Changes in nymphal morphometric values and tarsal microstructures during postembryonic development in the Neotropical harvestman Heteromitobates albiscriptus (Opiliones: Gonyleptidae)

Abstract

The postembryonic development of Opiliones (Arachnida) includes three phases: larval, nymphal (with four to eight instars), and adult (when molts cease). The present study aimed to describe the postembryonic development of Heteromitobates albiscriptus (Mello-Leitao, 1932) (Gonyleptidae) including both a morphometric study and SEM analysis of two structures present in the tarsus of nymphs and adults: the "tarsal aggregate pores" (TAPs) and the "tarsal perforated organ" (TPO). The nymphal phase includes five stages, which can be easily recognized by morphometric values. In contrast to the pectinate tarsal claws found in legs III-IV of adults (the main synapomorphy of the genus Heteromitobates in the subfamily Goniosomatinae), nymphs bear smooth claws. First nymphs lack TAPs and TPOs. TAPs seem to have a precisely defined position in both prolateral and retrolateral faces of the tarsus. The number of pores in TAPs grows from three or four among second nymphs to around 20 among adults, and measure around 2.15 mu m in diameter with no clear difference between ages. An additional field of pores on legs III-IV ("ventral tarsal aggregate pores" vTAPs) was detected only among adults. The plates at the base and the apex of the TPOs differ from the ones in between. The length of the TPO and its number of plates increase with each molt. However, there is no discernible pattern of growth throughout the postembryonic development when taking into account both the average size of the plates (ranging between similar to 7-11 mu m) and the ratio of TPO length to tarsus length.