This article is a supplement to my article "Collophorus - ventral tube of springtails". It is inspired by work carried out by Chinese researchers (Chong-Guang Chen, Tong Chen, Bao-Zhen Hua, and Tao-Ruan Wa) from Yangling University (Shaanxi) ... " Structure and functions of the ventral tube of clover springtail sminthurus viridis" Following an exchange with Bao-Zhen Hua, the latter authorized me to use the illustrations * of his article, I thank him very much.
Despite the studies carried out on the ventral tube, the peculiarities of its fine structure have not, to date, revealed all their functions. The researchers therefore dissected the collophore of a Sminthurus viridis (springtail of the family Sminthuridae, common in France) under an optical microscope.
Eversible vesicles :
Illustration A : Eversible vesicles of Sminthurus viridis. Diagram of a ventral view of an adult, showing the vesicles that can reverse out of the ventral tube. (a) Distal part, (b) basal part, (c) ventral tube, (d) furca.
Illustration B : Profile of an adult, with the eversible vesicles extracted from the ventral tube (Scale bar = 0.05 mm).
Fine structure of the eversible vesicles :
The two eversible vesicles, located at the distal end of the ventral tube, can extend from the apical opening of the tube. They are endowed with many tiny papillae and also have a kind of "central gutter" connected to these papillae. This open channel extends into the body cavity. The eversible vesicles can reach all parts of the body and thus perform various functions: absorb moisture; absorb water; clean the surface of the body and fix the body of springtail on a smooth surface.
The fine structure seen under an optical microscope (above - scale bars = 0.02 m).
A: Eversible vesicle. (a) Distal part - (b) basal part - (c) ventral tube
B: Detail on the arrangement of the papillae on the basal part
C: Papillae on the distal part. Eversible vesicles connect four muscle rods in the abdominal cavity.
After dissection of higher magnifications (illustration below), show:
Fig. 1: Liquid transmission ducts from the small papillae to the central duct.
Fig. 2: Central duct on the distal part, where the papillae connect.
Fig. 3: (a1, a2, b1 and b2), muscle rods of the eversible vesicles. (c1 and c2): interface with the digestive system. (d) ventral tube, on the surface of the body.
The structure of the ventral tube differs according to the orders. It is generally short in Poduromorphs whose way of life and the sectors of the ground where they evolve limit their possibility of jumping. Conversely, it is considerably long in Symphypleones, which are generally found on the surface with enough clearance to allow jumps. On the distal part, each eversible vesicle can carry up to 200 papillae. While there is much less of it on its basal part. Each papilla has a transverse duct (arrows illustration 1) connected to the "gutter" (arrow illustration 2) which is enveloped in a thick and powerful muscle tissue capable of stretching up to 10 times its folded size (Scale bar black e = 0.02 mm)
Role of vesicles during jumps :
Many species of springtails are able to perform jumps using the forked appendage (furca) located on their fourth abdominal segment (abd.4). Many of them also have a collophore located on the ventral aspect of the first abdominal segment (abd.1). This collophore is presumed to be the result of a fusion of old appendages located on their first abdominal segment (abd.1). Some researchers believe that their ability to turn inward is due to the hydraulic pressure within their hemocoel (a cavity in the body envelope or/and storage organ in some invertebrates). The ends of the vesicles have the sort of circular suckers that allow them to attach when the springtail moves on smooth or relatively uneven surfaces. In some species, the vesicles of the ventral tube can extend more than twice the length of the body and be used for automatic righting after a jump.
Methodology: To analyze springtail jumps, the researchers used a hyaline plastic film (glass transparency) placed at different heights (5, 10, 15 and 20 cm) to intercept the direction of their jumps. Whenever a springtail touched the plastic wrap, the first parts of its body to touch it and adhere to its smooth surface were the vesicles. When the springtail takes off from the ground, it simultaneously expands its eversible vesicles, until they reach the support. Upon landing, the vesicles retract into the ventral tube immediately after the legs have stabilized on their support. On particularly smooth surfaces, the tiny papillae help the springtails to stay attached. Thus, a good number of springtails can remain fixed for a long time upside down on a glass plate. It can be deduced that the eversible vesicles play a role for the safety and stabilization of the body in certain complex environments. The results of behavioral observations show that the interceptions are roughly the same at different heights.
