Mandible elemental composition and mechanical properties from distinct castes of the leafcutter ant Atta laevigata (Attini; Formicidae)

Abstract

Leafcutter ant colonies are divided into castes with the individuals performing different tasks, based mostly on size. With the mandibles, the small minims care for the brood or the fungus, whereas the larger minors and mediae cut and transport plant material, with the ant size positively related to the material size. The mechanical properties and composition of the mandible cuticle have been previously tested in the soldiers as the largest caste, revealing that the cutting edges contained high contents of the cross-linking transition metal zinc (Zn). With regard to the smaller castes, no data are present. To study how the mandible size and function relates to its mechanical properties, we here tested the mandibles of minims, minors and mediae by nanoindentation. We found that the hardness (H) and Young's modulus (E) values increased with increasing ant size and that the mandible cutting edges in each caste have the highest H- and E-values. To gain insight into the origins of these properties, we characterized the elemental composition by energy-dispersive X-ray analysis, revealing that minors and mediae possessed higher content of Zn in the cutting edges in contrast to the minims containing significantly less Zn. This shows, that Zn content relates to higher mechanical property values. Additionally, it shows that all of these parameters can differ within a single species.

Keywords: Young's modulus; biomechanics; cuticle; hardness; mouthparts; nanoindentation.

Figure 1.

(a) Light microscopic images of one head from each caste (media, minor, minim). (b) Embedded and polished mediae mandible used for the EDX and nanoindentation experiments. The white spots highlight the localities of these tests at the exocuticle. The mandible was divided into three different regions (cutting edge, outer edge, curvature to the base). Each region was divided into three or five areas (a, b, c, d, e) and tested from distal (left) to proximal (right). (c,d) SEM images of one media mandible in dorsal and ventral view. (e,f) SEM images of one minor mandible in dorsal and ventral view. (g,h) SEM images of one minim mandible in dorsal and ventral view. Abbreviations: C, condyle; Ce, cutting edge; Ctb, curvature to basis; d, distal; p, proximal; m, medial; l, lateral; Oe, outer edge. Scale bars: a, 2 mm; b–f, 300 µm; g–h 100 µm.

Figure 2.

Results of EDX analyses: proportions of the individual elements and Ae (sum of Ca, Cl, Cu, Fe, K, Mg, Mn, P + Pt, S, Si, Zn), given in atomic %, sorted to the castes.

Figure 3.

Results of EDX analyses: proportions of the individual elements and Ae (sum of Ca, Cl, Cu, Fe, K, Mg, Mn, P + Pt, S, Si, Zn), given in atomic %, sorted to the castes and regions (cutting edge, outer edge, curvature to the basis).

Figure 4.

Results of nanoindentation: hardness and Young's modulus (both in GPa) for each caste. (a) Results from all regions and areas pooled together. (b) Results sorted to the regions (cutting edge, outer edge, curvature to the basis).