Water Balance and Humidity Preference in Three Species of Ticks1

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Abstract
Comparisons were made between the abilities of 3 species of ticks, Amblyomma americanum (L.), A. maculatum Koch, and Dermacentor variabilis Say to resist dehydration and maintain water balance. Of the 3 species, A. americanum appears most dependent on moist microenvironments because of its greater susceptibility to percentage losses of total body water and drop in hemolymph volume at low humidities. This species appears to counteract these losses as adults by seeking environments with relative humidities above its critical equilibrium humidity (CEH). It does so very efficiently after water losses of ca 10–15% of its beginning weight. Although A. maculatum loses percentage water reserves less rapidly, it apparently is also dependent on relatively moist habitats because of its high critical equilibrium humidity (92–93% rh). By comparison, D. variabilis loses percentage water reserves at about the same rate as A. maculatum; but because of its intermediate size (6.74 mg and 5.41 mg in females and males, respectively, versus 9.80 mg and 8.70 mg in A. maculatum and 4.47 and 2.77 mg in A. americanum), it loses the least absolute amounts of water when exposed to the same desiccating conditions. The hemolymph volume percentage in females of D. variabilis is little different in ticks held at 93% and 30% rh, despite an approximate 10% greater weight loss in ticks held at 30% for 16 days. Large numbers of this species also seek out humidities above the CEH (84–85% rh) when the percentage weight losses approach 10–15% of body weight. The hemolymph volume percentage was lower in A. maculatum (at all experimental conditions) than in both A. americanum and D. variabilis. The above data suggest that there may be some correlation between the “normal” geographical distributions of the 3 species studied and their abilities to resist dehydration and maintain water balance.

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