Search the Community

About a year ago I made a post where I made a post on the forum where I shared some of my more interesting fossil insects I had found at the Florissant Fossil Beds in Colorado. For those uninitiated, the Florissant Formation is an Eocene (Priabonian Stage, ~34 Ma) lägerstatte notable for exceptional preservation of plant and arthropod fossils. My original post can be found here: One of the fossils I showed off in that post was of an insect larva which I had interpreted as a fly (order Diptera) larva, speculating that it may be a botfly larva because it possessed setae which resembled those of a botfly. I had never been super confident in this interpretation, and over the last year I’ve showed the fossil to several knowledgeable friends, paleontologists, and entomologists. What has emerged is a different consensus that I am much more confident with (and is far more intriguing than what I had originally thought!): this fossil is in fact the dorsal view of a caddisfly larva (order Trichoptera). There are several anatomical characteristics present which I had failed to notice initially. To start, I was viewing the fossil backwards. I had interpreted that anterior end as the posterior end and vice versa. Once I had this corrected, it became clear that the insect’s head and thorax are both clearly definable and both eyes are visible on the head. Additionally, small portions of the legs are visible sticking out from the thorax as well. I originally did not see any legs which had informed my original interpretation, but in this fossil most of the legs are tucked underneath the body and not visible. Here’s the fossil. The anterior end faces to the left. Here’s a picture of a modern caddisfly larva sans its protective casing. It is oriented in the same fashion as the fossil, for comparison: Caddisflies are closely related to butterflies and moths (order Lepidoptera) but they have larvae which live in bodies of water. Many caddisfly species’ larvae build little casings out of silk and pebbles/debris to protect themselves, but this is not universal. The larva seen here is not within a casing, so it either died in a rare instance in which it was not in its case or it is of a species which does not build cases. Adult caddisflies have been reported from Florissant, as have fossils of isolated larval casings, but I was unable to find a description of a body fossil of a caddisfly larva. Not to say one doesn’t exist, they just seem to be obscure. So there you have it: a really interesting and unique find from this fossil locality. Just needed another half dozen or so sets of eyes to figure out what was actually going on there!