Last updated on 2002.08.20 by Frans Janssens
Checklist of the Collembola: Some notes on the Ultrastructure of the Cuticula. Microtuberculae.

Jean-Auguste Barra, Laboratoire de Zoologie, Université Louis Pasteur, Strasbourg, 67000, France
Frans Janssens, Department of Biology, University of Antwerp (RUCA), Antwerp, B-2020, Belgium

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We propose a new model for the formation of the square headed microtubercles of the epicuticula of Collembola.


Fig.1. Formation of square headed microtubercle
After Lawrence & Massoud (1973)
Lawrence & Massoud (1973), cited from Palissa (2000:4,fig.6), model the secondary square microtubercles as a straightforward fusion of two primary triangular microtubercles (fig.1). However, we have found some evidence that the secondary square microtubercles are formed by a combination of four primary triangular microtubercles.

Fig.2. TEM of tangential section of epicuticula of
Isotomurus palustris
Barra (1973 unpublished)
A tangential ultra-thin section of the epicuticular microtuberculae of Isotomurus palustris (Barra 1973 unpublished) (fig.2) shows three types of transparancy, an indication of three different types of epicuticular material. This section is taken at a slight cuticular depression: the upper left corner and the lower right corner section occurs at the microtubercular basis and trough the interconnecting ridges, while in the center the section occurs at a level above the ridges. Checking a tangential microtubercular section that is taken sufficiently high above the epicuticular 'ground' surface (fig.2: A) one can easily distinguish:
1. a very light, electron translucent material a at the sides of the square head; this material continues centrally away from the walls of the square, forming a kind of inwards triangular section. This material has the same level of opacity as the material of the ridges (e.g., see fig.2 upper left corner).
2. a very dark, electron dense material b at the corners of the square head.
3. at the core of the microtubercle, a less dark, less electron dense material c in the form of a cross.

It seems that the secondary square microtubercle is formed by a combination of four primary triangular microtubercles. The four triangular microtubercles do not 'fuse' together: they can still be recognised as isolated triangles in the tangential sections of the square microtubercles. The ridges that interconnect the four triangualar microtubercles are reduced in length and diameter, but are still available and serve a function as boundary between material b and c.

It is worth noting that this mechanism of combining primary triangular microtubercles into secondary microtubercles is possibly a generic mechanism for the formation of polygon microtubercles. Note that in fig. 2. there are a few aberrant triangular microtubercles present among the square microtubercles. Note that the secondary triangular microtubercle (fig.2: B) is formed by three primary triangular microtubercles.

Fig.4. Section of square microtubercle
Fig.3. Section of square microtubercle
Barra (1973 unpublished)
We will now define a new model for the secondary square microtubercle based on one of the tubercles in fig.2. Tubercle A is enlarged in fig.3. The model of the square microtubercle is based on the following generic model of an interconnecting ridge. The main body of the ridge is modelled as a cylinder. At each end of the cylinder, a torus is placed transversally. The radius of the torus is proportional to the radius of the cylinder. While the torus is not a part of the ridge, it simulates the wall of the microtubercular pillar that is penetrated by the ridge. As each ridge forms a bridge between two microtubercles, in our model we consider the interconnected pillar walls as being part of the ridge model. Constituting microtubercles boils down to placing properly dimensioned ridge models in their appropriate arrangement.

A preliminary simulation of fig.3 can be seen in fig.4, in which the tangential section is simulated by clipping the model with a horizontally displaced plane. The 'internal to the square microtubercle' interconnecting ridges are reduced in size compared to the 'external to the square microtubercle' interconnecting ridges. The four 'internal' primary triangular microtubercles are obviously recognised. The dark patches at the square corners appear to correspond, according to the model, with transmicrotubercular (wax?) channels.