The world of insects is full of strangeness. And this is confirmed by a recent discovery concerning dinidorids, a family of bedbugs widespread in the tropical regions of Africa and Asia. In fact, it was believed that the females of these insects had a hearing organ – something similar to our ears – on their hind legs, and that they used it to listen to the males’ amorous calls and notice predators. The reality, however, is even stranger: in fact, they are not “ears”, but a symbiotic organ in which insects grow fungi, which they then use to defend their eggs from predators. This was revealed by a study carried out by researchers at the Japanese National Institute of Advanced Industrial Science and Technology, which recently appeared in the magazine Science.
Ears on paws?
Insects, as is known, possess a multiplicity of sensory organs: composite eyes for vision, antennas for smell, tactile receptors and, in many cases, specialized organs for hearing. Crickets, for example, have developed the equivalent of our eardrums on their front legs, perfect for detecting males’ courtship songs and sensing approaching enemies. Similarly, cicadas, grasshoppers, moths, and mantises have evolved tympanic organs, located primarily on the abdomen and thorax.
In the past it had been discovered that female bedbugs of the Dinidoridae family they have elongated organs on their hind legs, covered by a structure very similar to a tympanic membrane. Given the morphological similarity with the “ears” present in other insect species, it was established that they were auditory organs. But no one had yet wasted time studying how they worked, and how they were used by these insects. This is clearly where the authors of the new study come into play.
The discovery
Japanese researchers conducted a detailed study on a Japanese species of bedbug (Megumenum gracilicorne), discovering an unexpected detail: the structures present on the hind legs of females that had reached sexual maturity were covered by hyphae, the filaments making up the vegetative body of the fungi, in this case belonging to the Cordyceps family.
By studying insects, researchers have discovered that fungi come from the environment, and are selectively acquired by females with each new generation. And that when they lay their eggs, these insects actively, and with extreme precision, transfer the fungi stored in the organs on their hind legs directly onto the eggs themselves. The eggs are covered by fungal hyphae, creating a sort of microscopic blanket, which helps to improve their chances of survival: in fact, they form a physical barrier that prevents them from getting close to one of their main natural enemies, parasitoid wasps that lay their eggs inside those of other insects so that the larvae, once hatched, can feed on them.
Coevolution
The researchers tested the hypothesis using eggs covered in hyphae and eggs that had their fungal covering artificially removed, showing that the former had an extremely higher chance of survival when brought into contact with wasps. Japanese scientists also examined several other species of bedbugs from the Dinidoridae family, and in all cases, they found the same characteristic: all these species possess the structures on the hind legs specific to females and all demonstrate the distribution behavior of the fungus on the eggs. This – they explain – suggests that the organ on the hind leg and its use to guard the mushrooms and use them in the protection of the eggs must have evolved in their common ancestor, and therefore represent a defensive strategy deeply rooted in the evolution of the family.
From an evolutionary point of view – the researchers write in the study – the fungal symbiosis of bedbugs of the Dinidoridae family represents a magnificent example of coevolution. Bedbugs have not simply acquired the ability to cultivate fungi: they have developed specialized morphological structures to house these organisms, innate behaviors to transfer them to eggs, and a relationship with the environment that allows them to acquire fungi again each generation. Mushrooms, for their part, receive protection and spread through bedbug eggs. It’s a partnership that has had time to perfect over millions of years of shared evolution.