The "Other" Small Minnow Mayflies (Baetidae) of Summer

I have found four species of small minnow mayflies (Baetidae) in our streams over the summer.  Though it's a bit premature to do a "wrap-up" of these summer findings, that's what this is intended to be.  Clearly, new species will be noted if they are found.

The four small minnow species I've found are Acentrella nadineae, Baetis pluto, Baetis intercalaris, and Heterocloeon curiosum.   Since I focussed on A. nadineae in my last entry, this entry will look at the other three species describing key characteristic,  then noting where and when samples of each have been found.

I. Baetis pluto


The photo at the top of the page is a young Baetis pluto; in the picture below we find one that's a bit more mature.

Baetis pluto is an elegant mayfly with sleek lines and rich colors.  Key features include: 1) the head colors look like longitudinal stripes; 2) tergum (the dorsal side of an abdominal segment) 5 is always pale -- 6 and 7 are completely dark; and 3) the middle tail (caudal filament) is about 3/4 the length of those one either side.  The tolerance value is 3.4.

II. Baetis intercalaris


immature specimen

mature specimen

I've seen a lot of these in our streams.  My first sighting was on May 11th at Powells Creek; my most recent sighting was on August 4th at Buck Mt. Creek.  Again, there are three key features that lead to this identification: 1) the tracheation in the gills is well developed; 2) there are "parentheses" marks, ( ) on the leading edges of the abdominal segments; and 3) the tails (caudal filaments) have dark banding, at the base, the middle, and the tips.  The tolerance value is 5.0.

III. Heterocloeon curiosum


immature specimen

mature specimen

Like A. nadineae, this is a "two-tailer," but only one of the key features for identification is visible in the photos -- the spot of grey pigmentation in the center of each of the gills.  The other feature that we have to see is the "procoxal gills," small finger-like gills that stick out from the base of the front legs; this requires microscope work (see the photo below).  The tolerance value is 2.1.

Now let me give you a run down, stream by stream, of what I've seen and when I've seen it.

A. Buck Mt. Creek

1. A. nadineae: 5/30, 6/30, 7/2, 8/5
2. B. pluto: 6/10
3. B. intercalaris: 6/30, 7/2, 8/5

B. Powells Creek

1. A. nadineae: 8/4
2. B. pluto: 5/11
3. B. intercalaris: 5/11, 8/4

C. Doyles River

1. A. nadineae: 6/23, 7/14
2. B. intercalaris: 7/14

D. Moormans River

1. A. nadineae: 6/24, 7/19
2. B. intercalaris: 6/24
3. H. curiosum: 6/24

E. Rapidan River

1. A. nadineae: 6/6, 8/4

F. Lynch River

1. A. nadineae: 7/2
2. B. intercalaris: 7/2

F. Rivanna River


1. H. curiosum: 7/11, 7/25
2. Oh, yes -- and an unidentified, tiny, tiny, Baetis nymph (three tails) inadvertently found (didn't see it until I looked closely at the photo) when taking a picture of an Emerald dragonfly!

Now that is one "tiny" insect!  (Click on the photo to enlarge it.)