the larva. The yolk is surrounded by a thin translucent membrane, the vitelline membrane (mv). And the egg is surrounded by a thick protective envelope, the chorion (ch), which has been secreted by the follicular cells in the ovariole. The yolk is made up of yolk globules (gv) of diverse chemical nature (proteins, etc.), located in a cytoplasmic network, the cytoplasmic reticulum (ret). This is connected to a thin layer of peripheral cytoplasm devoid of yolk globules, the periplasm (pe), or cortex. A special plasm, or oosome (oos), rich in ribonucleoproteins, is formed at the posterior pole of the egg. It is involved in the larva formation process during the egg incubation phase.
In addition to the yolk, the egg contains a fertilization nucleus, located in a thickened area of the yolk mass, called the maturation plasm (pm). When the oocyte left the ovariole to pass into the genital tract, meiosis was blocked in metaphase I. When the oocyte passed in front of the opening of the spermatheca, spermatozoa were released and entered the oocyte through the apical openings of the latter, the micropyles (m). Meiosis completes rapidly in the maturation plasm. Fusion of the two nuclei (amphimixis) restores the diploid number of chromosomes.
Sagittal section of the egg just after laying (example of the Drosophila egg, according to A. HAGET).
REVEALING THE YOLK UNDER A STEREOMICROSCOPE
When I counted eggs in immature figs under a stereomicroscope, I noticed that the chorion came apart quite easily when I scraped the unhatched egg with one of the tips of the dissecting forceps, while holding it with a rounded-tipped pick. It then appears, either the yolk enclosed in the vitelline membrane, which turns out to be surprisingly solid (egg recently laid, or not fertilized), or a larva (egg laid for a longer time, incubation well advanced). In some cases, I use the contents of the egg to determine the uniqueness or the plurality of clutches in a fig.
Silba adipata McAlpine: 4 eggs, 1 of which with the yolk fully revealed after scraping the chorion; on its left, the removed chorion.
(note: the yolk, enclosed in the vitelline membrane, is translucent).
Silba adipata McAlpine: egg with the yolk partially revealed, after scraping the chorion.
(note that the yolk is translucent, unlike the chorion which is whitish).
NUMBER OF EGGS CARRIED BY A MATURE FEMALE
OBSERVATION 1
On June 5, 2021, as I walked around a fig tree to try to spot ongoing Silba adipata McAlpine ovipositions, I managed to crush an individual of the species with a sharp flick against a torn off petiole base on which it was standing motionless, absorbed in its latex consumption task.
Silba adipata McAlpine: female crushed by a sharp flick against a torn off petiole base.
It was a female and, under the impact of the finger, a compact mass of eggs, that I recognized as an ovary, was extracted from the abdomen which split open.
Silba adipata McAlpine: mass of eggs (ovary) extracted from the abdomen of a female crushed by a sharp flick.
Silba adipata McAlpine: mass of eggs (ovary) extracted from the abdomen of a female crushed by a sharp flick.
OBSERVATION 2
In order to find out how many eggs at most a Silba adipata McAlpine mature female can carry at a time, I started in the spring of 2021 to open the abdomen of captured females. During the first examinations of the abdomen contents (under stereomicroscope), I found no eggs, or I found a few eggs which seemed incompletely developed.
But on June 17, 2021, when examining a large Silba adipata McAlpine female (5 mm long), my attention was drawn to two rounded and rather elongated reliefs, slightly lighter in color, located on the ventral side of the abdomen, on either side of the midline. I recognized the two ovaries, and associated their size with a mature state of the female. When I carefully opened the abdomen with two sewing needles, using maximum stereomicroscope magnification (45X), I revealed two bulky eggs masses.
Still under maximum magnification, I succeeded in breaking these masses into several smaller clusters and I counted the well-formed eggs, which I most often removed one by one, sometimes two or three at a time, at the tip of a needle. My count was therefore complete and precise: 92 eggs. This one, however, included ten well-individualized eggs, but which seemed to me to be smaller than the others. These may have been eggs that would not have completed their formation until the following day. In any case, they were well-chorioned eggs. I therefore think that 92 eggs should be retained, which denotes the existence of 92 ovarioles for the 2 ovaries, i.e. 46 ovarioles per ovary .
Regarding these ten smaller chorioned eggs, I note that FLETCHER in 1987, and CHOU et al. in 2012, have shown on species of the family Tephritidae that egg development in ovarioles is asynchronous. It is likely that the same is true for Silba adipata McAlpine, although the latter belongs to a different family (Lonchaeidae). Reference 1: FLETCHER B. S., 1987, The biology of Dacine fruit flies, Annual Review of Entomology 32, 115-144. Reference 2: CHOU M-Y, MAU R. F. L., JANG E. B., VARGAS R. I., PINERO J. C., 2012, Morphological features of the ovaries during oogenesis of the Oriental Fruit Fly, Bactrocera dorsalis, in relation to the physiological state, The Journal of Insect Science 12, 1-12.
It is possible that the number of well-formed eggs carried by a small female (3.5 mm) is lower than that which I found for the large female (5 mm). I do not know in what exact proportion, but if we apply a simple rule of three, a 3.5 mm female would have 32 ovarioles per ovary. Note: G. P. FITT has shown the existence of a clear positive correlation between the number of ovarioles per ovary of females of Bactrocera (Dacus) tryoni Froggatt and Bactrocera jarvisi Tryon and their size (wing size). Reference: FITT G. P, 1990, Comparative fecundity, clutch size, ovariole number and egg size of Dacus tryoni, D. jarvisi, and their relationship to body size, Entomologia Experimentalis et Applicata 55, 11-21.
Note: the prior detection of well developed ovaries seems to me a good process to avoid opening an immature female. I also noticed that the abdomen was relatively thick, but was not swollen. The swollen abdomens, more translucent than usual, are not specific to females, and result from an abundant consumption of sugary food.
INCUBATION
Incubation of the egg is the phase during which the larva forms inside it. It begins as soon as the egg has been laid. According to the observations of F. SILVESTRI, the incubation of a Black Fig Fly's egg lasts 8 days in April, but only 3 days in summer. It therefore takes 3 to 8 days, depending on the season, for the larva to hatch from the egg. Reference: SILVESTRI F., 1917, Sulla Lonchaea aristella Beck. (Diptera : Lonchaeidae) dannosa alle infiorescenze e fruttescenze del caprifico
e del
fico, Bollettino del Laboratorio di Zoologia Agraria in Portici, vol.12, pp. 123 -146.
Silba adipata McAlpine: 1 larva and 3 chorions under an ostiolar scale.
(the larva has just been born from one of the chorions; length: 0.80 mm).
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