Mayflies et al.

One of my activities at the Pennypack Preserve is to act as a volunteer Streamkeeper.  Streamkeepers are participants in the Delaware River Watershed Initiative who monitor streams within the watershed.  The main purpose is to provide data to evaluate the effects of various ongoing funded watershed restoration projects.   However, another benefit of participation is to monitor the health of your local stream, in our case Pennypack Creek.  Various participants monitor sites both above and below the Pennypack Preserve.  My sites are within the preserve. 

One of the best ways to determine the health of a stream is to see what lives in it.  Highly polluted streams can support very little wildlife, but healthy streams will have an abundance of aquatic creatures.  These can include vertebrates such as fish, aquatic reptiles, and birds.  However, aquatic invertebrates are more accessible to study; all you have to do is turn over some rocks.  I have been interested in documenting mayflies in Pennypack Creek since I started as a Streamkeeper in 2016.  According to our former executive director, Dr. David Robertson, there were no mayflies in the Pennypack Preserve when he came here over 25 years ago, and none were reported in a limited survey by the Philadelphia Water Department in 2009.  However, we have been seeing mayfly nymphs in the creek since 2016.  Last Friday I brought my camera to take pictures of aquatic invertebrates, particularly to document what species of mayfly was present.  I had previously made an, it turns out, inaccurate identification based on a poor quality photograph, so I wanted to see if I could do better.

Mayflies, like all insects, have larval stages (called nymphs in aquatic insects) and adult, winged stages.  The mayfly nymphs live underwater, typically under rocks and cobbles in the stream, where they mostly eat diatoms and algae. Mayflies undergo incomplete metamorphosis, meaning that the larva resembles the adult in having six legs and an insect-like body structure. They do not pupate. The nymph will undergo several molts as it grows. At the last molt, it will swim to the surface, the cuticle will split, and the winged adult will emerge and fly into the nearby vegetation. Winged adults undergo a second molt on land before mating, laying their eggs, and dying. Compare this to butterflies which undergo complete metamorphosis. The butterfly larva (caterpillar) does not look like the adult and has to form a pupa in which metamorphosis occurs to produce the winged adult.

Mayfly nymphs are not super abundant in Pennypack Creek, so it took me awhile to find some.  This one was collected (and returned) at the bottom of the Rosebush Trail.

You can tell it is a mayfly nymph by the three long tails which are characteristic (a few species have only two).  It has a flat head with eyes on top of the head, rather than on the side, which identifies it as a member of the family Heptageniidae.   The fuzzy structures on each side of the abdomen are the gills through which it breathes.  The dark structures on the back extending from the junction of the second pair of legs back to the abdomen are wing buds, which contain the developing wings.  This tells us that later this month, at the next molt, it will emerge as a winged adult.  Based on when it emerges, and  by comparing to photos, this mayfly species is most likely Stenacron interpunctatum.  Stenacron mayflies have an intermediate sensitivity to pollutants.  They can also tolerate the lower oxygen levels which occur in creeks, such as Pennypack, which warm to over 70F in summer.  Other more fastidious mayflies will not live in these conditions.  However, I am assuming that the appearance of these mayflies in Pennypack Creek during the last decade or so is a sign that efforts to improve water quality in this urban/suburban stream have borne some fruit.

This photo is of a damselfly nymph which I collected near the bottom of the Management Trail Spur.  When I spotted it I thought it was a mayfly nymph.  Superficially, when seen on a damp rock, they look similar, but under magnification the differences are apparent.

Like mayflies, damselflies undergo incomplete metamorphosis.  They appear to have 3 short tails, but these are actually the gills through which it breaths.  Gills at the tip of the abdomen are characteristic of damselfly nymphs, while mayfly gills are on the side of the abdomen.  There are no wing buds, so there will be further molts as the damselfly grows to maturity.  Some damselflies lay their eggs in streams, while others utilize ponds.  Damselfly nymphs also have intermediate sensivity to pollution.

The next photo shows a dragonfly nymph, collected near the Rosebush Trail.

This nymph was about 1/2 inch long and probably has multiple molts to go through before emerging as a winged adult.  Like their relatives, the damselflies, dragonflies may lay eggs in either streams or ponds.

The last photo is of the aquatic sowbug, Asellus aquaticus.   Fisherman know these as cress bugs.

These were by far the most abundant invertebrates that I saw.  They were also the hardest to photograph, because they don’t stop moving.  They are not insects, but rather aquatic crustaceans.  They are relatively pollution tolerant.  Their high abundance, compared to mayfly nymphs, is still a sign that Pennypack Creek is an impaired stream given its location in a commercially developed environment.

I was surprised not to see any caddis larvae this spring, although I have seen them in the past.  Caddis flies are more closely related to moths.  They spin silk and do undergo complete metamorphosis.  The larva looks like a small caterpillar with only 6 legs in front.  The famous ones make tube-like structures which they live in until they pupate, but these varieties need cleaner water than found in Pennypack Creek.  The Pennypack varieties spin silk nets to capture debris in the stream on which they feed.  The adults of some species swim to the bottom to lay jelly-like blobs of eggs under the rocks.  I have seen these in past years, but not this year.  I don’t know if I am just looking in the wrong places or at the wrong time of year, but will continue to watch for them.

Pennypack Creek, despite being an urban/suburban stream, supports a variety of aquatic wildlife.  National environmental efforts such as the Clean Water Act, and local efforts, such as those by the Pennypack Ecological Restoration Trust and the Delaware River Watershed Initiative, are helping bring once polluted streams back to life.  Being aware of what lives in your local stream should be an encouragement to continue to support these efforts.

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