Much of mite biology is clouded in mystery—even the delineation of their body segments. A new study upends a 60-year-old model for the proper location of mite “waists.” Shown here is a scanning-electron microscope image of a Proteonematalycus wagneri female mite. (Image by Sameul Bolton, Ph.D.)

By Samuel Bolton, Ph.D.

Samuel Bolton, Ph.D.

Most people are surprised to find out that mites live in more places than just inside their mattress or on their pets. But what we acarologists know about mites is, comparatively speaking, not so much more, for there is still a tremendous amount that we have yet to discover about these arthropods.

For example, our knowledge of global mite biodiversity is so meager that estimates of the total number of undescribed species of mites range across nearly two orders of magnitude—from 500,000 to 40 million. And our ignorance extends to some fairly basic aspects of mite biology. There are still competing ideas over the correct body plan for all mites. (See video, below.)

There is even a controversy over where one major body region ends and another begins. This particular controversy interests me because it illustrates how an influential idea can persist long after evidence comes to light that shows it is likely in error. When a bad idea becomes highly influential, often because the originator is influential or because the idea has aesthetic appeal, it can endure for long enough to become entrenched within the culture of a scientific community.

Mites are arachnids, and that means that they have a body that is divided into a prosoma (the limb-bearing region at the front) and opisthosoma (the limbless region at the back). To keep things simple, I will call the border between the prosoma and the opisthosoma the “waist.” This is apt because in most arachnids there is a waist-like constriction between the prosoma and opisthosoma, which makes it very easy to tell where the prosoma ends and the opisthosoma begins. But almost all mites lack such a visible waist.

In 1963, a well-known acarologist, Leendert van der Hammen, published a hypothesis on where the waist is positioned in mites. He proposed that the waist is delineated by a furrow, present in some mites, that runs obliquely from the top of the body to an area just behind the rear pair of legs (see Figure 1, top). However, there are other mites, such as Micropsammus, that have a body with a vertical furrow that looks a lot more like a waist (see Figure 1, bottom). The dorsal part of the vertical furrow is in a different segmental position to that of the oblique furrow. It is therefore not possible that the vertical furrow has reorientated to become the oblique furrow or vice versa, and so only one of these furrows can be the waist.

A model of a mite (top) shows the oblique furrow that Leendert van der Hammen thought was a “waist,” or the division between prosoma (the limb-bearing region at the front) and opisthosoma (the limbless region at the back). The image of a Micropsammus sp. mite (bottom), however, has a vertical furrow that looks a lot like a waist. (Image by Sameul Bolton, Ph.D.)

Most acarologists treat van der Hammen’s oblique furrow as the true waist. However, van der Hammen’s interpretation was based on oribatid mites, which have highly modified morphologies for defense, and so the oblique furrow seems more likely to be the result of a defensive modification than a true waist. Why, then, is his interpretation still widely accepted? One reason is that this is another example of a persistent and influential idea that is long overdue for retirement. Another reason is that almost all species of mites lack visible body segments. The waist is a segmental border that divides the prosoma from the opisthosoma. Without a series of clearly delineated segmental borders, it is difficult to know which of the two furrows is definitely a waist.

There is one mite, however, that does very clearly show its body segments, especially on the part of the body where the waist is. Proteonematalycus wagneri, which has been collected on no more than a handful of occasions, has been examined only very rarely. The description of P. wagneri, which is more than 30 years old, includes drawings of a segmented body that starkly contradicts van der Hammen’s interpretation. Drawings can sometimes be misleading, though. In a paper published in February in PLOS ONE, I analyze new detailed images of P. wagneri, which more clearly illustrate the flaw in van der Hammen’s hypothesis and offer a new model for mite body segmentation.

As seen in this image of a Proteonematalycus wagneri adult female mite, the oblique furrow is absent and so it cannot be the true waist. (Image by Sameul Bolton, Ph.D.)

The image in Figure 2 shows that P. wagneri has a segmental border that is in exactly the correct position and orientation to correspond with the vertical furrow of Micropsammus (Figure 1, bottom). That border is the true waist, not only because it divides the prosoma from the opisthosoma, but also because there is no sign of the oblique furrow. If you can clearly see the body segments but the oblique furrow is nowhere to be seen, that can only mean that the oblique furrow does not correspond with a segmental border, and so van der Hammen was clearly wrong about that furrow being the waist.

But why is it so important to know where the waist is? Well, as I mentioned above, the waist delineates the boundary between two major body regions, the prosoma and opisthosoma. If the oblique furrow were the true waist, it would mark out mites as very unusual compared to other arachnids. In an important way, Proteonematalycus wagneri shows that mites are not quite as weird as we had thought.

In fact, the position of the waist was correctly determined more than a century ago . But over the past half century, countless papers, including my own, have mislabeled characters as opisthosomal when they are prosomal. Almost 60 years of confusion and debate, all caused by one very influential paper that was written by one very influential acarologist. Oh, what a mitey waist.

Samuel Bolton, Ph.D., is curator of Acari at the Florida State Collection of Arthropods, in the Florida Department of Agriculture and Consumer Services’ Division of Plant Industry. Email: samuel.bolton@fdacs.gov.