Signs of Spring 9: Insecta vs. Vertebrata!

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Beetles vs. Toads:

Photo by Osaka University, Wikimedia Commons

Bombardier beetles are a diverse (500 species) subset of carabid (“ground”) beetles that are found on every continent except Antarctica. These beetles, when disturbed, are able to spray a hot, caustic defensive secretion out of a gland at the tip of their abdomens. In these remarkable glands, bombardier beetles mix hydroquinone (a chemical produced by skin glands in all species of carabid beetles) with hydrogen peroxide (a chemical that is a common waste product of energy metabolism) in an abdominal sack that is lined with specific enzymes (catalase and peroxidase). The enzymes catalyze an extremely vigorous, exothermic (heat producing) reaction that generates 1,4-benzoquinone (a very toxic and irritating chemical) that is then explosively ejected out through the opening of the gland. An individual beetle has sufficient stores of hydroquinone and hydrogen peroxide to produce twenty benzoquinone ejections (more than enough to deter a whole array of potential predators!).

Two scientists from Kobe University in Japan just published a study that explored the effectiveness of these bombardier defense mechanisms in resisting predation by two species of toads. Toads are generalized, opportunistic predators, and any prey item that is small enough to fit in their mouths is likely to be taken. One of the toad species in the study was from a habitat in which bombardier beetles were common (and were, thus, a regular part of the toad’s potential prey cohort), while the other toad species was from a habitat in which bombardier beetles did not occur.

Japanese common toad. Photo by Y. Koide, Wikimedia Commons

When either type of toad ingested a bombardier beetle, the beetle responded with a ejection of hot, benzoquinone (the scientists could hear the explosive “pops” coming from inside the toad). The toad, then, just under half of the time, vomited up the bombardier beetle (although this sometimes took over an hour and half to occur!). The remarkable thing about the regurgitated beetles, though, was that they all were still alive (and lived for another two weeks (the time period of the experiment) after being vomited up. Even 107 minutes of emersion in the secretions of the toad’s digestive tract did not kill the bombardier beetles! Whether the beetles have developed some specific resistance to the toad’s digestive secretions or if the impact of the released benzoquinone inhibited normal digestive activity is not known.

There were some very distinct and very logical size relationships in the results of this study: Larger toads of both species vomited up beetles less often than smaller toads (probably reflecting the reduced impact of a benzoquinone dose on a larger digestive systems), and larger beetles were more likely to be vomited up than were smaller beetles (probably reflecting the larger doses of benzoquinone that they could produce).

There were also some interesting ecological (and, possibly, evolutionary) relationships highlighted in this study: Toads that were from a habitat that contained bombardier beetles vomited up the beetles much less often than toad’s without prior bombardier beetle exposure. These results suggest either an acquired (learned) tolerance to the benzoquinone in the previously exposed toads or, possibly, an evolved resistance to the caustic secretions of the beetles.

This paper was published in Biology Letters (February 7, 2018).

Midges vs. Frogs:

Tungara frog. Photo by B. Gratwicker, Wikimedia Commons

In another amphibian/insect interaction study, Dr. X. Bernal of Purdue University noted that blood sucking midges were densely clustering around the males of the Central American tungara frog while the frogs were in their mating pools. Exploring this more closely, Dr. Bernal determined that the midges were detecting and zeroing in on the male frog’s mating songs in order to locate them for their blood meals. Blood feeding flies (like midges or mosquitoes) are known to detect and follow chemical cues to their potential blood hosts, but sound has never been shown to be a possible attractant. Interestingly, the same features of a male tungara frog’s call that are most likely to attract female frogs also attracts the most blood feeding midges! These midges, by the way, are not only significant blood feeders on these male frogs but also carry trypanosome blood parasites that may be preferentially infecting, and debilitating the male frogs!

Dr. Bernal’s findings can be found in papers published in Ethiology (January 6, 2016) and the Journal of Vector Ecology (June 5, 2015).

And, finally, one more interesting insect interaction with a “higher” life form:

Caterpillars vs. birds.

We all know that birds eat caterpillars, and, overall, we are glad that they do! Caterpillars are very hard on leaves and would probably defoliate all  of our forests if left to feed unchecked. One caterpillar, though, the North American walnut sphinx moth caterpillar, has developed a very unique defense mechanism by which it tries to avoid becoming some bird’s dinner. When this caterpillar is pecked at by a bird it is able to contract its body wall muscles and compress itself (“like an accordion,” one researcher commented) forcing air out through a series of holes in the sides of its body. The sound generated by this sudden air expulsion is quite loud (about 85 decibels 5 cm away from the caterpillar) and also quite distinctive. The sound mimics the avian alarm call referred to as the “seet” call and not only startles the hunting bird but tells it that danger is near!

Researchers at the University of Montana demonstrated that this caterpillar generated alarm sound does indeed affect potential caterpillar-eating birds like a true avian community distress call. It’s very hard to both eat and take cover!

(These results were presented at the International Symposium on Acoustic Communication by Animals (2017)).



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