Taxy and variations of leg setae and solenidia in Tetranychus urticae (Acari, Prostigmata)
1998 - Volume: 39 Issue: 3 pages: 233-255
Keywordsevolution chaetotaxy solenidiotaxy development asymmetry meristic variations Tetranychus urticae
In actinotrichid mites, setae and solenidia (mechanoreceptor and/or chemoreceptor phaneres) may show meristic variations in the form of asymmetrical absences. Numerical variations of leg setae and solenidia were studied in a population of Tetranychus urticae (White Eyes I strain) on adult and immature individuals as well as on pupae. Females had 131 leg phaneres on either side of the body, males had 5 more solenidia. On the whole, 35 phaneres were found variable in both sexes whereas 12 additionnal phaneres varied only among females and 35 others among males. Most phaneres showed little variation. By contrast, 10 setae and 4 male solenidia varied in more than 5% of individuals. Such frequent variations reveal a low ’priority’, i.e. an evolutionary trend for a complete suppression in the future. With few exceptions, measured Variations conform to the 3 models of regression described by Granjean (1954a): vertical, ascendant and descendant. Variations of many setae can be related to the model of descendant regression, and this is clearly unusual in Actinotrichida. Also uncommon were variations shown by some phaneres (e.g. the seta ev N1 in femur IV) since they can be related to two models, suggesting an inter-individual variability of regressive processes in the population studied. Departures in size and location were less frequent than numerical variations. However, to our knowledge, the left/right inconstancy of disjunctions or basculations we detected in some paired setae (e.g. the pair v - v’’ of tibia I) has never been reported in any other Actinotrichida.
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