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Ontogenetic differences and interspecific variation in the tarsal aggregate pores on leg IV of cosmetid harvestmen (Opiliones: Laniatores)

Abstract Morphological studies of harvestmen usually investigate the structures of adults, seldom including data regarding ontogenetic differences in the microanatomy of the appendages. In gonyleptoidean harvestmen, adults have clusters of pores on the distal-most tarsomeres of legs III and IV that are known as tarsal aggregate pores (TAPs). In gonyleptid harvestmen, these pores occur on the prolateral and retrolateral surfaces as groups adjacent to the tarsal claws. In this study, we used scanning electron microscopy (SEM) to compare the microanatomy of the TAPs occurring on leg IV of adults and antepenultimate nymphs for three common species of Neotropical cosmetid harvestmen, nymphs of an additional seven cosmetid morphospecies, and adults and antepenultimate nymphs of three species of gonyleptoidean harvestmen. The distal tarsomere of leg IV of adult cosmetid harvestmen features two pairs of aggregate pores including dorsal TAPs and ventrolateral vTAPs. The TAPs of cosmetid nymphs have denticulate borders and are not closely associated with trichomes. We observed interspecific variation among cosmetid harvestmen with respect to the number of pores composing the TAPs. The TAPs on the prolateral and retrolateral surfaces occur in similar positions and are generally symmetrical with respect to overall shape and the number of pores. The functional significance of the TAPs and vTAPs of cosmetid harvestmen will require additional empirical evaluation.