Various phases of a jump :
Fig.A : The reversible vesicles stretch when the springtail propels itself to jump
Fig.B : The eversible vesicles are fully extended.
Fig.C : When the springtail lands on a surface, the vesicles are the first to reach the surface. They are then fixed to the support thanks to the "circular suction cups" with which they are equipped.
Fig.D : If the springtail has taken good support with its legs and if these allow it to fix properly, then the vesicles retract into the ventral tube. Note also that the furca is positioned parallel in a folded position under the abdomen. The ventral tube might even help direct the springtail jump.
The ventral tube, a grooming instrument :
In my article on the legs of springtails, I featured a video that showed the use of the legs when grooming. But the legs are not an exclusive tool. Indeed, the vesicles also play a role during the toilet. Springtails have a strong propensity for cleanliness. They do their cleaning job tirelessly when they are stationary. From observations, springtails typically clean their antennae more than 80 times per hour for 2 to 5 seconds each time. Researchers have also found that the most frequently cleaned parts are the antennae. An experiment with Sminthurus viridis also suggests that the vesicles are used to carry water droplets, retained by body hair, to the mouth. Given that springtails inhabit the ground, on which they encounter a multitude of particles of various kinds, they use the properties of their ventral tube to frequently eliminate these particles or the water droplets which attach themselves to their body or disturb certain of their sense organs.
(A) and (B): Collembola cleanses its body with the eversible vesicles.
(C): The reversible vesicle cleans an antenna.
(D): springtail inserts its eversible vesicles into the soil to absorb moisture (scale bar = 0.05 mm).
The movement of the eversible vesicles :
Anatomical experiments show the presence of four retractable muscle rods in the body.These muscle rods control the eversible vesicles and help them expand and retract rapidly. Eversible vesicles move very quickly; for example, they usually take less than three seconds to fully expand and retract when cleaning the body.
Fig.A: Extension movements of the vesicles. The eversible vesicles absorb water directly.
Fig.B: The springtail stands on the vesicles that are reversible in water.
Fig.C: The eversible vesicles help the mouth parts to absorb liquid food.
Fig.D: The eversible vesicles cleanse the head.
The vesicle tissues are soft, flexible and easy to control by springtail, which allows them to perform their various functions. Their hyaline appearance results from an absence of pigmentation. Previous morphological studies, mainly focused on the base of the ventral tube, from dead specimens, did not show the eversible vesicles which were, most of the time, retracted in the ventral tube and the body cavity. That is why they were rarely observed with ordinary methods. It was therefore difficult, in such conditions, to reveal their structure.
The researchers finally found that the Sminthuridae are the springtail family best endowed with eversible vesicles. Other families indeed have very short vesicles, and some even have a very reduced or retracted ventral tube. This tells us that studying the shape of the eversible vesicles may be useful for the classification and phylogenetic analyzes of springtails.
Prospects: Investigations into the fine structure of vesicles can also help to understand the habits of springtails and to better adapt their integration into programs to control certain crop pests **.
* The black and white diagrams accompanying this article have been retouched in PhotoshopCC and colored by me, in order to respect the graphic spirit of the other articles on this site.
** In this regard, (01-01-2021) I am currently in contact with researchers who are conducting a study on the use of a particular species of springtails which preferentially feed on a fungus that pests plants and seeds (fusarium). The study focuses on the introduction of these springtails on cultivated land with the objective of replacing them with phytosanitary treatments. When the researcher in charge of this study communicates his results to me, they will be the subject of a new article on this site.