Previously on Trichopterology...about 5 years previously, I talked about some very cool caddisflies that live in tide pools. These marine caddisflies feed on soft corals, and also use it to construct their cases. And the females of at least one species, Philanisus plebeius, oviposit into the body cavity of sea stars. Now, I'm not quite sure you'd call this relationship parasitic, because I don't know if the sea stars are harmed at all by the oviposition or the eggs. As soon as the larvae hatch, they leave the sea star through its stomach and out its mouth, and start munching on coral. They seem to be more of a commensalist incubation chamber than a host in a parasitic relationship. It's unfortunate that there hasn't been any more recent research in the literature, nor photographs because I would really love to show you all one of their cases.
Out of the three main indicator aquatic insect groups (caddisflies, mayflies, and stoneflies or EPTs), Trichoptera seem to have the widest range of niche. They range from free moving predators to plant shredders, to filter feeders, to scrapers and grazers. You can find them in tiny spring seeps and large rivers, in temporary pools and the wind swept shores of the Great lakes, and in aquatic habitats ranging from fully freshwater to marine. This diversity of habitats and feeding guilds is a testament to their wondrous use of silk, building cases out of practically every kind of material that can be found in aquatic habitats, or spinning silk into webs and nets.
Marine caddisflies are pretty weird, they have a semi-parasitic lifestyle and they live in habitats that are avoided by all other insects. But, there are other weirdos.
I've talked previously about the tethered casemakers, Limnocentropodidae, which connect their cases to the substrate with a sturdy silk stalk, sometimes tethering to other cases in long aggregations during pupation.
Then there's the Atriplectidae, which really deserve a blog post of their own. They're sometimes called the 'vulture caddis' due to their specialized telescoping head. Much like a vulture, they feed on carrion, but in this case it's other arthropods. The long 'neck' allows them to stay outside the corpse and insert only their head for feeding.
There are several species of caddisflies which spend most of their lives out of the water, in moist habitats. This includes the Platte River caddisfly, Ironoquia plattensis, which undergoes a terrestrial estivation period as larvae during the summer. There's also a British species, Enoicyla pusilla the land caddis, which feeds on dead oak leaves in humid forests and spends most of it's lifecycle out of water. A stranger habit is that of the retreat maker Xiphocentron sturmi. Typical of it's family, it makes a network of tubes appressed to a substrate, in this case rotting wood. What's not so typical is the tunnels are out of water, and weirder yet is it's "chrysalis". When X. sturmi finish larval development, they build a hanging structure that looks sort of like a tiny lemon on a rope, and pupate inside of it.
But really, these are all sideshows compared to the main attractions, a caddisfly-sponge mutualism and an honest-to-god caddisfly parasitoid.
Ceraclea is a genus of caddisflies in the family Leptoceridae, the long horned caddisflies. As the name implies, most leptocerids have long antennae in both adults and larvae. Ceraclea is unusually for a number of reasons, first of which is that their antennae are much shorter than other leptocerids. Another reason is that several species feed on freshwater sponges. I wouldn't suggest trying sponge for yourself, though. It would be like eating fiberglass, since the sponge skeleton is made of tiny glass bars called spicules. These sponge feeding caddisflies are able to ingest both the soft tissues and the spicules without damaging their guts because they have a super tough midgut. They're really feeding on the zooanthellae, endosymbiotic algae that live within the sponge tissue.
The sponge outwardly seems to be the host in a parasitic relationship, since the caddis feeds on and damages host tissue but doesn't consume the whole colony. But according to research from 2003, the sponge benefits as well. Electron micrographs of Ceraclea fulva cases showed that they are composed of a tightly bound series of silk bridges attached to sponge spicules. Furthermore, pieces of living sponge attach to the cases, especially in the late larval instars. Since sponges often spread by fragmentation, the combination of larval integration of living sponge fragments into its case as well as fragmentation during feeding means that the sponge can spread to new habitats with help from the caddisfly larva. Mutualisms are rare enough in aquatic insects that this is the only example I know where both species benefit. There are other aquatic insects that feed on sponges, the spongillaflies for example. But these are parasites, and are not dispersal agents for the sponge like Ceraclea cases.
|Case of C. fulva; 'S' indicates living sponge tissue (Corallini & Gaino 2003)|
At the other end of the spectrum from mutualism, you have parasitoids. This life history includes many groups of terrestrial insects, like the tachinid flies on which I am currently working. Most aquatic parasitoids are not truly aquatic, since they have no special adaptations for the aquatic environment. There is at least one species of truly aquatic chironomid midge which is an ectoparasitoid of caddisfly pupae. And, there are at least a few species of microcaddisflies that do the same.
Orthotrichia species, like all members of the family Hydroptilidae (microcaddisflies), spend 4 out of five of their larval molts as free living. In the final instar, hydroptilid larvae undergo hypermetamorphosis, greatly inflating their abdomens and building a portable case. This is believed to be an evolutionary link between the free living habit and the true casemaking habit, not quite a casemaker but not completely free living either. Orthotrichia in particular builds a tiny purse shaped case out of sand grains. The case is open at both ends and unlike true casemakers can be used bidirectionally.
|Orthotrichia spp. larvae; 1. in pillbox case; 2 & 3. with host pupae; note distended abdomen (Wells 2005)|
In a few unusual Australian Orthotrichia, the initial case is much smaller and pillbox like. The larvae are swept by the current into the nets of filter feeding caddisflies, which they somehow escape and get enclosed within the pupal case of their host. From there, these Orthotrichia construct their normal case, and begin feeding on the caddisfly pupa. This continues until the Orthotrichia abdomen is big and swollen, and the host is no more than a pupal husk. Having taken over it's host's pupal case, the Orthotrichia larva spins it's own cocoon and pupates. The adults are apparently larger than non-parasitic Orthotrichia, which could be in part due to the easy and massive food supply provided by a net spinning caddisfly pupa.
Even the sea star ovipositing marine caddisflies don't seem too bizarre when I consider all the other Trichoptera oddities. Parasitoids bring to mind tachinid flies and brachonid wasps, not aquatic insects. But I guess that caddisflies prove once again that if there's an aquatic habit, they'll find some way of making it work.
Wait...who am I kidding? Marine insects, feeding on soft corals, ovipositing into sea star incubation chambers? Nothing can beat that level of weird.