Silba adipata McAlpine

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Author: François DROUET.
Photographs: François DROUET.
(unless indicated).
All rights reserved.

 

 

The larva

 

 

In this chapter, I give the Black Fig Fly larva description, according to the following plan: morphology, size, cephalic capsule, pharyngeal armature, ventral surface, respiratory system.

I provide the results of my own observations, but to guide and reenforce these, I referred to the description of Filippo SIVESTRI. 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 author distinguishes two instars for the Black Fig Fly larva : the newly born larva, the characteristics of which persist until the 2 mm size, then the larva which he calls the adult larva. For the descripttion, I stick hereafter to the second instar (the adult larva).

 

MORPHOLOGY

 

The larva is formed of 11 segments (3 for the thorax, 8 for the abdomen), plus a cephalic capsule (head) which is retractable inside the first thoracic segment. In the past, the anterior end of the flies larva was called the "head", but contemporary entomologists classify the flies larva among the acephalic larvae and identify a "cephalic capsule".

In the photograph 1, below, the cephalic capsule is not visible, because its is retracted inside the first thoracic segment. The latter is easily recognizable having a different pointed shape compared to the other segments - on the right in the photograph.
 

Black Fig Fly larva with the cephalic capsule retracted inside the first thoracic segment.

Black Fig Fly larva with the cephalic capsule retracted inside the first thoracic segment.
(from left to right, we can count 11 segments).

 

In the photograph 2, below, the tiny cephalic capsule is visible, protuding from the first thoracic segment - on the right in the photograph. It is not visible to the naked eye.
 

Black Fig Fly larva with the tiny cephalic capsule protubing from  the first thoracic segment.

Black Fig Fly larva with the tiny cephalic capsule protubing from  the first thoracic segment.
(the first thoracic segment being the single segment with a pointed shape).

 

On the last segment of the larva, we can see two brown dots, which are respiratory orifices called the posterior stigmas - on the left in the two photographs above. They are not visible to the naked eye.

Inside the two first thoracic segments, we observe by transparency a black Y-shaped anatomical piece composed of the pharyngeal armature extended by the mandibles (mouth hooks). The pharyngeal armature is a skeleton on which certain muscles are attached, in particular those of the pump which sucks food.

 

SIZE

 

According to my measurements, the length of the larva is a little less than 1 mm  at birth, and about 7 mm  at full development (when it leaves the unripe fig). Knowing that when we cut and open an old infested fig, the largest larvae are most frequently 6.5 mm  long, not having quite completed their development.
 

Silba adipata McAlpine larva maximum length (7 mm).

Silba adipata McAlpine larva maximum length (7 mm).
 

Regarding the smallest larvae that I observed in the fig central cavity, the minimum length that I measured is 1.5 mm. The smaller larvae (1 mm long) having escaped my observation during my researches in the central cavity of the figs, yet numerous. But I regularly observe under the ostiolar scales larvae which have just left the egg, or are in the process of doing so, the size of which is slightly less than 1 mm.

The discovery in the fig central cavity of a 1.5 mm long larva is an extremely rare observation. When a fig is opened or fragmented to look for larvae, those of very small size (1-2 mm) remain inside the half-fig or fragments, flattened against the flowers, almost motionless due to their very low vivacity. I noted 1.5 mm long larvae observations on April 16, 2020 and May 7, 2021, in breba figs of the 'Grise de la Saint-Jean' variety. The second observation was made possible only because the 1.5 mm long larva was stuck to a 4 mm long larva, when the latter fell on the observation dish, during the fig fragmentation with the fingers. At these times of the year, the size of 1.5 mm corresponds to a 2-day-old larva.
 

Silba adipata McAlpine larva 1.5 mm long.

Silba adipata McAlpine larva 1.5 mm long (vey rare observation).

 

CEPHALIC CAPSULE

 

RETRACTILITY

The cephalic capsule is retractile, and is often fully inserted into the body. More précisely in the first thoracic segment, which has a pointed shape, unlike the other ten larva segments. When the cephalic capsule is retracted, the first thoracic segment can be misinterpreted as what would constitute a head for the larva due to its pointed shape. But it is not so, we only see the thorax and the abdomen of the larva.
 

Black Fig Fly larva: cephalic capsule protuding from the first thoracic segment.

Black Fig Fly larva: cephalic capsule protuding from the first thoracic segment.
(note the two mouth hooks born by the tiny cephalic capsule).
 

The cephalic capsule is housed at the tip of the first thoracic segment in a sort of open sac (pocket), and projects forward when released.
 

Black Fig Fly larva: opening of the first thoracic segment pocket housing the cephalic capsule.

Black Fig Fly larva: opening of the first thoracic segment pocket housing the cephalic capsule.

 

Black Fig Fly larva: cephalic capsule projected out of the first thoracic segment.

Black Fig Fly larva: cephalic capsule projected out of the first thoracic segment.

