Parasitic beetle that attacks ants reveals unusual brain

The beetle Paussus favieri has a brain with olfactory and motor coordination areas in proportions unusual among insects studied so far. The finding results from the first three-dimensional description of the brain of this species, made by researchers using X-ray microtomography. The insect lives inside the nests of the ant Pheidole pallidula and depends on chemical, structural, and behavioral adaptations to remain in the host colony (DOI 10.3390/insects17070701). The species does not exist in Brazil.

Research by Italian scientists analyzed three specimens of Paussus favieri. Two were males and one was female. The insects were collected in Spain, near Tarifa, in nests of Pheidole pallidula. Two specimens underwent microtomography. One male also underwent classical histological analysis with hematoxylin and eosin staining.

Antennal lobes

The study showed significant development of the antennal lobes. These regions process chemical information. In females, the antennal lobes accounted for approximately thirty-six percent of the relative volume of the main neuropils. In males analyzed by microtomography, they accounted for approximately thirty-two percent. The average exceeded that recorded in the species used for comparison, including Tribolium castaneum, Aethina tumida, Drosophila melanogaster, Apis mellifera, and Schistocerca gregaria.

Researchers link this result to the importance of chemical perception for the species. Paussus favieri exploits chemical cues from its host ant. The beetle uses trails associated with the venom gland of Pheidole pallidula queens to locate colonies. It can also acquire the queens' chemical signature through direct contact.

Central body

The central body also showed a high proportion. This region is part of the central brain complex and participates in functions related to the coordination of movements, navigation, and complex behaviors in insects. In Paussus favieri, the central body accounted for more than twelve percent of the relative volume of the principal neuropils in both sexes evaluated.

The expansion of this area may be related to the beetle's repertoire within the nest. The species uses defensive secretions directed by the elytra (Coandă effect) and acoustic signals. Studies cited in the article indicate the ability to mimic the sounds of worker ants and queens of the host ant. This strategy allows the parasite to solicit trophallaxis and explore vibrational communication channels of the colony.

The optic lobes had a relatively smaller share compared to the reference species. The reduced visual capacity is consistent with a lifestyle associated with the ground and the interior of nests. The optic lobes represented 35.4 percent of the volume of the main neuropils in females and more than 41 percent in males, before the average comparison with other regions.

Pedunculate bodies

The stalked bodies, linked to sensory integration and learning in insects, had a smaller relative volume than in Tribolium castaneum. The female presented about fourteen percent. The male presented about twelve percent. The researchers associate this reduction with the specialized ecology and restricted diet of Paussus favieri, composed of larvae, pupae, and tender individuals of Pheidole pallidula.

Microtomography also revealed asymmetries not previously described for the species. One involved the optic lobes. The other occurred in the pedunculated bodies. In males, the left medial lobe showed greater extension and protruded over the central body. In females, this characteristic did not appear in the same way. The study also recorded asymmetry in the male's antennal lobes, with the right lobe larger than the left.

Scientists highlight the need for further analysis with more specimens. The species has low availability in the field and restricted phenology. Even so, the data indicate a relationship between extreme ecological specialization and brain organization. The work opens a basis for research on the neuroanatomy of myrmecophilous beetles and on the evolution of parasites of social insects.