nanonaturalist: @midnight-mod submitted: Hey! …

nanonaturalist:

@midnight-mod submitted:

Hey! Could I get some help IDing these little fellas? My mom found them in a lake in Texas, they’re about 5-6cm long. She’s a flyfisherman so she’s always really excited about learning about aquatic larvae in areas she fishes! I’ve been digging through lists and guides of aquatic invertebrates/larvae and haven’t found anything quite like them, care to help me out?


These fat babies are water scavenger beetle larvae. I’ve never been blessed with seeing them in person, but I have seen the adults. They can be small, but they can also get very large! Here are a couple from Austin:

image

Tropisternus sp.

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Hydrophilus ovatus 

These were both seen in March 2018. Sorry it took me SO LONG to answer you life is stressful!

December 1, 2018

@indelliblemercinary, that’s a great question (and thanks for pointing that out–I edited my post to say they are beetle larvae to avoid confusion). I think you will find that many immature aquatic insects take on these shapes, which means they can look very similar to one another. I mentioned I’ve never seen water scavenger beetle larvae before, but I have seen predaceous diving beetle larvae before:

This guy is a lot younger than the babies in the submitted photo (this photo had to be taken through a microscope!), but the shape of the head and the mouthparts (those are beetle jaw pincers!) are very similar. If you are familiar enough with beetle larvae, you will notice that the legs, head shape, eyes, and “neck” are different enough that they are not the same type of beetle. 

But let’s look at dragonflies and damselflies, because those are very interesting to compare! I’d like to note that the immature forms of dragonflies and damselflies are known as nymphs because they don’t go through complete metamorphosis (there is no pupa stage, they just molt one more time and come out as a full adult). All beetles go through complete metamorphosis, so their immature phases are called larvae, even when they are aquatic.

I have never seen a living dragonfly nymph (I think they tend to keep themselves buried!), but I have seen their exuvia (the exoskeletons they leave behind after they molt). The exuvia will show you exactly what they looked like, minus some of the coloring. Notice the differences between this guy’s body shape and the beetle larvae: you can actually see the little flaps where his wings were forming, his legs are much longer, he has much larger eyes, and he doesn’t have the “pincer” jaws. Before that final molt, the nymph climbs out of the water so they can hang and let their wings harden, and sometimes you can get lucky and find one near a pond or lake.

Here is a live damselfly nymph! A little different from the dragonfly (we have the tail gills, and a longer body), but still fairly similar when you compare it to the beetle larvae. See the wing buds on his back? 

You are more likely to notice beetle larvae and dragonfly/damselfly nymphs when they are older and ready to become adults, but it’s important to keep in mind that they start out very tiny! As they develop, dragonfly and damselfly nymphs will molt several times, and look more and more like adults each time–but this means they start out looking more “larva-like.” Those wing buds don’t start to become visible until later stages in their development. For beetle larvae, they can start out very thin, and then get very fat as they feed, which can make identifying them tricky!

Aquatic insects are challenging, immature life stages of insects are challenging, so immature life stages of aquatic insects are really hard! I am definitely not an expert on these topics, so always feel free to ask me if I’m sure about something!

December 2, 2018