Sunday, January 3, 2016

Clavate Tortoise Beetle

I've had these photos of a Clavate Tortoise Beetle, Plagiometriona clavata around since this summer and I've been meaning to post them. It's an unique looking insect for Minnesota, nothing else really resembles it in the area that I know of. I've always found them on Bittersweet Nightshade, Solanum dulcamara, but they are known to feed on other plants in the nightshade family.
The Clavate Tortoise Beetles' odd appearance comes from its enlarged and flattened out elytra and pronotum. Elytra are the hardened fore wings found in all beetles (order Coleoptera) that extend back over the hind wings, which are used for flight, and the abdomen. In beetles, the pronotum covers the thorax, and in the Clavate Tortoise Beetles, it covers the head too. I think this feature is shared by all Tortoise Beetles (not to get all taxonomical on you, but Tortoise Beetles are classified in the Tribe Cassidini which is in the Leaf Beetle Family, Chyrsomelidae. Tribes are tough for me to keep track of, as they don't fit into the classic mnemonic King Phillip's Court Orders Fabulously Good Sandwiches - Kingdom, Phylum, Class, Order, Family, Genus, and Species).
Many species of Tortoise Beetles feature iridescent and metallic colors. Clavate Tortoise Beetles missed out on this fun though. To me, their coloration and texture resemble bird droppings. The dark pattern against the translucent background also has a a teddy bear appearance. So snugly bird droppings I guess.
Pictured above is a larva of the Clavate Tortoise Beetle. The larva is the slightly bristly green lump. The brown on top is a shield made from its own droppings and shed exoskeletons which is used to protect itself from potential predators. The fecal shield is held by an appendage at the end of its abdomen. Rather than acting as a camouflage, the shield contains toxins from the nightshade and deters potential. But I wonder about the adults; does their appearance provide camouflage by mimicking bird droppings? I didn't find any information about the appearances of adult Clavate Tortoise Beetles and its functions. No one else speculated that they might be a mimic of bird droppings. Other species of Tortoise Beetles can press themselves down tight to whatever surface they may be on when threatened by a predator, and some can change their color to some degree. There appears to be a study or two about the Clavate Tortoise Beetle larva, but not the adult stage.
I've seen these beetles now and then for years, but I just learned what they are this past summer. I was pretty excited, I shared the information with anyone I could when there happened to be Bittersweet Nightshade around.