188 MITES AND TICKS 



functionally interconnected. Resistance to desiccation at low 

 humidities is achieved by a dual mechanism: active secretion and 

 the physical retention of water by the wax-layer. 



The unfed sheep tick when at rest adopts either a questing atti- 

 tude with the fore legs extended, or an attitude of repose with the 

 legs folded. The ticks respond to gravity, humidity, temperature, 

 smell, light and tactile stimuli. They climb upwards towards the 

 tips of the leaves of the grass in which they live until they become 

 desiccated when they become active and move downwards again to 

 the moist air near the ground where they recover their water 

 balance by active secretion through the cuticle: then they climb 

 upwards again. If a suitable host happens to pass they are attracted 

 by the moisture and scent of its body, and the vibrations caused by 

 its movements, and respond by active questing. After feeding, the 

 engorged ticks drop to the ground and are then strongly photo- 

 negative (Lees, 1948). 



The main stimulus governing the ascent of grass blades by 

 larvae of the Australian cattle tick, Boophilus micropluSy is positive 

 phototaxis to moderate light intensities: the ticks shelter from 

 direct sunlight. In the field they are found to be more exposed in 

 the early morning when they are often at the tops of grass stalks. 

 The strongest questing behaviour occurs in response to odours, 

 but vibration, air currents, interrupted illumination, warmth and 

 moisture all play a part in the reactions to the host (Wilkinson, 

 1953). 



Argasid ticks are also of considerable economic and medical 

 importance. They occur on a wide variety of hosts including rep- 

 tiles, birds and mammals and in habits somewhat resemble bed 

 bugs. As a rule they are nocturnal, feeding moderately and at fre- 

 quent intervals. When fasting they are flat and readily creep into 

 narrow crevices. Argas persicus is not only a troublesome domestic 

 pest in some places but it also infests poultry all over the world, 

 causing much damage by its bites and sometimes transmitting a 

 spirochaete disease. Cattle are susceptible to the spinose ear tick 

 and relapsing fever is conveyed to humans by the notorious Or- 

 nithodorus moubata. 



The Thrombidiformes show a great diversity of form, life 

 history and behaviour and but a few selected examples can be 



