Attractivity or Repellence: Relation between the Endophytic Fungi of <i>Acalypha</i>, <i>Colocasia</i>and the Leaf-Cutting Ants—<i>Atta sexdens</i>

Symbiotic relations are present in the nature and can contribute to the success of the organisms involved. Endophytic fungi live within the leaf tissues without causing any harm to the host plant, and some of them can be a defense mechanism against the attack by the leaf-cutting ants. Ants of the genus Atta are known as leaf-cutting ants and have an obligatory association with the fungus Leucoagaricus gongylophorus, cutting pieces of leaves and bringing them back to the colony to the fungus. The present study aimed to find out the endophytic fungi community of an attractive plant (Acalypha wilkesiana) and a less attractive plant (Colocasia esculenta) to the ants Atta sexdens. We found out that the communities are different in quantity and in composition and 73% of the isolated fungi were from A. wilkesiana, which has fungi known as attractive to leaf-cutting ants, such as genus Colletotrichum, Pestalotiopsis, Phomopsis and Xylaria. On the other hand, in C. esculenta, there was found the genus Fusarium, known to be reject by the leaf-cutting ants, and less fungal diversity than in the attractive plant A. wilkesiana. Therefore, our data suggest that attractivity or repelence of a plant to the leaf-cutting ants could be related to presence or ausence of determinated fungi more than the quantity of fungi present in a leaf.

Morphology and Mechanical Properties of Vibratory Organs in the Leaf-cutting Ant （Atta cephalotes）

The ultrasonic scalpel has a number of excellent properties; however, its use in in vivo surgery is limited since the scalpel is not flexible enough. Changing the mechanism of ultrasonic vibration can allow the ultrasonic scalpel to bend. This paper reveals the mecha- nism of vibration generation of leaf-cutting ants, which is based on the microstructural and mechanical properties of special organs that produce the vibrations. Mierostructural characteristics of cross-sections of the vibratory organ of Atta cephalotes were observed using scanning electron microscopy. It was found that the scraper perfectly matches the file plate dorsoventrally; however, the file teeth cannot catch the scraper. An exploration of the kinematics of the file-scraper device was subsequently carried out to reveal a face-to-face contact mode, facilitating a gentler engagement process. For the first time, the mechanism of vibration generation of leaf-cutting ants was inves- tigated using a laser micrometer and high-speed camera. Results reveal the file-scraper device significantly amplifies the input frequency by 125 times, and magnification depends mainly on the tooth spacing and speed of engagement. Finally, nanoindentation tests were performed on file and scraper samples. The results show that they have similar mechanical properties, which greatly reduces friction and wear. This paper may provide theoretical guidance for the develooment of bionic vibration generators.

Acromyrmex ameliae sp. n. （Hymenoptera： Formicidae）： A new social parasite of leaf-cutting ants in Brazil

The fungus-growing ants （Tribe Attini） are a New World group of〉 200 species, all obligate symbionts with a fungus they use for food. Four attine taxa are known to be social parasites of other attines. Acromyrmex （ Pseudoatta） argentina argentina and Acromyrmex （Pseudoatta） argentina platensis （parasites of Acromyrmex lundi）, and Acromyrmex sp. （a parasite of Acromyrmex rugosus） produce no worker caste. In contrast, the recently discovered Acromyrmex insinuator （a parasite of Acromyrmex echinatior） does produce workers. Here, we describe a new species, Acromyrmex ameliae, a social parasite of Acromyrmex subterraneus subterraneus and Acromyrmex subterraneus brunneus in Minas Gerais, Brasil. Like A. insinuator, it produces workers and appears to be closely related to its hosts. Similar social parasites may be fairly common in the fungus-growing ants, but overlooked due to the close resemblance between parasite and host workers.

Bioinspired Scraper-File Type Frequency-Doubling Ultrasonic Exciter

In the natural world,leaf-cutting ants cause vibrations through their mutual scraping of file-scraper organs.In this study,we designed a Biomimetic Ultrasonic Exciter(BUE)that imitates leaf-cutting ants.The operating characteristics of the BUE were studied through experimental testing and finite element simulations.The results showed that the BUE could generate stable ultrasonic vibrations,and that the excitation frequency only needed to be half the Output Frequency(OF).This frequency-doubling phenomenon was conducive to achieving BUE miniaturization.To further explore the phenomenon of frequency-doubling vibration output,this study designed scrapers of five different sizes,conducted excitation and first-order natural frequency measurement tests,and the corresponding finite element simulations.It was found that each scraper could operate in frequency-doubling mode,but the operating frequency and natural mode frequencies did not correspond with one another.To further explicate experimental and simulation results,a two-degrees-of-freedom vibration model was developed.It was evident that the contact relationship between the dentate disc and scraper introduced strong nonlinear factors into the system,accounting for the frequency-doubling phenomenon and the difference between the BUE’s operating and mode frequencies.The BUE could be expected to facilitate the production of high-power micro-ultrasonic generators and has potential application value in the fields of mechanical processing,industrial production,and medical health.

Influence of physical and chemical factors during foraging and culture of the symbiont fungus in Atta sexdens rubropilosa （Hymenoptera： Formicidae）

The aim of this paper is to investigate the influence of physical and chemical factors on transport and use of substrate for Atta sexdens rubropilosa workers. Three types of rectangular fragments were used to study the physical influence factors： filter paper with paraffin, filter paper without paraffin and polyester film. To study the chemical factors, some fragments were impregnated with organic extract of orange albedo, others were soaked with soybean oil and for the remaining ones nothing was applied. The fonowing parameters were evaluated： （i） attractiveness of substrate for transport and number of loading workers per treatment; （ii） foraged material incorporation; （iii） rejection by numbers of fragments deposited in the garbage or beside the fungus garden. All the polyester film fragments carried out to the fungus garden were subsequently rejected. We verified that chemical factors of the substrate were more quickly detected by the workers, whereas physical factors were used as a criterion in the decision-making to reject or accept the substrate collected.