benthic macroinvertebrates Brett's Bugs

So I started with one of my favorite little beetles, the Haliplids, but really, honestly… the aquatic insect with which the general public is most fascinated is probably the Odonata (dragonflies and damselflies). This seems to exclude fly-fisher-persons from the category of general public (because of their obsession with mayflies, caddisflies, and stoneflies). But, lets just consider fly-fishers exceptional members of the general public, that are out numbered by people with a certain nostalgia for the lazy summer days of their childhood. So powerful is the connection of Odonata with these memories, that many sylized dragonflies are emblazoned on many house items are commonly available (Wal Mart, k-Mart, Tiffany & Co.).   I believe most people associate these images with some un-nameable inner peace, but maybe that’s just me.  I do know one woman, who thinks dragonflies are nothing more than sexual predators, but she has issues (which may or may not be justified; who am I to judge?).

The images associated with this post are from the Wyoming Educational Benthic Imaging Project, funded by the Wyoming Assoication of Conservation Districts, who along with EcoAnalysts, retains a limited copy right to the images. As the artist I retain full copy rights. if you’d like to use them, please let me know. Just please be sure to give credit where credit is due.

This dragonfly family is the Gomphidae; known as club-tails or Snake-tails because of the adult’s expanded abdomen, somewhat resembling the “hood” of a cobra. Honestly, it was so long ago when I examined these specimens, I don’t remember their genus. Still, there are generalizations about the gomphids that can be drawn from discussing these specimens–some of which apply to most (or all) dragonflies and damselflies.  For those curious specifically about dragonflies in Montana and Wyoming, I cam tell you that the most commonly collected genus of Gomphidae in the region appears to be Ophiogomphus sp.; this could be largely because of where people are sampling (riffle areas in valley streams).

The fact that these dragonflies covered with bits of fine detritus tells something about their way of life. The gomphid dragonflies are burrowers; they burrow into sand or silt and wait just below the surface for a prey animal to wriggle into striking range.  The complete and total coating of this nymph with sediments probably reflects the the nature its habitat. Specifically, the sediment were this critter was collected is probably much more organic than the habitats used by the upper specimens.  Seeing this specimen with hairs, antennae and even its eyes coated with a layer of “dirt” might suggest this critter was in a habitat that is somewhat inhospitable. However, this specimen was very large–nearly complete larval development–suggesting that this degree of sediment did not affect the    Other species very likely could not survive this amount of organic sediment. For example, the function of mayfly gills would very likely be impeded by a similar coating; resulting in suffocation.

Educators: here is a thought question to ask your students: Why do you think the dragonfly did not suffocate, where many mayflies would?

This specimen gave me a nice chance to photgraph something that is usually a little hard to photograph: the feeding structures of larval dragonflies. [NOTE: larval dragonflies are sometimes incorrectly called "nymphs," but this term refers to animals with "incomplete metamorphosis" (e.g., true bugs, grasshoppers, cockroaches) not those that are paurometabolus (mayflies, stoneflies, dragonflies, damselflies).] The principle adaptation of dragonfly larvae to the predatory life style is the modified labium (lower lip). This structure takes many forms in different insect species, but in dragonflies it is greatly elongated and hinged. The picture here shows the structure, viewed from under the head.  Often, the details of these structures do not photograph well, becasuse of a lack of contrast (yellow-white, on yellow-while, with low depth of field= poor photo). however, the sediment on this specimen allowed for a wide range of contrasts. Imagine this large plate-like structure folded at the lower-right corner of the photo, so that its total length is about 2 times what you see here.  Larval dragonflies, are slow movers; they are lie-in-wait predators. Unlike predatory stoneflies which chase down their prey dragonflies slowly stalk prey and wait for it to come into range.  Although, dragonflies are slow movers, they can rapidly compress their body, resulting in a brief increase in hydrostatic pressure, that cause the labium to launch forward with amazing speed–hardly visible.  The hooks (upper left of picture) puncture the prey and pull it back to mouth where it is held to be chewed and eaten at leisure.

This feeding style is ubiquitous among the Odonata (both dragonflies and damselflies).   Different species and families have different modifications of the labium and this one of the predominant structures used to differentiate larvae taxonomically and systematically.

Educators: here is a thought question to ask your students: How would growth of larval dragonflies change the kinds of food available to them?