Cannibalistic Lycaenids: Should Have Seen it Coming

Today we were quite disappointed to find that our newly molted Gray Hairstreak (Strymon melinus) chrysalises was being eaten by a fifth instar conspecific!

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The family Lycaenidae is quite famous for the relationship its caterpillars have with ants. Most species produce honeydew to attract and appease ants who may inadvertantly act to protect the larvae by warding of insect predators and parasitoids. Honeydew is comprised of a combination of sugars (glucose, fructose, and sucrose) and amino acids in set ratios that may only attract a specific species of ant. The larvae is protected from the ants themselves (who may normally bite at or attack caterpillars) through the evolution of a unique body form: a tiny head and legs that can be retracted underneath the short and stout body, the slug-like gliding movement while in retracted, extremely thick exoskeleton (sometimes chitinized!), and in some cases appeasing pheromones or sounds. 

In quite a few species, larvae are carnivorous (an highly cannibalistic) such as those that use honeydew to get ants to carry them into their nests. Once inside the nest, larvae start attacking ant grubs. It is not difficult, then, to see why lycaenid caterpillars are notoriously cannibalistic in the lab, when housed together. 

The lycaenid Strymon melinus, the Gray Hairstreak, is part of a group that is not so aggressive; they are certainly not carnivorous by choice. The ant-larvae relationship, as far as we know, is not quite so extreme. The two fourth instars that we collected on mallow (Malva) ate foliage like normal caterpillars and did not behave aggressively towards each other. The newborns, living in the same petri dish, have not eaten each other yet (at least we think!). 

The scary part of the story unfolded today when we discovered a fourth instar rolled up around and feeding on a chrysalis that pupated today, both of which were confined in a small petri dish with a very dry leaf of mallow that was probably unappetizing. The opportunistic caterpillar clearly took advantage of the soft, teneral chrysalis and bit a huge hole at the thorax which was spewing an ungodly amount of blood when we arrived on the scene. Seeing this, we quickly made the decision to remove the other fourth instar inside the dish which had turned brown yesterday (is going to pupate) out to a dish of its own. 

- Brian