Key structures of insects, such as their mandibles, become worn by the use and this leads to performance constraints, increase of metabolic costs, and decrease of individual’ both lifespan and survival. Studying this phenomenon is interesting in species that build nests as it demands a significant amount of mandible labor; however, the effect of this selective factor is less obvious in species like social vespids that perform different tasks at different ages. By comparing mandible wear between individuals of different ages in 18 colonies of 13 species of neotropical social wasps (Vespidae: Polistinae), we aimed to understand (1) the distribution of mandible wear across ages, and (2) the association between colony size and the coefficient of variation of mandible wear as an indicator of specialization. Despite a general positive trend between mandible wear and age, there are numerous exceptions and no age seems to concentrate this phenomenon. Mandible wear variation was evenly distributed throughout the worker ages, and was not correlated with colony size. Our study suggests that individuals distribute mandible demanding tasks throughout their life to reduce structural attrition which may improve worker’s general output. We proposed that, to extend both life expectancy and general performance of the workers, wasps may spread the more demanding tasks over longer and less intense periods.

We studied 18 colonies of 13 species of neotropical Polistinae wasps selected as representatives of major clades of the neotropical Polistinae phylogenetic hypotheses (Wenzel and Carpenter 1994; Piekarski et al. 2018; Menezes et al. 2020). The number of individuals studied per nest were as follows: Mischocyttarus sp (n=15), Polistes erythrocephalus (nest A, n=15; nest B, n=12; nest C, n=15), Agelaia pallipes (n=13), Parachartergus apicalis (n=15), Parachartergus fraternus (n=16), Synoeca septentrionalis (n=14), Metapolybia aztecoides (n=12), Protopolybia exigua (n=13), Charterginus fulvus (n=15), Polybia occidentalis (nest A, n=14; nest B, n=15; nest C, n=15), Polybia velutina (n=14), Polybia gorytoides (n=14), and Polybia emaciata (nest A, n=9; nest B, n=14). We studied a total of 250 individuals. We avoid within-colony sampling bias since we captured the nests at night with all individuals using a plastic bag. After collection individuals were transferred to 96% ethanol.

We dissected mandibles from the individuals using a Leica S8APO stereomicroscope. Mandibles were cleaned using cavitation for 2 min at 26°C with an ultrasonic cleaner FS20D (Fisher Scientific), then they were dehydrated by soaking in ethanol 96% for 3min and subsequently transferred to Xylol for 10 min. Gold-coated mandibles were photographed in a mesial position using a scanning electron FEI Quanta200 microscope at 25-30 kV (Fig 1). Measurements were taken from scaled SEM images using ImageJ version 1.50b (https://imagej.nih.gov/ij/). Every measurement is the mean of the left and right mandibles of each individual. Mandible terminology follows (Silveira and dos Santos 2011).

Variables

Species codes: Mis=Mischocyttarus sp., P.rA, P.rB, P.rC =Polistes erythrocephalus three colonies A,B,C, A.pal=Agelaia pallipes, Par A=Parachartergus apicalis, Par B=Parachartergus fraternus, S.sp= Synoeca septentrionalis, Mt=Metapolybia aztecoides, Pro=Protopolybia exigua, Char=Charterginus fulvus, P.ocA, P.ocB, P.ocC=Polybia occidentalis three nests A,B,C, P.vl=Polybia velutina, P.gor=Polybia gorytoides, and P.mA, P.mB= Polybia emaciata two nests A,B. A, B and C letters refer to different nests within a species.

Csize: refers to the number of individuals recorded from the colonies studied

Age: We use individual´s wear at the distal margin of the wings as an indicator of both relative age and shifting from within colony activities (young) to the foraging activity stage (old). A third intermediate category (middle) was used as indicator of the initial stages of foraging activities.

CV: We used the coefficient of variation (CV) of mean denticle wear per age within a colony as indicator of task specialization. CV was calculated as the ratio between standard deviation and the average values per nest multiplied by 100.