Tuesday, June 21, 2011
Why Are Summer Insects So Small?
I will get to my topic -- but first things first. I am sooo excited about finding this stonefly! This is a common stonefly, genus Neoperla. This is the first time I've seen one that's approaching maturity, and the first time I've gotten live photos. And the stream I went to today -- Powells Creek in Crozet -- is the only place I have seen this genus of common stonefly. How do we know that it's Neoperla? It's an easy one: we look at the head. There are only two ocelli -- all other Perlids (common stoneflies) have three that form a triangle at the back of the head. On a Neoperla nymph, there is no "anterior" ocellus. Here's a close-up.
An ocellus -- and I'm quoting Peckarsky (p. 424) is "A small visual organ with only one lens. In many arthropods, ocelli occur in addition to compound eyes."
Another look --
Two other nice photos and then I'll get down to work.
1) Flatheaded mayfly: genus Maccaffertium.
and 2) (surprise, surprise), a common stonefly, genus Perlesta. (Readers must be sick of seeing this insect by now!)
So, why are summer insects so small? I've been thinking of doing an entry on "The Summer Doldrums" since we find fewer insects during the summer in most of our streams, and the insects we find are often tiny. We get spoiled by the nice, big stoneflies and mayflies that we find in the winter and spring.
Evidence. 1) I'm finding two genera of small minnow mayflies in all of my streams -- a "three-tailed" Baetis, and a two-tailed Acentrella. I found both genera today at the Powells. All the small minnow nymphs that I'm finding are much smaller than the "two-tailed" Baetis that I found in the winter. Consequently, they're hard to pick up without harming them, and I'm having trouble getting good photos. 2) The genus of flatheaded mayfly that I see all over the place in the summer is Leucrocuta: these nymphs are tiny. 3) I know that in a month or two I'll be seeing some "Small Square-gilled" mayflies (family: Caenidae): they're very small. 4) And I know that at the end of the summer when I hit the Rivanna I'll find some "Little Stout Crawlers" (Tricos): I don't have to tell fly fishermen that this mayfly is small. 5) True, I'm still finding some decent sized stoneflies (common stoneflies: genus Perlesta, and now genus Neoperla). But Perlids (common stoneflies) mostly hatch in late June and early July. After that, most of the stoneflies I'm going to see will be small.
What's the explanation? Well, of course we have a new generation of mayflies and stoneflies, even some caddisflies, so naturally they will be small. But there's another factor that we could easily overlook. In the summer, there is less dissolved oxygen in the water for the insects that live in the streams. Flows have dropped -- sometimes significantly, and water temps are much higher than they were in the winter and spring: all of this means a decrease in oxygen levels.
As a result -- and I think this is really interesting -- aquatic insects have evolved and adapted to this condition by having the smallest families, genera, and species show up in the summer. The smaller the body, the less oxygen needed to keep it alive! The Acentrella small minnow mayflies I'm seeing right now are "tiny": I know for a fact that another species of Acentrella shows up in our streams in the fall, and they're much bigger-- easy to see and to pick up.
I think I'm right in saying as well that the families, genera, and species of mayflies that we see in the summer (not sure about stoneflies and caddis) are those with the higher tolerance values, i.e. the ones that can survive in lower oxygen levels.
This information comes from a book that I read, and I wish I could remember the source. I can't. So, my apologies to the author for not giving credit where credit is due. Also, if there are professional entomologists reading this entry who wish to comment on this -- please do.