As Delaware River water levels recede after recent rains and the river returns to its clear flowing nature, an evening stroll and scan of its serene surface reveals an interface alive with an unfolding drama. Multitudes of aquatic insects are struggling to shed their larval skins and lift off from the surface as they make a transition to winged adults, and opportunistic fish are rising to the occasion and devouring them. Local flyfishers tune in to this ensuing drama as keenly as the feeding fish.

Aquatic insects play an important role in the health of the Delaware and other rivers on many levels. Some break down leaves and woody debris (coarse particulate organic matter) that has washed into the river, as they feed on the microbes that colonize this material. Others collect smaller pieces (fine particulate organic matter) drifting downstream for food, utilizing underwater nets they’ve spun, or catching these particles as they stream by with baseball-glove-like appendages as skillfully as an all-star center fielder.

Still others graze on algae growing on riverbed rocks, while some are predators of other aquatic insects. A few even turn the tables by preying on small fish.

Collectively, they facilitate an efficient transfer of energy and nutrients through various trophic [relating to the feeding habits of different organisms in a food chain or web] levels of the river’s ecosystem and surrounding environs. Fishflies and their kin are major predators on other aquatic insects, forming an important link in the aquatic food chain. They in turn provide food for fish, birds and mammals too.

Fishflies and other larger aquatic insects in the order Megaloptera (fishflies, alderflies, dobsonflies) are unlike most other aquatic insects in that they go through complete metamorphosis, undergoing a pupal stage between the transition from aquatic larvae to flying adults. Larvae in this group that are ready to pupate (known as prepupae) crawl from the water onto the shore, where they may seek shelter under a log or vegetation. In two weeks or less, from this pupal stage emerges a winged adult, with a few days remaining in its life cycle to mate and (if a female) return to the river to lay eggs that will begin a new generation.

To learn more about aquatic insects in general, visit rivers.snre.umich.edu/www311/aqinsects_06.htm or https://www.freshwater-science.org/NABLinks/Aquatic-insects.cfm.

[Don Hamilton is the Natural Resources Chief, NPS Upper Delaware Scenic and Recreational River.]