 

SIZE

The cephalic capsule is of tiny size compared to that of the constituent segments of the larva, in particular of the first thoracic segment which shelters it. For the untrained eye, there is a risk of confusion with the terminal part of the first thoracic segment. We can notice that the first thoracic segment is in fact made up of two parts: a first part, pointed but wide, which follows the second thoracic segment; a second part, very short and much narrower after a clearly identifiable tightening, at the end of which appears the cephalic capsule when it is not retracted. The narrowed terminal part of the first thoracic segment should not be confused with the cephalic capsule.

So, in the photograph below, the cephalic capsule is the tiny rounded appendage that appears at the very end of the larva anterior part. The pharyngeal armature rear points protrude into the second thoracic segment. If we move towards the larva rear from the cephalic capsule, we distinguish the terminal part of the first thoracic segment, very short and narrowed, which stops at the level where the two pharyngeal armature rear points (forming a V) meet; then the first part of the first thoracic segment, in the shape of a point but clearly wider.
 

Silba adipata McAlpine larva: tiny cephalic capsule protuding from the first thoracic segment.

Silba adipata McAlpine larva: tiny cephalic capsule protuding from the first thoracic segment.
(do not confuse it with the narrowed terminal part of the first thoracic segment).

 

DESCRIPTION OF F.SILVESTRI

F. SILVESTRI provides the two figures below for the Black Fig Fy cephalic capsule (which he calls the head). He highlights a bilobed form at the top, two antennae, a mouth, and two mandibles (mouth hooks). 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.
 

Black Fig Fly larva: cephalic capsule (viewed from below).     Black Fig Fly larva: cephalic capsule (viewed laterally).

Black Fig Fly larva: cephalic capsule (viewed from below, left; wiewed laterally, right).
A = antenna, dorsal part. B = antenna, ventral part. C = pre-oral sensilla (receptors). D = mandible (mouth hook). E = lower lip.
Credit : F. SILVESTRI.

 

MOUTH

F. SILVESTRI indicates that the mouth is located on the ventral part of the retractile cephalic capsule. The mouth orifice is not visible to the naked eye.

The author specifies that the oral lobes are located on the ventral surface and partly on the side, and that they present dense transverse lines (see the figures provided above). I was able to observe the oral lobes lines under a stereomicroscope (see photographs below).
 

Black Fig Fly larva: oral lobe with transverse lines.

Black Fig Fly larva: oral lobe with transverse lines.

 

Black Fig Fly larva: oral lobe with transverse lines.

Black Fig Fly larva: oral lobe with transverse lines.

 

MOUTH HOOKS

Above the lower lip are two mouth hooks. These play in the vertical plane and, according to certain entomologists, would also serve as locomotion organs. On the side of the inward part of the mouth hooks, the oral lobes end in a slightly lobed blade, also represented by F. SILVESTRI in the figures above.

Below, a photograph showing the two mouth hooks..
 

 Black Fig Fly larva mouthhooks protubing from the first thoracic segment.

Black Fig Fly larva mouth hooks protubing from the first thoracic segment.

 

Black Fig Fly larva mouth hooks protubing from the first thoracic segment.

Black Fig Fly larva mouth hooks protubing from the first thoracic segment.
 

Looking closely at this second photograph, we can see the lower lip, between the two mouth hooks, below the exit points of these.
 

Black Fig Fly larva: lower lip and mouth hooks.

Black Fig Fly larva: lower lip and mouth hooks.
(for the lower lip, see the F. SILVESTRI's figures above).

 

ANTENNAE

F. SILVESTRI indicates that the cephalic capsule bears two antennae, located above the mouth hooks, each at the top of one of the two lobes.
 

Black Fig Fly larva: two antennae, each of them at the top of one cephalic capsule lobe.

Black Fig Fly larva: two antennae, each of them at the top of one cephalic capsule lobe.
 

The author specifies that each antenna consists of a point (composed of two very short articles) in its dorsal part, and of a slight protuberance in a circle in its ventral part (see figures provided above, and photograph below).
 

Black Fig Fly larva: details of cephalic capsule antennae.

Black Fig Fly larva: details of cephalic capsule antennae.
(see areas  A and B of the F. SILVERSTRI 's figures above).

 

Black Fig Fly larva: the antennae, two tiny points at the anrterior extremity.

Black Fig Fly larva: the antennae, two tiny points at the anrterior extremity.

 

PHARYNGEAL ARMATURE

 

The pharyngeal armature (at the anterior end of the larva) is a chitinous skeleton to which the muscles of the pharyngeal pump (which sucks food) and those of the mandibles (mouth hooks) attach.

In the photographs, from a certain magnification, it appears by transparency as a V-shaped black anatomical piece. And the whole composed of the pharyngeal armature extended by the mandibles appears as a Y-shaped black piece. But neiher each of the two elements, nor the whole, are distinguishable with the naked eye. When we look at the larva, we simply perceive a tiny black point at the anterior extremity of it.
 

