I wrote about a Very Hungry Chironomid in a post back in March after watching a midge larva munch its way through a patch of diatoms in the River Ehen. I’ve spent the time between then and now trying to capture the image in paint. This is partly to remind myself of the artificiality of all that we look at under the microscope, because so much disruption is involved in getting microscopic organisms from their natural habitats on river beds to a microscope slide. But it is also serves partly as a meditation on the organisms themselves, what they look like in their natural habitat, and how they interact with the organisms around them.
My problems are intensified because it is a long time since I looked at insect larvae in any detail. The last time was, I am fairly sure, practical classes as a second year undergraduate, and I had to do a lot of background reading to remind myself just how different the insect mouthparts are to the vertebrates. I had my own videos and others I found on YouTube as source material, as I tried to work out how a midge larva would work its way through a patch of diatoms. My sketchbook now has page after page of sketches, annotated with diagrams cut from papers and textbooks on insect mouthparts, and I exchanged emails with Les Ruse, a colleague who is an expert on this group.
A chironomid (non-biting midge) larva feeding on diatoms in the River Ehen.
My picture is an approximation of the view that I saw back in early March. Though only a couple of millimetres long, a chironomid is still two orders of magnitude larger than the algae on which it was feeding. This is approximately the same size ratio as a cow and the grass on which it feeds and, consequently, it is difficult to capture both the intricacy of the diatoms and the immensity of the larva. The picture shows the two prolegs at the front of its body. These are unjointed stumpy projections which are capable of limited movement, and enable the larva to drag itself along. The front prolegs have a ring of hooks with which it held onto the stalks of the diatom as it pulled itself into a position where its mandibles could shear through the stalks. The rear-view here is deliberate as it is hard to see the other mouthparts in sufficient detail to draw them, forcing me to search for books and papers with illustrations that I could use as source material.
A sketch of the mouthparts of a chironomid larva similar to that found in the River Ehen. The head is approximately 1/10th of a millimetre across
Even this was complicated because there are almost 600 species of chironomid recorded from Britain and Ireland alone, and mouthparts vary from species to species, depending on their habitat and food preferences. The diagrams in books are often very generalised so I had to go back and forth between these and my photographs and videos to try to work out how the various parts fitted together. The second image in this post is the last of the 13 pages of sketches that I made. The mandibles (a) are impressive shear-like organs on either side of the head which move obliquely and which, in this species, seemed to be hacking through the diatom stalks. The movement of the mandibles carried the food towards the mouth where other mouthparts, the maxillae (b), mentum (c), which roughly equates to the lower lip, and the labium (d), the upper “lip”, direct these into the mouth. Note the fine hair like structures on the maxillae which assist in this process. Finally, there are maxillae palps (e), which are jointed, mobile structures which sense the characteristics of potential foods. Compare this arrangement with our own mouths, where the jaws work vertically and where the taste receptors are inside rather than outside. It is no wonder that I needed 13 pages of my sketchbook to figure it all out.