Serious Mediterranean Katydid Cannibalism

Tonight at around 10 PM, we were in for a gruesome sight when we saw one of our sixth instar Mediterranean Katydids (Phaneroptera nana) cannibalizing a molting fifth instar!

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We have always had issues in the past with mild cannibalism in katydids when they are overcrowded, with both Mediterranean Katydids (Phaneroptera nana) and Mexican Bush Katydids (Scudderia mexicana), and it is a pretty disgusting sight. Usually, it isn't really too bad -- just the occasional gnawing of the tips of limbs and wings. We even once had a Mexican Bush Katydid cannibalism a fallen roommate back on 5/27 this year. But today, we had the chance of catching red-handed one of our Mediterraneans eating its comrade live in cold-blood.

A ton of interest has been sparked by the cannibalistic tendencies of some animals and in most cases, the verdict has been that cannibalism increases fitness by directly advantaging the perpetrator(s) who reap additional nutrients from the cannibalized. There is plenty of scientific literature available on this behavior, even in some insects such as the infamous praying mantis which makes sense since they are strictly carnivorous anyway. But much less has been studied about cannibalism in Orthoptera, most of which are normally herbivores. One species that has been studied extensively is a flightless katydid called the Mormon Cricket (Anabrus simplex). Basically, these katydids march on their feet in huge swarms and devour any vegetation they come across, similar to the destructive nature of locust swarms. But should any of its members become weak and stop marching with the rest of the group, they are immediately pounced on by nearby crickets and cannibalized. The long-standing hypothesis seems to be that running the risk of being cannibalized by sticking with the gang is better than going solo where the chance of regular predation is much higher. (Read Katydids and Bush-Crickets: Reproductive Behavior and Evolution of the Tettigoniidae by Darryl T. Gwynne for more information.)

What we have seen in our katydids is probably a similar principle. Like Mormon Crickets, cannibalism in Mediterranean katydids is clearly highly density dependent; the more individuals there are, the higher the chance of cannibalism. For example, Mormon Crickets would probably rarely cannibalize if they weren't in a swam and living solitary and our Mediterranean Katydids would probably also rarely cannibalize if they weren't so overcrowded (We are keeping over 40 of them in one little tank right now!). They just wouldn't have the chance to since it's not like they are actively searching for others to attack like a true predator. Furthermore, the taking advantage of weaker members in the Mormon Crickets is a similar idea to our katydids taking advantage of molting individuals who are also weak and unable to defend themselves. After all, most Orthopterans (especially Tettigoniidae) are opportunistic omnivores that will scavenge for dying or dead invertebrates in the wild. 

In any case, the damage has been done already and we do not have the space or time to completely prevent overcrowding and/or cannibalism by housing each individual separately. We have caught well over 50 Mediterranean Katydids this year and only 42 now remain in the tank. Though some may have simply escaped while changing food, many of them are presumably cannibalized. There are few thing that would make us think otherwise after seeing what happened tonight! At least we hope that those that have participated in the cannibalism gain some benefit out of it and help us produce some mighty fine eggs when we attempt to breed them later. 

- Brian