Stoneflies and the hatch.

Stoneflies, which belong to the order Plecoptera, are aquatic insects as juveniles. When mature they leave their watery existence and emerge as winged adults capable of flight. The nymphs, which are what stonefly larvae are called, goes through several molts. Young larvae tyically are detritivores or herbivores and feed on whatever is available, typically choosing seclusion as a means of self-preservation. As they get larger some species become predatory. The photo I took shows a more mature stonefly larva attacking a beetle larva. The first image (1.) shows them in close proximity to one another and the second image (2.) shows the stonefly in the process of feeding.

Some stoneflies can become very large, measuring over two inches in length. It may take two or three years to become that big. As they grow and live in that environment they are both hunting and being hunted. It turns out that many fish are fond of eating stoneflies. Avoiding fish isn't the only challenge they face; larger insects, some birds, and even the odd mammal will eat them when given the opportunity. The beetle larva being eaten is smaller than the predaceous mayfly because it is still very young; it may not have even had its first molt yet. When it becomes larger it is likely to turn the tables on things and start hunting them. Predaceous diving beetles, both as larvae and adults, are aquatic and highly aggressive.

Stoneflies live a long time as nymphs but usually only last a few days as adults. Stoneflies typically emerge from the water just before their last molt. It is not unusual for the larvae to exit the water at night en masse. When a group of aquatic insects collectively do this it is called a hatch. Resting on a nearby stone or branch out of the water they completely transform into an adult. The wings are initially soft and compact; air must be pumped into them to enlarge and prepare them for flying.

The adults now can search for a mate. Females tend to be larger than males. Most species do not eat as adults; their sole purpose is to mate and lay eggs. Fish eagerly await what comes next. The females must fly close to the water's surface to deposit her eggs. A single stonefly has little chance of laying her eggs without being first, but when thousands of them are doing it at the same time some will certainly be successful. The hatch is also a favourite time for fishermen as the fish tend to be readily striking in anticipation of an easy meal. Fly fishing aficionados will tie a fly matching the insects found in the hatch; their prey busily taking advantage of the swarming stoneflies busily depositing eggs on the water's surface.

Stoneflies are an important species not only because they represent a significant source of prey for fish but also because they are considered an indicator species. They are very sensitive to water pollution; their numbers are easily affected by toxins, low oxygen levels, or high water temperatures. Keeping our wetlands clean is important for us and all the species that live in and by it. The next time you see a stonefly remember to do your part in keeping the water clean.

Thanks for reading.     Ericspix     Eric Svendsen