The Ups and Downs of Stream Sampling: Benthic Macro Invertebrates

Post date: Jan 4, 2011 9:37:00 PM

Originally posted July 11, 2009

It is no secret that our subjects of study are not considered to be appealing by most. I will admit that if you told me even a few years ago that my future job would be to handle and count all sorts of creepy crawlies that I found in a river, I would never have believed it. However if you can get past the initial heebie-jeebies that usually accompany the thought of bugs, it can open up an entirely new and fascinating world for you that you likely didn’t even realize existed.

Some of the adaptations and behaviour Benthic Macro Invertebrates (BMI’s or Benthos for short) have developed in order to survive in this particular ecosystem are really quite interesting. For example, in its larval stage, the caddisfly Tricoptera will use materials found in its environment (such as sticks, stones, and other debris) to build its own house, which it drags along as a snail or hermit crab would. The dragonfly nymph (Anisoptera) has a strange and somewhat comical adaptation for movement. It propels its large body though the water by taking in and shooting out water from its posterior. There are many other weird and wonderful attributes that make BMI’s compelling, and there is so much more we don’t yet know or understand.

BMI’s come in all shapes and sizes. Like anything else, most of what we find through our sampling will be in the average category, which makes it all the more exciting when you find something unusual. Sometimes these anomalies come in the form of an insect of unusual size, type, shape, colouring or form. For example, on two separate occasions we found what we initially believed to be an albino stonefly nymph (Plecoptera). On both occasions the specimen was pure white other than two little black specs for eyes. Upon further research, we found out that it was more likely that the creatures we found had just recently molted, the natural process these nymphs use to grow. It is still a rarity to find a stonefly that has just molted, because it will only be white or pale for a day or two.

Though all of the specimens we sample have their rightful place in the stream ecosystem, and all have an inert value, some are more unappealing than others. Of all the benthos we encounter, the most unpleasant looking one is probably the cranefly larva (Tipulidae). I would consider this to be the true definition of a creepy crawly. These creatures do not have legs, and their head is usually completely retracted into their maggot-like body.

They use a series of ‘creeping welts’ to get around, and this movement just adds to their unappealing quality. The largest cranefly larva we have encountered so far was approximately the length and girth of my pinky finger. Though they are completely harmless, one can’t help but be in a sort of disgusted awe while watching it.

Thus far we have had some rather interesting encounters with the bugs we sample, and we have learned a lot. I have no doubt that we will come across many more strange and unusual creatures as the summer progresses, and I look forward to that.