What Are Fishflies?

Added: 2026-05-05

[00:00:00]This is a 30-part series giving an ecological overview of the insect orders. This episode will be looking at the Megaloptera, a small but striking order within the Holometabola, that is complete metamorphosis insects.

[00:00:18]These guys are almost entirely tied to freshwater systems.

[00:00:25]The name Megaloptera is derived from the ancient Greek, combining two elements to describe the physical characteristics of the insects in this order.

[00:00:34]First is megalo, meaning large or great, and then tera, meaning wing.

[00:00:42]This is in reference to their large, highly pigmented wings as adults, which often exaggerate their size, as can be seen in the dobsonflies, who have tiny abdomens which are hidden at rest due to the large wings.

[00:01:00]The Megaloptera are, across nearly all species, predatory in their larval stage. And, like the Neuroptera, they are rather fragile and functional in their adult stage.

[00:01:14]The larva can live for quite a long time, often from a few months to several years, depending on the species.

[00:01:21]And they tend to live in flowing freshwater systems, especially ones where they can burrow themselves into the substrate or hide under rocks with just their heads poking out.

[00:01:33]They like cool, well-oxygenated, and pollution-free bodies of water.

[00:01:38]And so, they're like stoneflies in that their presence often indicates a healthy aquatic ecosystem.

[00:01:46]At first glance, they might seem to be just like aquatic neuropterans, but there are some pretty distinct differences. So, although the two groups are actually quite related, as they are to the snakeflies as well, this is a distinct group that has traits that are all its own.

[00:02:07]In addition, this is one of two holometabolic orders which are entirely aquatic in their larval stage, the other being Trichoptera.

[00:02:15]But, these guys are distinct in that they are entirely predatory.

[00:02:20]The larvae of the Megaloptera, often called hellgrammites or alderfly larvae, are commonly used as fishing bait, whether they are real or not.

[00:02:31]They are elongated, armored, and equipped with strong mandibles designed for seizing and shredding prey.

[00:02:38]But, more importantly, they possess lateral abdominal filaments that function as gills, allowing them to extract oxygen directly from the water.

[00:02:49]Many aquatic insects have external gills just like this, but in hellgrammites, they look a bit different because they are derived from a larvaform holometabolic insect.

[00:03:01]Functionally, they do the same job as the gills in, say, mayfly larva.

[00:03:07]The one thing is that they may be more concealed and conducive to locomotion, as the Megaloptera larva swim by wriggling like a sea snake.

[00:03:19]This order is exceptionally small, with only about 300 to 400 known species.

[00:03:25]But, strangely, they are rather successful and found worldwide.

[00:03:31]Let's look at some of the diversity that is present in the Megaloptera, because these guys actually have three distinct common names referring to three distinct groups within the Megaloptera, that is the dobsonflies, fishflies, and alderflies.

[00:03:49]The Corydalidae, that is dobsonflies and fishflies, have larvae that are large, highly active aquatic predators.

[00:03:58]They consume insect larvae, small crustaceans, and even occasionally small fish.

[00:04:05]They both possess gill filaments and sometimes spiracles and can tolerate low oxygen.

[00:04:13]When they're getting ready to pupate, they leave the water, burrow into moist soil near streams, and then complete the process.

[00:04:22]They'll emerge as adults that are short-lived, weak feeders, and males in some species of dobsonflies, specifically, have enormous mandibles used in mating, but not feeding.

[00:04:36]The difference between dobsonflies and fishflies is minimal.

[00:04:41]Dobsonflies are usually just the name referred to the very large fishflies with prominent mandibles.

[00:04:49]Then there's the alderflies in the family Sialidae, and these larvae are smaller, more sedentary aquatic predators, and they often live in soft sediments.

[00:05:02]They breathe through a single terminal filament and cutaneous exchange, which means the oxygen diffuses in directly through the cuticle or exoskeleton.

[00:05:13]They pupate [music] in much the same way in damp soil near the water, and then as adults, they are also weak flyers, short-lived, and feed minimally.