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

 

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

 

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

 

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.

Black Fig Fly: pharyngeal armature (V-shaped) extended by the mandibles, the whole forming a Y.
(larva in ripe fig, note the larva damage in the infructescence).

 

lack Fig Fly: pharyngeal armature and mouth hooks.

Black Fig Fly: pharyngeal armature and mouth hooks.
 

F. SILVESTRI provides a drawing of the pharyngeal armature. 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. It seems interesting to me to produce it below to understand what we see in less detail in the photographs.

 

Black Fig Fly: pharyngeal armature.

Black Fig Fly: pharyngeal armature.
(on the left, seen from below, drawn stretched; on the right, lateral view with mandibles).

(credit :
F. SILVESTRI).

 

VENTRAL SURFACE

 

The larva's ventral surface is characterized by the presence of creeping welts (fleshy transverse ridges), which assist it in its movements.
 

Silba adipata McAlpine larva: creeping welts.

Silba adipata McAlpine larva: creeping welts (the last relef, on the left, is the anal area).
 

Below, a photograph showing the ventral surface of a Silba adipata McAlpine larva. From left to right in the photograph, three features can be highlighted, as follows.

The ventral surface of the three first segments (thorax) is smooth, they have no creeping welts. The creeping welts of the eight abdominal segments are located on their anterior part, and are increasingly big from the first to the last of them. The last segment shows two protuberances: a creeping welt in its anterior part, and the anal area in its posterior part. The anal area is circular and has several lobes.
 

Silba adipata McAlpine larva: ventral surface..

Silba adipata McAlpine larva: ventral surface.

 

Silba adipata McAlpine larva: anal area.

Silba adipata McAlpine larva: anal area (circular, with several lobes, preceded by the creeping welt of the last segment).

 

Silba adipata McAlpine larva: anal area.

Silba adipata McAlpine larva: anal area (circular relief just on the second millimeter graduation, on the left).

 

RESPIRATORY SYSTEM

 

SPIRACLES

The mature larva has two pairs of spiracles (respiratory orifices): an anterior pair, and a posterior pair (not visible to the naked eye). The newly born larva has only two spiracles (the posterior ones).The anterior spiracles are located on the first thoracic segment (in its posterior part, at the limit with the second segment). The posterior spiracles are located on the upper part of the larva rear, and are much larger than the anterior ones. 
 

Silba adipata McAlpine larva : the four spiracles.

Silba adipata McAlpine larva: the four spiracles.

 

Silba adipata McAlpine larva: right anterior spiracle.

Silba adipata McAlpine larva: right anterior spiracle.
(location: posterior part of  the first segment, at the limit with the second segment).

 

Silba adipata McAlpine larva: posterior spiracles.

Silba adipata McAlpine larva: posterior spiracles.
 

According to the F.SILVESTRI's observations, each spiracle consists of an atrium which opens to the exterior, and a chitinized internal structure .And the author provides the two drawings below. 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.

F. SILVESTRI gives some details. The anterior spiracle protudes little from the surface of the body, and has 8 or 9 lobes, sometimes 7. The posterior spiracle is carried by a very short cylindrical protuberance, and has three lateral fissures-shaped openings. Each of the openings is elongated and provided on its inner limit with small pea-shaped protuberances forming a filter for the air. On the spiracle blade, there are four branched bristle-like appendages.
 

                   

Silba adipata McAlpine larva: on the left, anterior spiracle; on the right, posterior spiracle.
 
(credit:
F. SILVESTRI). 

 

TRACHEAS

Two dorsal longitudinal trunks (main tracheas) extend between the anterior and posterior spiracles (one on each side). From the main tracheas, a complex tracheal branches network supplies air to the muscles and the internal organs.

In the photograh below, we can follow the left main trachea from the left anterior spiracle to the left posterior one.
 

Black Fig Fly (Silba adipata McAlpine) larva: main tracheas.

Silba adipata McAlpine larva: main tracheas.
 

In this second photograph, we can see the exact location of the left anterior spiracle on the first thoracic segment. And we can follow the main trachea starting from it, to join the posterior spiracle (not visible).
 

Silba adipata McAlpine larva: left main trachea and left anterior spiracle.

Silba adipata McAlpine larva: left main trachea and left anterior spiracle.
 

This third photograph shows the junction of the right main trachea with the corresponding anterior spiracle.
 

Silba adipata McAlpine larva: junction of the right main trachea with the corresponding anterior spiracle.

Silba adipata McAlpine larva: junction of the right main trachea with the corresponding anterior spiracle.
 

And the photograph below shows some ramifications of the right main trachea.
 

Silba adipata McAlpine larva: sone ramifications of the right main trachea.

Silba adipata McAlpine larva: sone ramifications of the right main trachea.

 

 

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