Temporal polyethism, life expectancy, and entropy of workers of the ant Ectatomma vizottoi Almeida, 1987 (Formicidae: Ectatomminae)

We investigated the changes in the behavioral repertoire over the course of life and determined the life expectancy and entropy of workers of the ant Ectatomma vizottoi. Newly emerged ants were individually marked with model airplane paint for observation of behaviors and determination of the age and life expectancy. Ants were divided into two groups: young and old workers. The 36 behaviors observed were divided into eight categories. Workers exhibit a clear division of tasks throughout their lives, with young workers performing more tasks inside the colony and old workers, outside, unlike species that have small colonies. This species also exhibits an intermediate life expectancy compared to workers of other species that are also intermediary in size. This supports the hypothesis of a relationship between size and maximum life expectancy, but it also suggests that other factors may also be acting in concert. Entropy value shows a high mortality rate during the first life intervals

Division of labour and its regulation in a primitively eusocial wasp

Social insect species that exhibit overlap of generations, cooperative brood care and reproductive caste differentiation have been termed eusocial. Among these, those that also exhibit morphological caste differentiation are termed highly eusocial while those that have morphologically identical queen and worker castes are termed primitively eusocial. Because queens and workers are not morphologically differentiated in primitively eusocial species, caste differentiation is often post-imaginal. The processes by which some individuals succeed in becoming reproductive queens while others end up as sterile workers in primitively eusocial species is of obvious interest. Such post-imaginal reproductive caste differentiation is absent in highly eusocial species because reproductive caste differentiation in these species is usually pre-imaginal. On the other hand, further differentiation of the workers into different kinds of task specialists is a phenomenon that has been extensively studied in highly eusocial species (Wilson, 1971). There has seldom been an opportunity to study both post-imaginal reproductive caste differentiation as well as post-imaginal division of non-reproductive labour in the same species because these two phenomena usually do not occur in the same species. Ropalidia marginata is an old world tropical primitively eusocial wasp widely distributed in southern India. Although classified as primitively eusocial by the traditional criteria of overlap of generations, cooperative brood care, reproductive caste differentiation and absence of morphological differentiation between queens and workers (Gadagkar, 2001a,b), R. marginata appears to have acquired some features resembling highly eusocial species. One such feature concerns the mode of division of non-reproductive labour among the adult wasps. R. marginata exhibits a remarkably well-developed honeybee like age polyethism. Workers show a strong tendency to feed larvae, build the nest, forage for pulp and forage for food, in that order, as they age. The frequency and probability of performance of different tasks is strongly influenced by the age of the individual (Naug and Gadagkar, 1998a). Like in the honeybees, age polyethism in R. marginata is very flexible and this flexibility comes from the fact that workers are allocated tasks based more on their relative age rather than their absolute age. This can be demonstrated experimentally by creating artificial colonies containing only young individuals (young cohort colonies) or containing only old individuals (old cohort colonies). In young cohort colonies, precocious foragers, i.e., some individuals who begin to forage at an early age when they would not do so in normal colonies, compensate for the absence of older individuals (Naug and Gadagkar, 1998b). In old cohort colonies, hard working nurses, i.e., some individuals who feed larvae at rates higher than they would in normal colonies, compensate for the absence of younger individuals (Agrahari and Gadagkar, submitted). Juvenile hormone modulates post-imaginal reproductive division of labor in primitively eusocial species and promotes the production of queens (e.g., Polistes) while it modulates age polyethism and promotes the production of foragers in highly eusocial species (e.g., the honeybee). Since R. marginata shows both post-imaginal regulation of reproductive division onabor as well as age polyethism, it is a particularly interesting model system to study the effect of juvenile hormone. A single, topical application of 100 ~g. of juvenile hormone-III per female wasp accelerates ovarian development of wasps held in isolation. Similar application to wasps released back on to their natal nests has no effect on their rate of behavioral development as witnessed from the age of first performance of feed larva, build, bring pulp and bring food. It appears, therefore, that in R. marginata, juvenile hormone has retained its function of modulating reproductive division of labor and has not acquired the function of modulating age polyethism (Agrahari and Gadagkar, 2003)

Developmental biomechanics and age polyethism in leaf-cutter ants.

Many social insects display age polyethism: young workers stay inside the nest, and only older workers forage. This behavioural transition is accompanied by genetic and physiological changes, but the mechanistic origin of it remains unclear. To investigate if the mechanical demands on the musculoskeletal system effectively prevent young workers from foraging, we studied the biomechanical development of the bite apparatus in Atta vollenweideri leaf-cutter ants. Fully matured foragers generated peak in vivo bite forces of around 100 mN, more than one order of magnitude in excess of those measured for freshly eclosed callows of the same size. This change in bite force was accompanied by a sixfold increase in the volume of the mandible closer muscle, and by a substantial increase of the flexural rigidity of the head capsule, driven by a significant increase in both average thickness and indentation modulus of the head capsule cuticle. Consequently, callows lack the muscle force capacity required for leaf-cutting, and their head capsule is so compliant that large muscle forces would be likely to cause damaging deformations. On the basis of these results, we speculate that continued biomechanical development post eclosion may be a key factor underlying age polyethism, wherever foraging is associated with substantial mechanical demands

Juvenile hormone downregulates vitellogenin production in Ectatomma tuberculatum (Hymenoptera: Formicidae) sterile workers

In the ant Ectatomma tuberculatum (Olivier 1792), workers have active ovaries and lay trophic eggs that are eaten by the queen and larvae. Vitellogenins are the main proteins found in the eggs of insects and are the source of nutrients for the embryo in the fertilized eggs and for adults in the trophic eggs. In social insects, vitellogenin titres vary between castes and affect reproductive social status, nursing, foraging, longevity, somatic maintenance, and immunity. In most insects, vitellogenin synthesis is mainly regulated by juvenile hormone. However, in non-reproductive worker ants, this relationship is poorly characterized. This study determined the effects of juvenile hormone on vitellogenin synthesis in non-reproductive E. tuberculatum workers. Juvenile hormone was topically applied onto workers, and the effect on vitellogenin synthesis in the fat body and vitellogenin titres in the haemolymph were analysed by ELISA and qPCR. Juvenile hormone downregulated protein synthesis and reduced vitellogenin titres in the haemolymph, suggesting that in workers of E. tuberculatum, juvenile hormone loses its gonadotrophic function