Contribution to the prelarva status: The moulting cycle of the calyptostasic prelarva of the trombiculid mite Leptotrombidium orientale (Acariformes: Trombiculidae)
1999 - Volume: 40 Issue: 3 pages: 265-274
The calyptostasic prelarva of the trombiculid mite Leptotrombidium orientale (Schluger, 1948) and its moulting were investigated by means of transmission and scanning electron microscopy. Even before the formation of the prelarva, a particular embryonic cuticle above ectoderm cells is secreted; this soon detaches, leaving an exuvial space where haemocytes migrate. Prelarval cuticle is deposited above short microvilli of hypodermal cells and, in the completely developed state, is very similar to the cuticle of quiescent proto- and tritonymphs. It is composed of a thick, clear procuticle with curved pore canals and a narrow epicuticular layer, completely lacking setae. The hypodermal layer of the prelarva consists of flattened epithelial cells, which undergo mitotic divisions. The prelarva, as an individual instar with cuticle closely applied to the hypodermis, exists for a very short period and is completely concealed within the eggshell, being only a pharate instar. Within the eggshell, the prelarva undergoes the next moult, expressed again by detachment of cuticle, migration of haemocytes into the exuvial space, reorganization of hypodermis and, finally, secretion of an essentially new strongly-folded, cuticle of the active larva. At this time, the eggshell splits and the prelarval exuvium with the pharate larva inside appears. After some time, the latter hatches actively from the prelarval cuticle. Since the egg at the later stages is not an embryo, but only a cover for the pharate prelarva as well as for the larva, the processes considered can not be attributed to an embryonization and the prelarva cannot be regarded as an embryonic instar.
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