After finding a female Arrenurus buccinator, I searched the same ditch for a male. I caught two long-tailed mites, one turned out to be A. buccinator, but the other had a clearly different tail and turned out to be Arrenurus securiformis. The name is derived from Latin securis, axe and of course refers to the axe-shaped end of the tail, Viets (1936) compares that with a fish tail. This species is also quite common in the Netherlands (Smit & vd Hammen, 2000).

The two water mites look so much alike that I kept wondering whether I wasn't dealing with more or less grown specimens of the same species. There will probably be a close relationship between the two species, see the comparison picture below.

Note the similar characteristics, such as the color patterns, the spurs on the hind legs, the armor, the front edge that is somewhat concave in both specimens. And of course the long tail: both species belong to the subgenus Megaluracurus, which is sometimes used (the "long-tailed", see A. globator). However, there are also clear differences to be noted. In the A. securiformis specimen (left) the front corners are different than in the A. buccinator specimen, this is because in the latter the side edges of the body run more parallel, this makes the rear corners in this specimen more pronounced than in A. securiformis.

The tails have a clear difference: the buccinator is widest in the middle. And the ends are different, see the detail image below.

Despite the obvious differences, I can imagine how the ends of both types may originally have had the same basic plan.