[00:05:26]It's basically the same strategy, but slightly more subdued.

[00:05:32]The hunting strategy of the Megaloptera is about sitting and waiting and then initiating brute force.

[00:05:39]They allow the water to bring food to them, and then grab it, never releasing, using powerful muscles to subdue and kill their prey.

[00:05:49]They have poor eyesight, so they really act upon feeling their environment.

[00:05:56]The strategy they exhibit, the ambush brute force strategy, is tried and true in water and works at several trophic levels. It's employed by fish, crustaceans, amphibians, and even say crocodiles.

[00:06:13]If I had to say what differentiates the Megaloptera from other complete metamorphosis insects, it would be that they employ the sit-and-wait ambush strategy in conjunction with complete metamorphosis.

[00:06:29]In this effort, like the Neuroptera, the larval stage is often more consequential than the adult stage, but the adult stage still sometimes has some interesting traits.

[00:06:40]Adult megalopterans have better eyes than the larva, and they use these for tracking down mates.

[00:06:48]Once mated, they will lay eggs on gelatinous clusters in vegetation, where the larvae will hatch and then simply drop down into the water.

[00:07:01]Unlike many other complete metamorphosis insects, where the larval and adult stage partition resources or form complementary systems, the Megaloptera show minimal cross-stage synergy beyond that which is inherent to their strategy.

[00:07:18]What's interesting is that this still works fairly well because of how nutrients are distributed differently in freshwater versus terrestrial ecosystems.

[00:07:28]Similarly to the dragonflies, because there are somewhat more, or at least more generalized nutrient [music] distributions in freshwater, it supports larger sizes when leaving the water.

[00:07:44]By capitalizing on high nutrient intake [music] in a place where nutrients are somewhat easier to come by than, say, on terrestrial ecosystems, the megalopterans are able to reach such large sizes in their adult stage.

[00:08:08]From an evolutionary standpoint, fossil evidence [music] suggests that the Megaloptera were once a more widespread Although not to the same degree that the fossil evidence [music] shows snakeflies or neuropterans being relict.

[00:08:26]Their strict dependence on clean, oxygenated water does place a hard constraint on where they can exist, which probably contributes to the low diversity found in this group.

[00:08:42]Regardless, where they do exist, they can be quite common.

[00:08:48]As insect groups continue to diversify, and indeed diversify back into freshwater, it has likely contributed to the Megaloptera's decline.

[00:08:59]It's fascinating just how many terrestrial strategies can be repurposed for aquatic environments across all animal groups.

[00:09:10]It does not necessarily mean that the Megaloptera is on its way out because of these [music] adapted strategies, but it does speak to the fundamentally iterative nature of evolution, and how a greater ability to diversify >> [music] >> often translates to a greater ability to invade novel ecosystems.

[00:09:31]Complete metamorphosis as a driver of diversification has allowed life stages [music] to expand into different niches.

[00:09:39]In the beetles, aquatic adaptation exists in some lineages in just the larval stage, and some in just the adult stage, >> [music] >> and some in both, that is, completely aquatic beetles.

[00:09:53]The hellgrammites, being a highly canalized, that is, specialized predator like the Neuroptera introduce a predatory adaptation under the umbrella of complete metamorphosis.

[00:10:07]And this is likely what has differentiated the niche in freshwater ecosystems.

[00:10:12]However, they are under the strain of a constantly shifting ecosystem.

[00:10:24]This is why despite their proven success, they are still considered a somewhat relict group like the snakeflies.

[00:10:32]It's just that freshwater ecosystems are that ephemeral and shifting.

[00:10:38]And especially so for complete metamorphosis insects.

[00:10:44]The Megaloptera represent a early and highly specialized experiment in aquatic predation within the Holometabola.

[00:10:52]One that succeeded locally, but never expanded broadly.

[00:10:57]It's a fascinatingly hardcore group that has brute forced its way into being extant.

[00:11:06]That being said, thanks for watching this episode of privilege bug facts.

[00:11:10]Stay tuned for more bug content just like this.

[00:11:14]Thanks.