Macroinvertebrate Update

A couple of years ago I did a post about macroinvertebrates at my Stream Keeper site on Pennypack Creek. We monitor macroinvertebrates because their presence and diversity is a good indicator of water quality and the general health of the stream. In this previous post, I talked about finding mayfly nymphs, along with other macroinvertebrates, but noted that I hadn’t seen any caddis nymphs. It turns out I just wasn’t looking hard enough, so I thought it would be a good time to update my post with what I am seeing at my site in Pennypack Creek.

Macroinvertebrates are small, but visible to the naked eye invertebrates. Mostly we are talking about the nymph or larval forms of flying insects which lay their eggs in water. The nymphs spend most of their lives in the water, but eventually metamorphose into flying insects to mate. The commonest macroinvertebrates we see include the larvae of midges and black flies. These insects undergo complete metamorphosis meaning that they start life as a caterpillar-like larva, but eventually form a pupa in which they transform into the adult insect.

The image on the left above is a black fly larva. The head is at the bottom with the distinctive bulbous tail at the top. These larvae live on the tops of the rocks and filter food particles from the flowing stream water. The picture above right is of a midge larva. The head is on the left with two prolegs. The tail on the right has hooks to attach to the rock surface where it also filters food. The red color comes from a hemoglobin-like substance which allows it to live in low oxygen conditions. Both these organisms are tolerant of low oxygen and pollution.

There is one mayfly species that I see regularly at my site; in fact it is quite common. This species, Stenacron interpunctatum, is also reasonably tolerant of conditions which is why it is so abundant in our suburban creek. Mayflies are primitive insects. Their larvae are called nymphs and undergo incomplete metamorphosis. This means that the larva resembles the adult and does not form a pupa. Instead they molt periodically, with the final molt producing the winged insect. Stenacron interpunctatum also has the unusual characteristic that the total population contains subpopulations which emerge to mate at different times. There may be up to three different populations in a stream with emergences ranging from late spring to late summer. Therefore, it is not uncommon to see more than one size nymph on the rocks. This is a small nymph photographed in April:

The three tails are distinctive in identifying it as a mayfly. It has no wing pads on the back, so it will need to molt a few more times before emerging as an adult. The picture below is of a freshly molted mayfly nymph, which is paler than the more mature nymph.

The next picture, taken in June 2019, shows a mayfly nymph which is ready to emerge.

The dark structures on the thorax are wing pads containing the collapsed wings. This nymph will eventually swim to the surface and the mature adult will emerge. These particular mayfly nymphs live on rocks in the stream and browse algae and diatoms from the rocks.

Caddis fly larvae are also found in Pennypack Creek. Caddis “flies” are actually more closely related to moths and butterflies. The larvae are capable of spinning silk which many species use to create underwater homes or refuges. We generally divide them into two types. Case-building caddis construct a silk case to which they attach small rocks or sticks. They live in the case and drag it around with them for protection. Case-building caddis tend to be more pollution-sensitive and are not commonly seen in the mainstream of Pennypack Creek. Free-living caddis do not build a case, but may spin a silk refuge which they attach to the rocks of the streambed for shelter. They may also spin nets to catch the food particles which they eat. The common caddis in Pennypack Creek are net-spinning caddis such as the one shown below.

It has 3 pairs of legs in front, and hooks on the back to hold it in its shelter, although it may also move around to collect food from its nets. A less common version is this attractive bright green net-spinning caddis.

The hair-like structures on the abdomen are gills. A different group of free-living caddis are the finger net caddis, named for creating a tubular silk pouch as a shelter. This one is in the genus Chimarra.

It looks similar to the net-spinning caddis, but is quite distinctive in appearance with a yellow body and orange head. The finger net caddis are less pollution tolerant than the common net-spinning caddis, and do indicate somewhat better water quality.

As mentioned above, case-making caddis are uncommon in the main stream of Pennypack Creek, so I was surprised to find these attached to rocks at my site.

These are abandoned caddis cases with tiny pebbles/sand grains attached to the cocoons. I showed these to our macroinvertebrate consultant, Nick Macelko, who identified them as the pupal cases of the common net-spinning caddis. It turns out that while some caddis larvae live in cases and others are free-living, all caddis when they pupate form cocoons which they cover with pebbles or other material. They remain in the cocoon for several weeks until the pupa emerges, floats to the surface, and molts into the adult flying insect. This raises a question about caddis evolution. One might speculate that all caddis were originally free-living and only formed cases as pupal cocoons. However, some eventually evolved to form their cases earlier in development and to carry them around for protection, while other species continued to live freely, and only to form the case when they pupated. The name caddis comes from the Middle English word cadice, which referred to ribbons. Ribbon salesmen were called “cadice men” because they covered their coats with samples of their ribbons. “Cadice” flies, of course, cover their cocoons with pebbles or sticks.

Pennypack Creek contains a variety of macroinvertebrates. Most are reasonably tolerant of conditions as would be expected in a suburban/urban stream. However, their presence does attest to the efforts, both local and national, to improve water quality. Thirty years ago, there were no mayflies or caddis in Pennypack Creek within the Pennypack Preserve. Preservation of the creek by the Pennypack Trust, and the effects of the Clean Water Act on upstream sources of pollution has resulted in improved water quality in our stream.


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