Temperature-induced alteration of cuticular lipids are not required for transition phenomenon in ticks

The linkage between a phase change in cuticular lipids and accelerated water loss is defined as critical transition temperature (CTT), a characteristic in ticks reputed for implying habitat preference and serving as a cue to regulate water levels during feeding. The CTT is based on activation energy (E_a) describing the effect of temperature (T) change on the rate of water loss (k) by the Arrhenius equation, k = A exp — E_a/RT. Typical CTT expression in American dog tick, Dermacentor variabilis, and lone star tick, Amblyomma americanum, occurs at 35–40 °C in the adult and 30–35°C in the larva (used for their lack of spiracles). Removing cuticular lipids from ticks produced higher evaporative losses, but this did not ablate or modify CTT or corresponding values of E_a. What changed was frequency (steric) factor A, simply as an integrated measure of increased water loss from the delipidizing treatment. This first analysis of the actual animal, rather than a cuticular lipid extract, demonstrates the failure of E_a to respond to artificially increased water loss rates, allowing for the role of cuticular lipids in tick water-proofing to be separated from CTT and the E_a.